Go チートシート

package main

import "fmt"

func main() {
    fmt.Println("Hello, World!")
    fmt.Printf("Name: %s, Age: %d\n", "Alice", 30)
}

// Run: go run main.go
// Build: go build -o app main.go
// Format: gofmt -w main.go

// Initialize a new module
// $ go mod init github.com/user/project

// go.mod file:
module github.com/user/project

go 1.21

require (
    github.com/gin-gonic/gin v1.9.1
    golang.org/x/sync v0.3.0
)

// Add dependencies:
// $ go get github.com/gin-gonic/gin
// $ go mod tidy  // clean up unused deps

// project structure:
// myproject/
//   go.mod
//   main.go          // package main
//   utils/
//     helper.go      // package utils
//     math.go
//   internal/
//     api/           // private (only importable within parent)

// main.go
package main

import (
    "fmt"
    "github.com/user/project/utils"
)

func main() {
    fmt.Println(utils.Add(1, 2))
}

// Exported names start with uppercase letter
// unexported names start with lowercase letter

package main

import "fmt"

// Package-level declarations
var version = "1.0.0" // inferred type
const Pi = 3.14159

func main() {
    // var with type
    var name string = "Alice"
    var age int = 30

    // Short declaration (only inside functions)
    city := "NYC"

    // Multiple assignment
    a, b, c := 1, 2, 3

    // Zero values (default)
    var x int      // 0
    var s string   // ""
    var ok bool    // false
    var p *int     // nil

    fmt.Println(name, age, city, a, b, c, x, s, ok, p)
}

// Numeric types
var i int = 42
var i8 int8 = 127
var u uint = 42
var f32 float32 = 3.14
var f64 float64 = 3.141592653589793

// Other types
var b bool = true
var s string = "hello"
var r rune = 'A' // int32 alias for Unicode code point
var by byte = 255 // uint8 alias

// Type conversion (explicit, no implicit conversion)
var n int = 42
var f float64 = float64(n)  // int -> float64
var s string = string(65)   // 65 -> "A" (rune to string)
var i2 int = int(f)         // float64 -> int (truncates)

// Type inference
var x = 42        // int
var y = 3.14      // float64
var z = len("hi") // int

package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "Hello, World"

    // Length (bytes, not runes!)
    fmt.Println(len(s)) // 12

    // Concatenation
    s1 := "Hello" + " " + "World"
    s2 := fmt.Sprintf("%s = %d", "age", 30)

    // Comparison
    fmt.Println("abc" == "abc") // true
    fmt.Println("a" < "b")      // true (lexicographic)

    // Iteration (by rune)
    for i, r := range "Go语" {
        fmt.Printf("%d: %c\n", i, r)
    }
}

import (
    "fmt"
    "strings"
)

s := "Hello, World"

// Case
strings.ToUpper(s)         // "HELLO, WORLD"
strings.ToLower(s)         // "hello, world"
strings.Title("hello world") // "Hello World"

// Search
strings.Contains(s, "World")  // true
strings.HasPrefix(s, "Hello") // true
strings.HasSuffix(s, "World") // true
strings.Index(s, "World")     // 7 (-1 if not found)
strings.Count(s, "l")         // 3

// Split & Join
parts := strings.Split("a,b,c", ",")  // ["a", "b", "c"]
joined := strings.Join(parts, "-")    // "a-b-c"

// Replace
strings.Replace(s, "l", "L", 1)  // "HeLlo, World" (1 occurrence)
strings.ReplaceAll(s, "l", "L")  // "HeLLo, WorLd"
strings.TrimSpace("  hi  ")      // "hi"

import (
    "fmt"
    "strconv"
)

// String to number
n, err := strconv.Atoi("42")     // string -> int
f, err := strconv.ParseFloat("3.14", 64)

// Number to string
s1 := strconv.Itoa(42)            // int -> string
s2 := strconv.FormatFloat(3.14, 'f', 2, 64) // "3.14"

// Printf formatting
fmt.Printf("Int: %d\n", 42)
fmt.Printf("Float: %.2f\n", 3.14159)
fmt.Printf("String: %s\n", "hi")
fmt.Printf("Bool: %t\n", true)
fmt.Printf("Type: %T\n", 42)     // int
fmt.Printf("Hex: %x\n", 255)     // ff
fmt.Printf("Pad: %05d\n", 42)    // 00042
fmt.Printf("Quote: %q\n", "hi")  // "hi"

// Sprintf returns formatted string
result := fmt.Sprintf("Name: %s, Age: %d", "Alice", 30)

// Raw string literal (backticks) - no escape processing
raw := `This is a
multiline string
with \n (literal backslash-n)`

// Interpreted string (double quotes) - processes escapes
interpreted := "Line1\nLine2\tTabbed"

// Raw strings are useful for:
// - Regex patterns
// - HTML/SQL templates
// - File paths on Windows
regex := `^d{4}-d{2}-d{2}$`

// String builder (efficient concatenation)
var sb strings.Builder
for i := 0; i < 1000; i++ {
    sb.WriteString("line\n")
}
result := sb.String()

import (
    "fmt"
    "unicode/utf8"
)

s := "Hello, 世界"

// Byte length vs rune length
fmt.Println(len(s))                    // 13 (bytes)
fmt.Println(utf8.RuneCountInString(s)) // 9 (runes/characters)

// Decode runes manually
for i := 0; i < len(s); {
    r, size := utf8.DecodeRuneInString(s[i:])
    fmt.Printf("%c ", r)
    i += size
}

// Range automatically decodes runes
for i, r := range s {
    fmt.Printf("%d:%c ", i, r)
}

// Rune slice (mutable)
runes := []rune("Hello")
runes[0] = 'J'
s2 := string(runes) // "Jello"

// Basic if/else
score := 85
if score >= 90 {
    fmt.Println("A")
} else if score >= 80 {
    fmt.Println("B")
} else {
    fmt.Println("C")
}

// If with initialization statement
if n := computeValue(); n > 100 {
    fmt.Println("big:", n)
} else {
    fmt.Println("small:", n)
}
// n is not accessible here (scoped to if)

// No parentheses needed, but braces are required

// C-style for
for i := 0; i < 5; i++ {
    fmt.Println(i)
}

// While-style (condition only)
n := 10
for n > 0 {
    n--
}

// Infinite loop
for {
    break
}

// Range (iterate slices, maps, strings)
nums := []int{10, 20, 30}
for index, value := range nums {
    fmt.Printf("%d: %d\n", index, value)
}

// Skip index or value with _
for _, value := range nums {
    fmt.Println(value)
}

// Range over map (unordered!)
m := map[string]int{"a": 1, "b": 2}
for key, val := range m {
    fmt.Println(key, val)
}

// Basic switch
day := 3
switch day {
case 1:
    fmt.Println("Mon")
case 2, 3, 4, 5:
    fmt.Println("Weekday")
case 6, 7:
    fmt.Println("Weekend")
default:
    fmt.Println("Invalid")
}

// Switch with no expression (like if/else chain)
switch {
case score >= 90:
    grade = "A"
case score >= 80:
    grade = "B"
default:
    grade = "C"
}

// Switch with init
switch os := runtime.GOOS; os {
case "linux":
    fmt.Println("Linux")
case "darwin":
    fmt.Println("macOS")
}

// Fallthrough (rare, goes to next case unconditionally)
switch 1 {
case 1:
    fmt.Println("one")
    fallthrough
case 2:
    fmt.Println("two") // executes even though value is 1
}

// Defer runs when function returns (LIFO order)
func main() {
    defer fmt.Println("third")
    defer fmt.Println("second")
    fmt.Println("first")
}
// Output: first, second, third

// Common: resource cleanup
func readFile(path string) error {
    f, err := os.Open(path)
    if err != nil {
        return err
    }
    defer f.Close() // runs when readFile returns

    // ... use f ...
    return nil
}

// Defer evaluates arguments immediately
i := 1
defer fmt.Println(i) // prints 1 (not 2)
i = 2

// Break and continue
for i := 0; i < 10; i++ {
    if i%2 == 0 {
        continue // skip even numbers
    }
    if i > 7 {
        break // exit loop
    }
    fmt.Println(i)
}

// Break with label (for nested loops)
outer:
for i := 0; i < 3; i++ {
    for j := 0; j < 3; j++ {
        if i == 1 && j == 1 {
            break outer // exits both loops
        }
    }
}

// Goto (rarely used, avoid)
i := 0
loop:
    if i < 5 {
        fmt.Println(i)
        i++
        goto loop
    }

// Select waits on multiple channel operations
select {
case msg := <-messages:
    fmt.Println("received:", msg)
case <-timeout:
    fmt.Println("timed out")
default:
    fmt.Println("no activity") // non-blocking
}

// Select with send
ch1 := make(chan int)
ch2 := make(chan int)
go func() { ch1 <- 1 }()
go func() { ch2 <- 2 }()

select {
case v := <-ch1:
    fmt.Println("ch1:", v)
case v := <-ch2:
    fmt.Println("ch2:", v)
}

// Select in a loop (event loop pattern)
for {
    select {
    case msg := <-ch:
        fmt.Println(msg)
    case <-time.After(5 * time.Second):
        return // timeout after 5s of inactivity
    }
}

// Function with multiple return values
func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, fmt.Errorf("divide by zero")
    }
    return a / b, nil
}

// Named return values
func split(sum int) (x, y int) {
    x = sum * 4 / 9
    y = sum - x
    return // "naked" return (uses named values)
}

// Usage
result, err := divide(10, 2)
if err != nil {
    log.Fatal(err)
}
fmt.Println(result) // 5

x, y := split(100)
fmt.Println(x, y)

// Variadic function (variable arguments)
func sum(nums ...int) int {
    total := 0
    for _, n := range nums {
        total += n
    }
    return total
}
fmt.Println(sum(1, 2, 3))       // 6
fmt.Println(sum(1, 2, 3, 4, 5)) // 15

// Spread a slice
nums := []int{1, 2, 3}
fmt.Println(sum(nums...)) // 6

// Closure (function that captures variables)
func counter() func() int {
    count := 0
    return func() int {
        count++
        return count
    }
}

next := counter()
fmt.Println(next()) // 1
fmt.Println(next()) // 2
fmt.Println(next()) // 3

// Functions are first-class values
func apply(f func(int) int, x int) int {
    return f(x)
}

func double(x int) int { return x * 2 }
func square(x int) int { return x * x }

fmt.Println(apply(double, 5)) // 10
fmt.Println(apply(square, 5)) // 25

// Function type
type MathFunc func(int) int

var fn MathFunc = func(x int) int { return x + 1 }
fmt.Println(fn(10)) // 11

// Map of functions
ops := map[string]func(int, int) int{
    "add": func(a, b int) int { return a + b },
    "sub": func(a, b int) int { return a - b },
    "mul": func(a, b int) int { return a * b },
}
fmt.Println(ops["add"](3, 4)) // 7

// init() runs before main(), once per package
// Multiple init() functions allowed, run in order declared
package main

import "fmt"

var config string

func init() {
    // Setup code: config, connections, validation
    config = "production"
    fmt.Println("init 1")
}

func init() {
    fmt.Println("init 2, config:", config)
}

func main() {
    fmt.Println("main")
}
// Output: init 1, init 2, main

// Anonymous function (IIFE)
result := func(x int) int {
    return x * 2
}(5) // immediately invoked, result = 10

// Goroutine with anonymous function
go func(msg string) {
    fmt.Println(msg)
}("async")

// Generic function with type parameter
func Map[T, U any](slice []T, f func(T) U) []U {
    result := make([]U, len(slice))
    for i, v := range slice {
        result[i] = f(v)
    }
    return result
}

nums := []int{1, 2, 3}
doubled := Map(nums, func(n int) int { return n * 2 })
// [2, 4, 6]

strs := Map(nums, func(n int) string {
    return fmt.Sprintf("num%d", n)
})
// ["num1", "num2", "num3"]

// Type constraints
type Number interface {
    int | int64 | float64
}

func Sum[T Number](nums []T) T {
    var total T
    for _, n := range nums {
        total += n
    }
    return total
}

// comparable constraint (for maps/slices keys)
func Contains[T comparable](slice []T, target T) bool {
    for _, v := range slice {
        if v == target {
            return true
        }
    }
    return false
}

type Rectangle struct {
    Width, Height float64
}

// Method with value receiver
func (r Rectangle) Area() float64 {
    return r.Width * r.Height
}

// Method with pointer receiver (can modify)
func (r *Rectangle) Scale(factor float64) {
    r.Width *= factor
    r.Height *= factor
}

// Method on non-struct types
type MyString string

func (s MyString) Shout() string {
    return strings.ToUpper(string(s)) + "!"
}

// Usage
r := Rectangle{Width: 10, Height: 5}
fmt.Println(r.Area()) // 50
r.Scale(2)
fmt.Println(r.Width)  // 20

s := MyString("hello")
fmt.Println(s.Shout()) // HELLO!

// Array (fixed length, rarely used directly)
var arr [3]int = [3]int{1, 2, 3}
arr2 := [...]int{4, 5, 6} // size inferred

// Slice (dynamic array, most common)
nums := []int{1, 2, 3}
nums = append(nums, 4)        // [1, 2, 3, 4]
nums = append(nums, 5, 6, 7) // [1, 2, 3, 4, 5, 6, 7]

// Make a slice with capacity
s := make([]int, 3, 10) // len=3, cap=10

// Slicing
sub := nums[1:4]  // [2, 3, 4]
first := nums[:2] // [1, 2]
last := nums[3:]  // [4, 5, 6, 7]

// Copy
dst := make([]int, len(nums))
copy(dst, nums)

// Length and capacity
fmt.Println(len(nums), cap(nums))

// Create a map
m := map[string]int{
    "Alice": 30,
    "Bob":   25,
}

// Using make
ages := make(map[string]int)
ages["Charlie"] = 35

// Access
fmt.Println(m["Alice"]) // 30

// Check existence (comma-ok pattern)
age, ok := m["David"]
if !ok {
    fmt.Println("not found")
}

// Delete
delete(m, "Bob")

// Iterate (random order)
for name, age := range m {
    fmt.Printf("%s: %d\n", name, age)
}

// Length
fmt.Println(len(m))

// Nested maps
matrix := map[string]map[string]int{}
matrix["row1"] = map[string]int{"col1": 1}

// Define a struct
type Person struct {
    Name string
    Age  int
    Address string // field names are exported (capitalized)
}

// Create instances
p1 := Person{Name: "Alice", Age: 30}
p2 := Person{"Bob", 25, "NYC"} // positional (not recommended)
p3 := new(Person)              // returns *Person, zero values
p3.Name = "Charlie"

// Anonymous struct (one-off)
config := struct {
    Port int
    Host string
}{
    Port: 8080,
    Host: "localhost",
}

// Struct embedding (composition)
type Employee struct {
    Person          // embedded (promotes fields)
    Salary   float64
}

emp := Employee{
    Person: Person{Name: "Dave", Age: 40},
    Salary: 50000,
}
fmt.Println(emp.Name) // "Dave" (promoted from Person)

// Pointer basics
x := 42
p := &x          // p is *int, points to x
fmt.Println(*p)  // 42 (dereference)
*p = 100         // modify x through pointer
fmt.Println(x)   // 100

// new() allocates and returns pointer
p2 := new(int)   // *int, value 0
*p2 = 42

// Pointers to structs
type User struct{ Name string }
u := &User{Name: "Alice"} // *User
u.Name = "Bob"            // (*u).Name shorthand
fmt.Println(u.Name)       // Bob

// nil pointer
var p3 *int // nil
// *p3 // panic: nil pointer dereference

// No pointer arithmetic (unlike C)
// p++ is illegal

// Filter
func filter(nums []int, pred func(int) bool) []int {
    result := []int{}
    for _, n := range nums {
        if pred(n) {
            result = append(result, n)
        }
    }
    return result
}

evens := filter([]int{1,2,3,4,5}, func(n int) bool {
    return n%2 == 0
}) // [2, 4]

// Insert at index
func insert(s []int, i, v int) []int {
    s = append(s, 0)
    copy(s[i+1:], s[i:])
    s[i] = v
    return s
}

// Remove at index
func remove(s []int, i int) []int {
    return append(s[:i], s[i+1:]...)
}

// 2D slice
matrix := make([][]int, 3)
for i := range matrix {
    matrix[i] = make([]int, 4)
}

import "sort"

// Sort a slice
nums := []int{3, 1, 4, 1, 5, 9, 2, 6}
sort.Ints(nums) // [1 1 2 3 4 5 6 9]
fmt.Println(nums)

// Sort strings
strs := []string{"banana", "apple", "cherry"}
sort.Strings(strs) // [apple banana cherry]

// Sort with custom comparator
type Person struct {
    Name string
    Age  int
}

people := []Person{
    {"Alice", 30},
    {"Bob", 25},
    {"Charlie", 35},
}

sort.Slice(people, func(i, j int) bool {
    return people[i].Age < people[j].Age // by age ascending
})

// Binary search (sorted slice)
idx := sort.SearchInts(nums, 4) // index of 4

// Sort package (Go 1.21+): slices.Sort
import "slices"
slices.Sort(people) // needs Less method or cmp func

// Interface: a set of method signatures
type Shape interface {
    Area() float64
    Perimeter() float64
}

// Implement implicitly (no 'implements' keyword)
type Circle struct {
    Radius float64
}

func (c Circle) Area() float64 {
    return math.Pi * c.Radius * c.Radius
}

func (c Circle) Perimeter() float64 {
    return 2 * math.Pi * c.Radius
}

// Use the interface
func describe(s Shape) {
    fmt.Printf("Area: %.2f, Perimeter: %.2f\n",
        s.Area(), s.Perimeter())
}

c := Circle{Radius: 5}
describe(c) // works! Circle implements Shape

// Empty interface (any type)
// Go 1.18+: 'any' is an alias for interface{}
func printAny(v any) {
    fmt.Println(v)
}

printAny(42)
printAny("hello")
printAny([]int{1, 2, 3})

// Type assertion
var i any = "hello"
s := i.(string)    // panics if not string
fmt.Println(s)

// Safe type assertion (comma-ok)
s, ok := i.(string)
if ok {
    fmt.Println("string:", s)
}

// Type switch
func describe(v any) {
    switch x := v.(type) {
    case int:
        fmt.Printf("int: %d\n", x)
    case string:
        fmt.Printf("string: %s\n", x)
    case []int:
        fmt.Printf("int slice: %v\n", x)
    default:
        fmt.Printf("unknown type: %T\n", x)
    }
}

// Compose interfaces
type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

// ReadWriter combines Reader and Writer
type ReadWriter interface {
    Reader
    Writer
}

// io.Reader and io.Writer are built-in interfaces
// Many types implement them: *os.File, *bytes.Buffer, net.Conn

func copyData(r io.Reader, w io.Writer) error {
    buf := make([]byte, 1024)
    for {
        n, err := r.Read(buf)
        if n > 0 {
            if _, err := w.Write(buf[:n]); err != nil {
                return err
            }
        }
        if err == io.EOF {
            break
        }
        if err != nil {
            return err
        }
    }
    return nil
}

type Counter struct {
    count int
}

// Value receiver: works on a copy, can't modify
func (c Counter) Get() int {
    return c.count
}

// Pointer receiver: can modify the original
func (c *Counter) Increment() {
    c.count++
}

c := Counter{}
c.Increment() // Go auto-takes address (&c).Increment()
c.Increment()
fmt.Println(c.Get()) // 2

// Interface implementation note:
// *Counter implements both Get() and Increment()
// Counter (value) implements only Get()
// So *Counter satisfies an interface requiring both

// Stringer interface (like toString() in other languages)
type Stringer interface {
    String() string
}

type Person struct {
    Name string
    Age  int
}

// Implement Stringer
func (p Person) String() string {
    return fmt.Sprintf("%s (%d years)", p.Name, p.Age)
}

p := Person{Name: "Alice", Age: 30}
fmt.Println(p)       // Alice (30 years) — uses String()
fmt.Printf("%v\n", p) // Alice (30 years)
fmt.Printf("%+v\n", p) // Name:Alice Age:30
fmt.Printf("%#v\n", p) // main.Person{Name:"Alice", Age:30}

// Error interface is similar:
// type error interface { Error() string }
// Implement Error() to make a type usable as an error

// Embed a struct (composition over inheritance)
type Animal struct {
    Name string
}

func (a Animal) Speak() string {
    return a.Name + " makes a sound"
}

type Dog struct {
    Animal // embedded — promotes Name and Speak()
    Breed  string
}

d := Dog{
    Animal: Animal{Name: "Rex"},
    Breed:  "Labrador",
}

fmt.Println(d.Name)      // "Rex" (promoted)
fmt.Println(d.Speak())   // "Rex makes a sound" (promoted)
fmt.Println(d.Animal.Name) // explicit access also works

// Override a promoted method
func (d Dog) Speak() string {
    return d.Name + " barks!"
}
fmt.Println(d.Speak()) // "Rex barks!"

// Embed an interface
type Logger struct {
    io.Writer // embed interface
}
// Logger now has Write() method, delegating to the embedded Writer

// Functions return errors as last return value
func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("cannot divide by zero")
    }
    return a / b, nil
}

// Always check errors immediately
result, err := divide(10, 0)
if err != nil {
    log.Fatal(err) // or handle gracefully
}
fmt.Println(result)

// fmt.Errorf for formatted errors
func validate(age int) error {
    if age < 0 {
        return fmt.Errorf("invalid age: %d (must be positive)", age)
    }
    return nil
}

// Sentinel errors
var ErrNotFound = errors.New("not found")
if err := lookup("key"); err == ErrNotFound {
    // handle not found
}

// Custom error type (implements error interface)
type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation error on '%s': %s", e.Field, e.Message)
}

// Use the custom error
func validateUser(name string) error {
    if name == "" {
        return &ValidationError{
            Field:   "name",
            Message: "cannot be empty",
        }
    }
    return nil
}

// Type-assert to access fields
err := validateUser("")
if ve, ok := err.(*ValidationError); ok {
    fmt.Println("Field:", ve.Field)   // "name"
    fmt.Println("Message:", ve.Message) // "cannot be empty"
}

import "errors"

// Wrapped errors with %w
var ErrNotFound = errors.New("not found")

func getUser(id int) error {
    if id == 0 {
        return fmt.Errorf("getUser(%d): %w", id, ErrNotFound)
    }
    return nil
}

// errors.Is: check if error matches (unwraps the chain)
err := getUser(0)
if errors.Is(err, ErrNotFound) {
    fmt.Println("user not found")
}

// errors.As: extract a specific error type
var ve *ValidationError
if errors.As(err, &ve) {
    fmt.Println("field:", ve.Field)
}

// errors.Unwrap: get the wrapped error
inner := errors.Unwrap(err)

// Panic: unrecoverable error (like throw)
func mustParse(s string) int {
    n, err := strconv.Atoi(s)
    if err != nil {
        panic(fmt.Sprintf("invalid number: %s", s))
    }
    return n
}

// Recover: catch a panic (only in deferred functions)
func safeCall() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("recovered:", r)
        }
    }()
    panic("something went wrong")
}

// Practical: safe goroutine
func safeGo(fn func()) {
    go func() {
        defer func() {
            if r := recover(); r != nil {
                log.Println("goroutine panic:", r)
            }
        }()
        fn()
    }()
}

// Wrap errors with context as they propagate up
func fetchUser(id int) (*User, error) {
    row := db.QueryRow("SELECT ... WHERE id = ?", id)
    var u User
    if err := row.Scan(&u.Name, &u.Age); err != nil {
        return nil, fmt.Errorf("fetchUser(%d): %w", id, err)
    }
    return &u, nil
}

func getUserProfile(id int) (*Profile, error) {
    user, err := fetchUser(id)
    if err != nil {
        return nil, fmt.Errorf("getUserProfile(%d): %w", id, err)
    }
    // ...
}

// At the top level, log the full chain
profile, err := getUserProfile(42)
if err != nil {
    log.Printf("error: %v", err)
    // Output: getUserProfile(42): fetchUser(42): sql: no rows
}

// golang.org/x/xerrors or pkg/errors for stack traces

// Complete pattern: cleanup + panic recovery
func processFile(path string) (err error) {
    file, e := os.Open(path)
    if e != nil {
        return e
    }
    defer func() {
        // Close file regardless of panic
        file.Close()

        // Recover from panic and convert to error
        if r := recover(); r != nil {
            err = fmt.Errorf("panic in processFile: %v", r)
        }
    }()

    // Do work that might panic
    data := parseFile(file) // might panic
    return saveData(data)
}

// Defer for resource cleanup (always runs)
func withDatabase(db *sql.DB, fn func(*sql.DB) error) error {
    tx, err := db.Begin()
    if err != nil {
        return err
    }
    defer func() {
        if err != nil {
            tx.Rollback()
        } else {
            tx.Commit()
        }
    }()
    return fn(tx)
}

package main

import (
    "fmt"
    "time"
)

func sayHello(name string) {
    for i := 0; i < 3; i++ {
        fmt.Println(name, i)
        time.Sleep(100 * time.Millisecond)
    }
}

func main() {
    // Launch goroutine with 'go' keyword
    go sayHello("Alice")
    go sayHello("Bob")

    // Anonymous goroutine
    go func() {
        fmt.Println("anonymous goroutine")
    }()

    // Wait for goroutines (simple approach)
    time.Sleep(1 * time.Second)
    fmt.Println("done")
}

// Goroutines are lightweight (~2KB stack, grows as needed)
// Millions of goroutines can run concurrently

// Unbuffered channel (synchronous)
ch := make(chan string)

// Send and receive
go func() {
    ch <- "hello" // blocks until received
}()
msg := <-ch // blocks until sent
fmt.Println(msg)

// Buffered channel (asynchronous)
buf := make(chan int, 3)
buf <- 1 // doesn't block (buffer has space)
buf <- 2
buf <- 3
// buf <- 4 // would block (buffer full)
fmt.Println(<-buf) // 1

// Close a channel (sender closes, never receiver)
close(buf)

// Range over channel (until closed)
for v := range buf {
    fmt.Println(v)
}

// Check if closed
v, ok := <-buf
if !ok {
    fmt.Println("channel closed")
}

// Select: multiplex channel operations
ch1 := make(chan string)
ch2 := make(chan string)

go func() {
    time.Sleep(1 * time.Second)
    ch1 <- "one"
}()
go func() {
    time.Sleep(2 * time.Second)
    ch2 <- "two"
}()

// Wait for first to arrive
for i := 0; i < 2; i++ {
    select {
    case msg1 := <-ch1:
        fmt.Println("received:", msg1)
    case msg2 := <-ch2:
        fmt.Println("received:", msg2)
    }
}

// Timeout with select
select {
case result := <-slowOperation():
    fmt.Println(result)
case <-time.After(3 * time.Second):
    fmt.Println("timeout!")
}

// Non-blocking receive (default case)
select {
case msg := <-ch:
    fmt.Println(msg)
default:
    fmt.Println("no message") // runs if no data
}

import "sync"

func main() {
    var wg sync.WaitGroup

    // Launch 5 goroutines
    for i := 0; i < 5; i++ {
        wg.Add(1) // increment counter
        go func(id int) {
            defer wg.Done() // decrement when done
            fmt.Printf("Worker %d started\n", id)
            time.Sleep(time.Duration(id) * 100 * time.Millisecond)
            fmt.Printf("Worker %d done\n", id)
        }(i)
    }

    wg.Wait() // block until all Done() called
    fmt.Println("all workers finished")
}

// Common mistake: passing loop variable to goroutine
// Always pass as parameter: go func(i int) { ... }(i)
// (Go 1.22+ fixes loop variable scoping, but still good practice)

import "sync"

// Mutex: mutual exclusion lock
type SafeCounter struct {
    mu    sync.Mutex
    count int
}

func (c *SafeCounter) Increment() {
    c.mu.Lock()
    defer c.mu.Unlock() // always unlock with defer
    c.count++
}

func (c *SafeCounter) Value() int {
    c.mu.Lock()
    defer c.mu.Unlock()
    return c.count
}

// RWMutex: multiple readers, one writer
type Cache struct {
    mu   sync.RWMutex
    data map[string]string
}

func (c *Cache) Get(key string) (string, bool) {
    c.mu.RLock()         // read lock (multiple allowed)
    defer c.mu.RUnlock()
    val, ok := c.data[key]
    return val, ok
}

func (c *Cache) Set(key, val string) {
    c.mu.Lock()          // write lock (exclusive)
    defer c.mu.Unlock()
    c.data[key] = val
}

// sync.Once: run exactly once
var (
    once sync.Once
    instance *Database
)

func GetDB() *Database {
    once.Do(func() {
        instance = &Database{}
    })
    return instance
}

// Worker pool
func worker(id int, jobs <-chan int, results chan<- int) {
    for j := range jobs {
        time.Sleep(time.Second) // simulate work
        results <- j * 2
    }
}

func main() {
    jobs := make(chan int, 100)
    results := make(chan int, 100)

    // Start 3 workers
    for w := 1; w <= 3; w++ {
        go worker(w, jobs, results)
    }

    // Send 5 jobs
    for j := 1; j <= 5; j++ {
        jobs <- j
    }
    close(jobs)

    // Collect results
    for a := 1; a <= 5; a++ {
        fmt.Println(<-results)
    }
}

// Fan-out/Fan-in
func fanOut(input <-chan int, workers int) <-chan int {
    output := make(chan int)
    var wg sync.WaitGroup
    wg.Add(workers)
    for i := 0; i < workers; i++ {
        go func() {
            defer wg.Done()
            for v := range input {
                output <- v * 2
            }
        }()
    }
    go func() { wg.Wait(); close(output) }()
    return output
}

import (
    "io"
    "os"
)

// Read entire file (small files)
data, err := os.ReadFile("input.txt")
if err != nil {
    log.Fatal(err)
}
fmt.Println(string(data))

// Open and read (large files)
file, err := os.Open("large.txt")
if err != nil {
    log.Fatal(err)
}
defer file.Close()

// Read in chunks
buf := make([]byte, 1024)
for {
    n, err := file.Read(buf)
    if n > 0 {
        // process buf[:n]
    }
    if err == io.EOF {
        break
    }
    if err != nil {
        log.Fatal(err)
    }
}

// Read line by line
scanner := bufio.NewScanner(file)
for scanner.Scan() {
    line := scanner.Text()
    fmt.Println(line)
}

// Write entire file (creates or truncates)
err := os.WriteFile("output.txt", []byte("Hello, World!"), 0644)
if err != nil {
    log.Fatal(err)
}

// Open for writing (with options)
file, err := os.OpenFile("log.txt",
    os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
if err != nil {
    log.Fatal(err)
}
defer file.Close()

// Write
if _, err := file.WriteString("log entry\n"); err != nil {
    log.Fatal(err)
}

// Buffered writer (efficient for many small writes)
writer := bufio.NewWriter(file)
for i := 0; i < 1000; i++ {
    writer.WriteString(fmt.Sprintf("line %d\n", i))
}
writer.Flush() // don't forget to flush!

// File modes: 0644 = owner read/write, others read
// os.O_APPEND, O_CREATE, O_WRONLY, O_RDONLY, O_RDWR, O_TRUNC

// Create directory
os.Mkdir("mydir", 0755)       // one level
os.MkdirAll("a/b/c", 0755)    // nested directories

// List directory
entries, err := os.ReadDir(".")
if err != nil {
    log.Fatal(err)
}
for _, entry := range entries {
    fmt.Println(entry.Name(), entry.IsDir())
}

// Walk directory tree
filepath.WalkDir(".", func(path string, d fs.DirEntry, err error) error {
    if err != nil {
        return err
    }
    fmt.Println(path)
    return nil
})

// File info
info, _ := os.Stat("file.txt")
fmt.Println(info.Size())      // bytes
fmt.Println(info.ModTime())   // last modified
fmt.Println(info.IsDir())

// Check existence
if _, err := os.Stat("file.txt"); os.IsNotExist(err) {
    fmt.Println("file does not exist")
}

// Remove
os.Remove("file.txt")
os.RemoveAll("directory") // recursive

// Environment variables
os.Setenv("API_KEY", "secret")
key := os.Getenv("API_KEY") // "" if not set

// With existence check
if val, ok := os.LookupEnv("HOME"); ok {
    fmt.Println("HOME:", val)
}

// All environment variables
for _, e := range os.Environ() {
    fmt.Println(e)
}

// Command line arguments
// args := os.Args // []string, args[0] is program name

// Using flag package
var port int
var host string
flag.IntVar(&port, "port", 8080, "server port")
flag.StringVar(&host, "host", "localhost", "server host")
flag.Parse()
fmt.Printf("Running on %s:%d\n", host, port)

// Run: go run main.go -port 3000 -host 0.0.0.0

import "os/exec"

// Run a command and get output
cmd := exec.Command("ls", "-la", "/tmp")
output, err := cmd.Output()
if err != nil {
    log.Fatal(err)
}
fmt.Println(string(output))

// Capture stdout and stderr separately
cmd = exec.Command("git", "status")
var stdout, stderr bytes.Buffer
cmd.Stdout = &stdout
cmd.Stderr = &stderr
err = cmd.Run()
fmt.Println("stdout:", stdout.String())
fmt.Println("stderr:", stderr.String())

// Pipe input to command
cmd = exec.Command("grep", "error")
cmd.Stdin = strings.NewReader("info\nerror\nwarn\n")
result, _ := cmd.Output()
fmt.Println(string(result)) // "error"

// Run with context (timeout)
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
cmd = exec.CommandContext(ctx, "sleep", "10")
err = cmd.Run() // killed after 5s

import "encoding/json"

type User struct {
    ID       int    `json:"id"`
    Name     string `json:"name"`
    Email    string `json:"email,omitempty"` // omit if empty
    Password string `json:"-"`               // never serialize
    Age      int    `json:"age,omitempty"`
}

// Struct to JSON (marshal)
user := User{ID: 1, Name: "Alice", Age: 30}
data, err := json.Marshal(user)
// {"id":1,"name":"Alice","age":30}

// Pretty print
pretty, _ := json.MarshalIndent(user, "", "  ")

// JSON to struct (unmarshal)
jsonStr := `{"id":2,"name":"Bob","email":"[email protected]"}`
var u User
err := json.Unmarshal([]byte(jsonStr), &u)
fmt.Println(u.Name) // "Bob"

// Struct tags control field names:
// json:"name"        -> rename
// json:"name,omitempty" -> skip if zero value
// json:"-"           -> skip entirely
// json:",omitempty"  -> keep name, skip if empty

// Parse arbitrary JSON into map[string]interface{}
jsonStr := `{"name":"Alice","scores":[90,85,92],"active":true}`

var result map[string]any
json.Unmarshal([]byte(jsonStr), &result)

// Type-assert to access values
name := result["name"].(string)
scores := result["scores"].([]any)
for _, s := range scores {
    fmt.Println(s.(float64)) // JSON numbers are float64
}

// Encode a map
m := map[string]any{
    "count": 42,
    "items": []string{"a", "b", "c"},
    "meta":  map[string]any{"page": 1},
}
data, _ := json.Marshal(m)

// Decode JSON stream
dec := json.NewDecoder(strings.NewReader(jsonStr))
var v map[string]any
dec.Decode(&v)

// Encode to stream
enc := json.NewEncoder(os.Stdout)
enc.SetIndent("", "  ")
enc.Encode(user)

// Implement MarshalJSON/UnmarshalJSON
type Money struct {
    Amount   float64
    Currency string
}

func (m Money) MarshalJSON() ([]byte, error) {
    return json.Marshal(map[string]any{
        "amount":   m.Amount,
        "currency": m.Currency,
        "display":  fmt.Sprintf("%.2f %s", m.Amount, m.Currency),
    })
}

func (m *Money) UnmarshalJSON(data []byte) error {
    var v struct {
        Amount   float64
        Currency string
    }
    if err := json.Unmarshal(data, &v); err != nil {
        return err
    }
    m.Amount = v.Amount
    m.Currency = v.Currency
    return nil
}

money := Money{Amount: 99.99, Currency: "USD"}
data, _ := json.Marshal(money)
// {"amount":99.99,"currency":"USD","display":"99.99 USD"}

// Time formatting
type Event struct {
    Time time.Time `json:"time"`
}
// time.Time marshals as RFC 3339 by default

import (
    "encoding/json"
    "net/http"
)

type Response struct {
    Status  string `json:"status"`
    Message string `json:"message"`
}

func main() {
    // Simple handler
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "Hello, %s!", r.URL.Path[1:])
    })

    // JSON API endpoint
    http.HandleFunc("/api/user", func(w http.ResponseWriter, r *http.Request) {
        user := User{ID: 1, Name: "Alice"}
        w.Header().Set("Content-Type", "application/json")
        json.NewEncoder(w).Encode(user)
    })

    // Start server
    fmt.Println("Server running on :8080")
    log.Fatal(http.ListenAndServe(":8080", nil))
}

// Request methods:
// r.Method  -> "GET", "POST", etc.
// r.URL.Query().Get("key") -> query params
// r.Body    -> request body (io.ReadCloser)

// Simple GET
resp, err := http.Get("https://api.example.com/users")
if err != nil {
    log.Fatal(err)
}
defer resp.Body.Close()

body, err := io.ReadAll(resp.Body)
fmt.Println(string(body))

// Custom request with headers
req, err := http.NewRequest("GET", "https://api.example.com/data", nil)
req.Header.Set("Authorization", "Bearer token123")
req.Header.Set("Accept", "application/json")

resp, err := http.DefaultClient.Do(req)
defer resp.Body.Close()

// POST with JSON body
user := User{Name: "Alice"}
body, _ := json.Marshal(user)
resp, err := http.Post(
    "https://api.example.com/users",
    "application/json",
    bytes.NewBuffer(body),
)
defer resp.Body.Close()

// With timeout
client := &http.Client{Timeout: 10 * time.Second}
resp, err = client.Get("https://slow-api.example.com")

// math_test.go (file must end with _test.go)
package math

import "testing"

// Test function: func TestXxx(t *testing.T)
func TestAdd(t *testing.T) {
    result := Add(2, 3)
    expected := 5
    if result != expected {
        t.Errorf("Add(2, 3) = %d; want %d", result, expected)
    }
}

// Table-driven tests (idiomatic Go)
func TestAddTable(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        expected int
    }{
        {"positive", 2, 3, 5},
        {"negative", -1, -1, -2},
        {"zero", 0, 0, 0},
        {"mixed", -5, 10, 5},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            result := Add(tt.a, tt.b)
            if result != tt.expected {
                t.Errorf("Add(%d, %d) = %d; want %d",
                    tt.a, tt.b, result, tt.expected)
            }
        })
    }
}

// Run: go test -v
// Run specific: go test -run TestAddTable/positive

// Benchmark function: func BenchmarkXxx(b *testing.B)
func BenchmarkFibonacci(b *testing.B) {
    for i := 0; i < b.N; i++ {
        Fibonacci(20)
    }
}

// Benchmark with allocations
func BenchmarkStringConcat(b *testing.B) {
    for i := 0; i < b.N; i++ {
        s := ""
        for j := 0; j < 100; j++ {
            s += "a"
        }
    }
}

// Run: go test -bench=.
// Output:
// BenchmarkFibonacci-8        300     4234567 ns/op
// BenchmarkStringConcat-8     200     6789012 ns/op   100 B/op   1 allocs/op

// Benchmark with setup
func BenchmarkProcess(b *testing.B) {
    data := setup() // setup not measured
    b.ResetTimer()  // start timing here
    for i := 0; i < b.N; i++ {
        process(data)
    }
}

// Memory allocations
func BenchmarkAllocs(b *testing.B) {
    b.ReportAllocs()
    for i := 0; i < b.N; i++ {
        make([]int, 100)
    }
}

// t.Helper() marks helper functions (better error locations)
func assertEqual(t *testing.T, got, want int) {
    t.Helper()
    if got != want {
        t.Errorf("got %d, want %d", got, want)
    }
}

// Interfaces for mocking
type DataStore interface {
    Get(key string) (string, error)
}

type MockStore struct {
    data map[string]string
}

func (m *MockStore) Get(key string) (string, error) {
    if v, ok := m.data[key]; ok {
        return v, nil
    }
    return "", errors.New("not found")
}

func TestService(t *testing.T) {
    store := &MockStore{data: map[string]string{"key": "value"}}
    svc := NewService(store)
    result := svc.GetValue("key")
    assertEqual(t, result, "value")
}

// Subtests with t.Run for setup/teardown
func TestWithCleanup(t *testing.T) {
    t.Cleanup(func() {
        // runs after test (LIFO order)
        os.Remove("tempfile")
    })
    // test code
}

// time package
now := time.Now()
future := now.Add(24 * time.Hour)
formatted := now.Format("2006-01-02 15:04:05")
parsed, _ := time.Parse("2006-01-02", "2024-01-15")

// regexp
re := regexp.MustCompile(`\d{4}-\d{2}-\d{2}`)
matches := re.FindString("date: 2024-01-15")
all := re.FindAllString("2024-01-15 and 2024-02-20", -1)

// context (cancellation, timeouts)
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
go doWork(ctx)
// cancel() or timeout stops doWork

// sync.Pool (reuse objects)
var bufPool = sync.Pool{
    New: func() any { return new(bytes.Buffer) },
}
buf := bufPool.Get().(*bytes.Buffer)
defer bufPool.Put(buf)

// sort (Go 1.21+ slices package)
slices.Sort(nums)
slices.Contains(nums, 42)
slices.Reverse(nums)

package main

import (
    "fmt"
    "sync"
)

func worker(id int, wg *sync.WaitGroup) {
    defer wg.Done() // signal completion when function returns
    fmt.Printf("Worker %d started\n", id)
    // ... do work ...
    fmt.Printf("Worker %d done\n", id)
}

func main() {
    var wg sync.WaitGroup

    for i := 1; i <= 5; i++ {
        wg.Add(1) // increment counter before starting goroutine
        go worker(i, &wg)
    }

    wg.Wait() // block until all goroutines call Done()
    fmt.Println("All workers finished")
}

package main

import (
    "fmt"
    "runtime"
)

func main() {
    // GOMAXPROCS: number of OS threads available to run goroutines
    fmt.Println("GOMAXPROCS:", runtime.GOMAXPROCS(0)) // defaults to CPU cores

    // Set it manually (rarely needed — default is optimal)
    runtime.GOMAXPROCS(2)

    // Goroutines are multiplexed onto OS threads (M:N scheduling)
    // - Blocking syscalls (I/O) don't block other goroutines
    // - The scheduler steals work across threads
    // - A goroutine blocked on channel/IO yields its thread

    // Number of goroutines currently running
    go func() { runtime.Gosched() }() // yield to other goroutines
    fmt.Println("Goroutines:", runtime.NumGoroutine())
}

package main

// BAD: goroutine leaks — this goroutine never exits
func leaky() <-chan int {
    ch := make(chan int)
    go func() {
        ch <- 42 // blocks forever if nobody reads from ch
    }()
    return ch
}

// GOOD: use context for cancellation
func safe(ctx context.Context) <-chan int {
    ch := make(chan int, 1) // buffered — sender never blocks
    go func() {
        select {
        case ch <- 42:
        case <-ctx.Done(): // exit when context is cancelled
            return
        }
    }()
    return ch
}

// GOOD: always ensure senders can exit
// 1. Buffer the channel so send doesn't block
// 2. Use select with a done/cancel channel
// 3. Use context.WithCancel/WithTimeout for lifecycle control

package main

import (
    "fmt"
    "sync"
)

var (
    instance *Database
    once     sync.Once
)

type Database struct{ name string }

func GetDB() *Database {
    // sync.Once guarantees the function runs exactly once,
    // even if called from many goroutines simultaneously
    once.Do(func() {
        instance = &Database{name: "production"}
        fmt.Println("Database initialized")
    })
    return instance
}

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            GetDB() // "Database initialized" prints only once
        }()
    }
    wg.Wait()
}

package main

import (
    "fmt"
    "sync"
)

func worker(id int, jobs <-chan int, results chan<- int, wg *sync.WaitGroup) {
    defer wg.Done()
    for job := range jobs { // exits when jobs channel is closed
        results <- job * job // process and send result
    }
}

func main() {
    jobs := make(chan int, 100)
    results := make(chan int, 100)
    var wg sync.WaitGroup

    // Start 3 workers
    for w := 1; w <= 3; w++ {
        wg.Add(1)
        go worker(w, jobs, results, &wg)
    }

    // Send 5 jobs
    for j := 1; j <= 5; j++ {
        jobs <- j
    }
    close(jobs) // signal workers to stop (range loop exits)

    wg.Wait()
    close(results)

    for r := range results {
        fmt.Println("Result:", r)
    }
}

package main

import "fmt"

func main() {
    // Unbuffered: send blocks until someone receives (synchronous)
    ch := make(chan int)
    go func() {
        ch <- 42 // blocks until main reads
    }()
    fmt.Println(<-ch) // 42

    // Buffered: send blocks only when buffer is full
    buf := make(chan int, 3)
    buf <- 1 // doesn't block (buffer has space)
    buf <- 2
    buf <- 3
    // buf <- 4 // would block — buffer full
    fmt.Println(<-buf) // 1 (FIFO)

    // Close a channel (sender closes, never the receiver)
    close(buf)

    // Check if closed (ok is false when closed and empty)
    val, ok := <-buf
    fmt.Println(val, ok) // 2 true

    // Range over channel until closed
    nums := make(chan int, 3)
    nums <- 10; nums <- 20; nums <- 30
    close(nums)
    for n := range nums {
        fmt.Println(n) // 10, 20, 30
    }
}

package main

import (
    "fmt"
    "time"
)

func main() {
    ch1 := make(chan string)
    ch2 := make(chan string)

    go func() {
        time.Sleep(1 * time.Second)
        ch1 <- "from ch1"
    }()
    go func() {
        time.Sleep(2 * time.Second)
        ch2 <- "from ch2"
    }()

    // select waits on multiple channel operations
    // picks the first one that's ready (random if multiple ready)
    for i := 0; i < 2; i++ {
        select {
        case msg := <-ch1:
            fmt.Println(msg)
        case msg := <-ch2:
            fmt.Println(msg)
        }
    }

    // Non-blocking receive with default
    select {
    case msg := <-ch1:
        fmt.Println(msg)
    default:
        fmt.Println("no message ready") // runs immediately
    }

    // Timeout with time.After
    select {
    case msg := <-ch1:
        fmt.Println(msg)
    case <-time.After(500 * time.Millisecond):
        fmt.Println("timed out")
    }
}

package main

import (
    "fmt"
    "sync"
)

// Fan-Out: distribute work across multiple goroutines
func fanOut(input <-chan int, n int) []<-chan int {
    outputs := make([]<-chan int, n)
    for i := 0; i < n; i++ {
        outputs[i] = process(input) // each worker reads from same input
    }
    return outputs
}

func process(input <-chan int) <-chan int {
    output := make(chan int)
    go func() {
        defer close(output)
        for v := range input {
            output <- v * v // transform
        }
    }()
    return output
}

// Fan-In: merge multiple channels into one
func fanIn(channels ...<-chan int) <-chan int {
    var wg sync.WaitGroup
    merged := make(chan int)

    output := func(c <-chan int) {
        defer wg.Done()
        for v := range c {
            merged <- v
        }
    }

    wg.Add(len(channels))
    for _, c := range channels {
        go output(c)
    }

    go func() {
        wg.Wait()
        close(merged)
    }()
    return merged
}

package main

import "fmt"

// <-chan int: receive-only (can read, cannot send)
// chan<- int: send-only (can send, cannot read)
func producer(out chan<- int) { // can only send
    for i := 0; i < 3; i++ {
        out <- i
    }
    close(out)
}

func consumer(in <-chan int) { // can only receive
    for v := range in {
        fmt.Println("consumed:", v)
    }
}

func main() {
    ch := make(chan int)

    // Directional restrictions are enforced at compile time
    go producer(ch) // bidirectional chan converts to send-only
    consumer(ch)    // bidirectional chan converts to receive-only

    // This prevents bugs: producer can't accidentally read,
    // consumer can't accidentally send or close
}

package main

import (
    "fmt"
    "time"
)

func main() {
    // Ticker: fires repeatedly at intervals
    ticker := time.NewTicker(500 * time.Millisecond)
    defer ticker.Stop() // always stop to release resources

    go func() {
        for t := range ticker.C { // ticker.C is a channel
            fmt.Println("Tick at", t)
        }
    }()

    // Timer: fires once after a duration
    timer := time.NewTimer(2 * time.Second)
    <-timer.C // blocks until timer fires
    fmt.Println("Timer fired!")

    // time.After: one-shot timer as a channel (no Stop needed)
    select {
    case <-time.After(1 * time.Second):
        fmt.Println("1 second elapsed")
    }

    // Reset a timer (cancel and reschedule)
    timer2 := time.NewTimer(5 * time.Second)
    timer2.Reset(100 * time.Millisecond)
    <-timer2.C
}

package main

import (
    "context"
    "fmt"
    "time"
)

func worker(ctx context.Context, id int) {
    for {
        select {
        case <-ctx.Done(): // cancelled or timed out
            fmt.Printf("Worker %d stopped: %v\n", id, ctx.Err())
            return
        default:
            fmt.Printf("Worker %d working...\n", id)
            time.Sleep(500 * time.Millisecond)
        }
    }
}

func main() {
    // WithCancel: manual cancellation
    ctx, cancel := context.WithCancel(context.Background())
    go worker(ctx, 1)
    time.Sleep(2 * time.Second)
    cancel() // stop the worker
    time.Sleep(500 * time.Millisecond)

    // WithTimeout: auto-cancel after duration
    ctx2, cancel2 := context.WithTimeout(context.Background(), 1500*time.Millisecond)
    defer cancel2() // always call cancel to release resources
    go worker(ctx2, 2)
    time.Sleep(2 * time.Second) // worker stops after 1.5s
}

package main

import (
    "context"
    "database/sql"
    "net/http"
)

// Context should be the FIRST parameter, named ctx
func GetUser(ctx context.Context, db *sql.DB, id int) (string, error) {
    // Pass ctx to all blocking operations so they cancel together
    var name string
    err := db.QueryRowContext(ctx, "SELECT name FROM users WHERE id=?", id).Scan(&name)
    return name, err
}

func handler(w http.ResponseWriter, r *http.Request) {
    // r.Context() is cancelled when the client disconnects
    ctx := r.Context()

    name, err := GetUser(ctx, db, 42)
    if err != nil {
        http.Error(w, err.Error(), 500)
        return
    }
    w.Write([]byte(name))
}

// BEST PRACTICE: pass context through every function in the call chain
// that might do I/O. This ensures a client disconnect or timeout
// cancels ALL in-flight work (DB queries, HTTP calls, etc.)

package main

import (
    "context"
    "fmt"
    "net/http"
)

// Define a custom key type to avoid collisions
type contextKey string

const userIDKey contextKey = "userID"

// Set a value in context
func authMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        userID := 42 // authenticate...
        // WithValue creates a new context with the value
        ctx := context.WithValue(r.Context(), userIDKey, userID)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

// Get a value from context (must type-assert)
func getUserID(ctx context.Context) (int, bool) {
    if v, ok := ctx.Value(userIDKey).(int); ok {
        return v, true
    }
    return 0, false
}

func handler(w http.ResponseWriter, r *http.Request) {
    if uid, ok := getUserID(r.Context()); ok {
        fmt.Fprintf(w, "User ID: %d", uid)
    }
}

package main

import (
    "context"
    "log"
    "net/http"
    "os"
    "os/signal"
    "syscall"
    "time"
)

func main() {
    server := &http.Server{Addr: ":8080", Handler: mux}

    // Listen for OS interrupt signals (Ctrl+C, SIGTERM)
    stop := make(chan os.Signal, 1)
    signal.Notify(stop, syscall.SIGINT, syscall.SIGTERM)

    go func() {
        log.Println("Server starting on :8080")
        if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
            log.Fatalf("Server error: %v", err)
        }
    }()

    <-stop // block until signal received
    log.Println("Shutting down...")

    // Give in-flight requests 10 seconds to finish
    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()

    if err := server.Shutdown(ctx); err != nil {
        log.Printf("Forced shutdown: %v", err)
    }
    log.Println("Server stopped gracefully")
}

package main

import (
    "context"
    "errors"
    "fmt"
    "time"
)

func slowOperation(ctx context.Context) (string, error) {
    // Check if already cancelled before starting
    if err := ctx.Err(); err != nil {
        return "", err
    }

    select {
    case <-time.After(3 * time.Second): // simulate slow work
        return "result", nil
    case <-ctx.Done(): // cancelled or deadline exceeded
        return "", ctx.Err()
    }
}

func main() {
    // Deadline: cancel at a specific time
    deadline := time.Now().Add(1 * time.Second)
    ctx, cancel := context.WithDeadline(context.Background(), deadline)
    defer cancel()

    result, err := slowOperation(ctx)
    if err != nil {
        if errors.Is(err, context.DeadlineExceeded) {
            fmt.Println("Operation timed out")
        } else if errors.Is(err, context.Canceled) {
            fmt.Println("Operation was cancelled")
        }
        return
    }
    fmt.Println(result)
}

package main

import (
    "encoding/json"
    "log"
    "net/http"
)

type User struct {
    ID   int    `json:"id"`
    Name string `json:"name"`
}

func getUser(w http.ResponseWriter, r *http.Request) {
    user := User{ID: 1, Name: "Alice"}

    w.Header().Set("Content-Type", "application/json")
    json.NewEncoder(w).Encode(user)
}

func createUser(w http.ResponseWriter, r *http.Request) {
    if r.Method != http.MethodPost {
        http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
        return
    }

    var user User
    if err := json.NewDecoder(r.Body).Decode(&user); err != nil {
        http.Error(w, "Invalid JSON", http.StatusBadRequest)
        return
    }
    user.ID = 99
    w.WriteHeader(http.StatusCreated)
    json.NewEncoder(w).Encode(user)
}

func main() {
    http.HandleFunc("/users/1", getUser)
    http.HandleFunc("/users", createUser)

    log.Println("Server on :8080")
    log.Fatal(http.ListenAndServe(":8080", nil))
}

package main

import (
    "bytes"
    "context"
    "encoding/json"
    "fmt"
    "net/http"
    "time"
)

func main() {
    // Simple GET
    resp, err := http.Get("https://api.example.com/users")
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close() // ALWAYS close the body

    var users []map[string]any
    json.NewDecoder(resp.Body).Decode(&users)
    fmt.Println(users)

    // Custom client with timeout (default has NO timeout!)
    client := &http.Client{Timeout: 30 * time.Second}

    // POST with JSON body and context
    body, _ := json.Marshal(map[string]string{"name": "Bob"})
    req, _ := http.NewRequestWithContext(context.Background(),
        "POST", "https://api.example.com/users", bytes.NewReader(body))
    req.Header.Set("Content-Type", "application/json")

    resp, err = client.Do(req)
    defer resp.Body.Close()
    fmt.Println("Status:", resp.Status)
}

package main

import (
    "log"
    "net/http"
    "time"
)

// Middleware wraps an http.Handler to add cross-cutting concerns
type Middleware func(http.Handler) http.Handler

func Logging(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        next.ServeHTTP(w, r) // call the wrapped handler
        log.Printf("%s %s %v", r.Method, r.URL.Path, time.Since(start))
    })
}

func Auth(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if r.Header.Get("Authorization") == "" {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Chain multiple middlewares
func Chain(h http.Handler, mws ...Middleware) http.Handler {
    for i := len(mws) - 1; i >= 0; i-- {
        h = mws[i](h)
    }
    return h
}

func main() {
    handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello!"))
    })
    // Apply middlewares: Auth -> Logging -> handler
    http.Handle("/", Chain(handler, Logging, Auth))
    http.ListenAndServe(":8080", nil)
}

package main

import (
    "html/template"
    "net/http"
)

func main() {
    // Serve static files (CSS, JS, images)
    fs := http.FileServer(http.Dir("./static"))
    http.Handle("/static/", http.StripPrefix("/static/", fs))

    // HTML templates
    tmpl := template.Must(template.ParseFiles("templates/index.html"))

    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        data := struct {
            Title string
            Items []string
        }{
            Title: "My Page",
            Items: []string{"Apple", "Banana", "Cherry"},
        }
        tmpl.Execute(w, data)
    })

    http.ListenAndServe(":8080", nil)
}

// templates/index.html:
// <h1>{{.Title}}</h1>
// <ul>{{range .Items}}<li>{{.}}</li>{{end}}</ul>

package main

import (
    "embed"
    "io/fs"
    "net/http"
)

//go:embed static/*
var staticFiles embed.FS

func main() {
    // Embed static files into the binary (no external files needed)
    // Access the embedded filesystem
    sub, _ := fs.Sub(staticFiles, "static")
    http.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.FS(sub))))

    // go:embed benefits:
    // - Single binary deployment (no external files)
    // - Files are compiled into the executable
    // - Works with templates, configs, migrations, etc.

    // Embed a single file
    //go:embed config.json
    // var configData []byte

    // Embed with pattern
    //go:embed templates/*.html
    // var templates embed.FS

    http.ListenAndServe(":8080", nil)
}

package main

import (
    "fmt"
    "sync"
)

type SafeCounter struct {
    mu    sync.Mutex
    count int
}

func (c *SafeCounter) Increment() {
    c.mu.Lock()
    defer c.mu.Unlock() // always unlock with defer
    c.count++
}

func (c *SafeCounter) Value() int {
    c.mu.Lock()
    defer c.mu.Unlock()
    return c.count
}

// RWMutex: allows multiple readers OR one writer
type SafeCache struct {
    mu   sync.RWMutex
    data map[string]string
}

func (c *SafeCache) Get(key string) (string, bool) {
    c.mu.RLock()         // read lock (multiple readers OK)
    defer c.mu.RUnlock()
    val, ok := c.data[key]
    return val, ok
}

func (c *SafeCache) Set(key, val string) {
    c.mu.Lock()          // write lock (exclusive)
    defer c.mu.Unlock()
    c.data[key] = val
}

package main

import (
    "fmt"
    "sync"
)

func main() {
    var m sync.Map

    // Store and Load (thread-safe, no locks needed)
    m.Store("name", "Alice")
    m.Store("age", 30)

    val, ok := m.Load("name")
    fmt.Println(val, ok) // Alice true

    // LoadOrStore: atomic get-or-set
    actual, loaded := m.LoadOrStore("name", "Bob")
    fmt.Println(actual, loaded) // Alice true (already existed)

    // Delete
    m.Delete("age")

    // Range over all entries
    m.Range(func(key, value any) bool {
        fmt.Printf("%v = %v\n", key, value)
        return true // continue; return false to stop
    })

    // Use sync.Map when:
    // 1. Keys are written once, read many times (caches)
    // 2. Multiple goroutines read/write disjoint keys
    // For most cases, a regular map + Mutex is simpler and faster
}

package main

import (
    "fmt"
    "sync"
    "time"
)

type Queue struct {
    items []int
    mu    sync.Mutex
    cond  *sync.Cond
}

func NewQueue() *Queue {
    q := &Queue{}
    q.cond = sync.NewCond(&q.mu) // cond is tied to the mutex
    return q
}

func (q *Queue) Put(item int) {
    q.mu.Lock()
    defer q.mu.Unlock()
    q.items = append(q.items, item)
    q.cond.Signal() // wake one waiting goroutine
}

func (q *Queue) Get() int {
    q.mu.Lock()
    defer q.mu.Unlock()
    for len(q.items) == 0 {
        q.cond.Wait() // atomically unlocks mu, sleeps, re-locks on wake
    }
    item := q.items[0]
    q.items = q.items[1:]
    return item
}

func main() {
    q := NewQueue()
    go func() {
        time.Sleep(1 * time.Second)
        q.Put(42)
    }()
    fmt.Println("Waiting for item...")
    fmt.Println("Got:", q.Get()) // blocks until item available
}

package main

import (
    "bytes"
    "sync"
)

var bufPool = sync.Pool{
    New: func() any {
        return new(bytes.Buffer) // create when pool is empty
    },
}

func Process(data []byte) string {
    // Get a buffer from the pool (or create via New)
    buf := bufPool.Get().(*bytes.Buffer)
    defer bufPool.Put(buf) // return it when done

    buf.Reset() // clear before reuse
    buf.Write(data)
    buf.WriteString("-processed")
    return buf.String()
}

// Benefits:
// - Reduces GC pressure by reusing objects
// - Avoids allocation overhead for short-lived objects
// - Pool entries can be garbage collected between GC cycles
//
// Use for: bytes.Buffer, json encoders, temporary slices
// NOT for: long-lived objects or when size is unpredictable

package main

import (
    "context"
    "fmt"
    "golang.org/x/sync/errgroup"
    "net/http"
)

// errgroup is like WaitGroup but propagates the first error
// and can cancel remaining goroutines on failure
func fetchAll(urls []string) ([]*http.Response, error) {
    g, ctx := errgroup.WithContext(context.Background())
    results := make([]*http.Response, len(urls))

    for i, url := range urls {
        i, url := i, url // capture loop variables
        g.Go(func() error {
            req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
            resp, err := http.DefaultClient.Do(req)
            if err != nil {
                return err // cancels ctx, stopping other goroutines
            }
            results[i] = resp
            return nil
        })
    }

    if err := g.Wait(); err != nil {
        return nil, err // first error returned
    }
    return results, nil
}

func main() {
    urls := []string{
        "https://example.com",
        "https://golang.org",
    }
    resps, err := fetchAll(urls)
    fmt.Println(len(resps), err)
}

package main

import "fmt"

// Type parameters in brackets: [T any] means T can be any type
func Map[T, U any](slice []T, fn func(T) U) []U {
    result := make([]U, len(slice))
    for i, v := range slice {
        result[i] = fn(v)
    }
    return result
}

func Filter[T any](slice []T, predicate func(T) bool) []T {
    var result []T
    for _, v := range slice {
        if predicate(v) {
            result = append(result, v)
        }
    }
    return result
}

func Reduce[T, U any](slice []T, initial U, fn func(U, T) U) U {
    result := initial
    for _, v := range slice {
        result = fn(result, v)
    }
    return result
}

func main() {
    nums := []int{1, 2, 3, 4, 5}
    squared := Map(nums, func(n int) int { return n * n })
    fmt.Println(squared) // [1 4 9 16 25]

    evens := Filter(nums, func(n int) bool { return n%2 == 0 })
    fmt.Println(evens) // [2 4]

    sum := Reduce(nums, 0, func(acc, n int) int { return acc + n })
    fmt.Println(sum) // 15

    // Works with any type
    words := []string{"go", "rust", "python"}
    lengths := Map(words, func(s string) int { return len(s) })
    fmt.Println(lengths) // [2 4 6]
}

package main

import "fmt"

// comparable: types that support == and != (built-in constraint)
func Contains[T comparable](slice []T, target T) bool {
    for _, v := range slice {
        if v == target {
            return true
        }
    }
    return false
}

// Custom constraint with a type set
type Number interface {
    int | int64 | float64 | float32
}

func Sum[T Number](nums []T) T {
    var total T
    for _, n := range nums {
        total += n
    }
    return total
}

// Multiple type parameters
func Pair[T, U any](first T, second U) struct{ First T; Second U } {
    return struct{ First T; Second U }{first, second}
}

func main() {
    fmt.Println(Contains([]string{"a", "b", "c"}, "b")) // true
    fmt.Println(Contains([]int{1, 2, 3}, 5))             // false

    fmt.Println(Sum([]int{1, 2, 3}))           // 6
    fmt.Println(Sum([]float64{1.5, 2.5}))      // 4

    p := Pair("Alice", 30)
    fmt.Println(p) // {Alice 30}
}

package main

import "fmt"

// Generic Stack — works with any type, type-safe
type Stack[T any] struct {
    items []T
}

func (s *Stack[T]) Push(item T) {
    s.items = append(s.items, item)
}

func (s *Stack[T]) Pop() (T, bool) {
    if len(s.items) == 0 {
        var zero T
        return zero, false
    }
    index := len(s.items) - 1
    item := s.items[index]
    s.items = s.items[:index]
    return item, true
}

func (s *Stack[T]) Len() int {
    return len(s.items)
}

func main() {
    // Type-safe stack of ints
    intStack := &Stack[int]{}
    intStack.Push(1)
    intStack.Push(2)
    val, _ := intStack.Pop()
    fmt.Println(val) // 2

    // Type-safe stack of strings
    strStack := &Stack[string]{}
    strStack.Push("hello")
    s, _ := strStack.Pop()
    fmt.Println(s) // hello

    // intStack.Push("oops") // compile error! type-safe
}

package main

import "fmt"

// ~ allows any type whose UNDERLYING type matches
// (e.g., type MyString string would match ~string)
type StringLike interface {
    ~string
}

func Join[S StringLike](items []S, sep string) string {
    result := ""
    for i, item := range items {
        if i > 0 {
            result += sep
        }
        result += string(item) // convert underlying type to string
    }
    return result
}

// Numeric constraint with ~
type Integer interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64
}

func Double[T Integer](v T) T {
    return v * 2
}

// Custom type based on int
type Score int

func main() {
    type MyString string
    items := []MyString{"a", "b", "c"}
    fmt.Println(Join(items, "-")) // a-b-c

    s := Double(Score(21))
    fmt.Println(s) // 42
}

package main

import "fmt"

// GENERICS: best for algorithms/data structures that work
// the same way regardless of type (collections, sorting, mapping)
func Max[T int | float64](a, b T) T {
    if a > b {
        return a
    }
    return b
}

// INTERFACES: best when different types need DIFFERENT implementations
// (polymorphism, dependency injection, mocking)
type Stringer interface {
    String() string
}

type Dog struct{ Name string }
func (d Dog) String() string { return "Dog: " + d.Name }

type Cat struct{ Name string }
func (c Cat) String() string { return "Cat: " + c.Name }

func Print[T Stringer](v T) {
    fmt.Println(v.String())
}

func main() {
    fmt.Println(Max(3, 7))       // 7
    fmt.Println(Max(3.14, 2.71)) // 3.14

    Print(Dog{"Rex"}) // Dog: Rex
    Print(Cat{"Whiskers"}) // Cat: Whiskers
}

// RULE OF THUMB:
// - Same logic, different types -> Generics
// - Different logic, shared contract -> Interfaces
// - Need runtime polymorphism -> Interfaces
// - Need compile-time type safety -> Generics

package main

import (
    "fmt"
    "reflect"
)

type User struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

func main() {
    u := User{Name: "Alice", Age: 30}

    // Get type and value at runtime
    t := reflect.TypeOf(u)
    v := reflect.ValueOf(u)

    fmt.Println("Type:", t.Name())        // User
    fmt.Println("Kind:", t.Kind())         // struct
    fmt.Println("NumFields:", t.NumField())

    // Iterate struct fields
    for i := 0; i < t.NumField(); i++ {
        field := t.Field(i)
        value := v.Field(i)
        tag := field.Tag.Get("json")
        fmt.Printf("%s (%s) = %v [json:%s]\n",
            field.Name, field.Type, value, tag)
    }
    // Name (string) = Alice [json:name]
    // Age (int) = 30 [json:age]
}

package main

import (
    "fmt"
    "reflect"
)

func main() {
    type Config struct {
        Port int
        Host string
    }

    cfg := Config{Port: 8080, Host: "localhost"}

    // MUST pass a pointer to modify (ValueOf of a struct is a copy)
    v := reflect.ValueOf(&cfg).Elem()

    // Check if a field is settable
    portField := v.FieldByName("Port")
    fmt.Println("Settable:", portField.CanSet()) // true

    // Modify fields by name
    portField.SetInt(9090)
    v.FieldByName("Host").SetString("0.0.0.0")

    fmt.Println(cfg) // {9090 0.0.0.0}

    // Set via interface (type-checked at runtime)
    v.FieldByName("Port").Set(reflect.ValueOf(7070))
    fmt.Println(cfg) // {7070 0.0.0.0}

    // Unexported fields are NOT settable (CanSet returns false)
}

package main

import (
    "fmt"
    "reflect"
)

type Calculator struct{}

func (c Calculator) Add(a, b int) int { return a + b }
func (c Calculator) Mul(a, b int) int { return a * b }

func main() {
    calc := Calculator{}
    v := reflect.ValueOf(calc)

    // Find a method by name
    method := v.MethodByName("Add")
    fmt.Println("Found:", method.IsValid()) // true

    // Call with arguments (each arg must be a reflect.Value)
    args := []reflect.Value{
        reflect.ValueOf(3),
        reflect.ValueOf(4),
    }
    results := method.Call(args)
    fmt.Println("3 + 4 =", results[0].Int()) // 7

    // List all methods
    t := reflect.TypeOf(calc)
    for i := 0; i < t.NumMethod(); i++ {
        m := t.Method(i)
        fmt.Printf("Method: %s, Type: %v\n", m.Name, m.Type)
    }
    // Method: Add, Type: func(main.Calculator, int, int) int
    // Method: Mul, Type: func(main.Calculator, int, int) int
}

package main

import (
    "fmt"
    "reflect"
)

// Convert any struct to a map using struct tags as keys
func StructToMap(obj any, tag string) map[string]any {
    result := make(map[string]any)
    v := reflect.ValueOf(obj)
    t := reflect.TypeOf(obj)

    // Dereference pointers
    if v.Kind() == reflect.Ptr {
        v = v.Elem()
        t = t.Elem()
    }

    for i := 0; i < t.NumField(); i++ {
        field := t.Field(i)
        // Use the tag value if present, else the field name
        key := field.Tag.Get(tag)
        if key == "" {
            key = field.Name
        }
        // Skip fields tagged with "-"
        if key == "-" {
            continue
        }
        result[key] = v.Field(i).Interface()
    }
    return result
}

type User struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
    Pass string `json:"-"` // excluded
}

func main() {
    u := User{Name: "Alice", Age: 30, Pass: "secret"}
    m := StructToMap(u, "json")
    fmt.Println(m) // map[age:30 name:Alice]
}

# Build an executable (outputs to current directory)
go build -o myapp              # custom output name
go build -o bin/myapp ./cmd    # build a specific package

# Build for different OS/arch (cross-compilation)
GOOS=linux GOARCH=amd64 go build -o myapp-linux
GOOS=windows GOARCH=amd64 go build -o myapp.exe
GOOS=darwin GOARCH=arm64 go build -o myapp-mac  # Apple Silicon

# Run without building an executable (compiles to temp, runs, deletes)
go run main.go                 # run single file
go run ./cmd/server            # run a package

# Install: builds and puts binary in $GOPATH/bin
go install ./cmd/myapp         # now 'myapp' is on your PATH

# Build flags
go build -v                    # verbose: print packages being compiled
go build -race                 # enable race detector (for testing)
go build -ldflags="-s -w"      # strip debug info (smaller binary)
go build -ldflags="-X main.Version=1.0.0"  # inject version at build time

# Initialize a new module
go mod init github.com/user/myproject

# This creates go.mod:
# module github.com/user/myproject
# go 1.21

# Add a dependency (auto-added to go.mod)
go get github.com/gin-gonic/gin@latest
go get github.com/lib/[email protected]  # specific version

# Tidy: add missing deps, remove unused ones
go mod tidy

# Download dependencies to local cache
go mod download

# Verify dependencies haven't been modified
go mod verify

# Vendor: copy deps into ./vendor (for offline builds)
go mod vendor

# Upgrade dependencies
go get -u github.com/lib/pq       # upgrade to latest minor
go get -u=patch github.com/lib/pq # upgrade patches only
go get github.com/lib/[email protected]  # pin specific version

# View dependency graph
go mod graph | head

# Run all tests in the current package
go test ./...

# Verbose output
go test -v ./...

# Run a specific test
go test -run TestAdd -v

# Run benchmarks
go test -bench=. -benchmem

# Race detector (find data races)
go test -race ./...

# Coverage
go test -cover ./...
go test -coverprofile=coverage.out ./...
go tool cover -html=coverage.out  # view in browser

# --- test file example (xxx_test.go) ---
# func TestAdd(t *testing.T) {
#     got := Add(2, 3)
#     if got != 5 {
#         t.Errorf("Add(2,3) = %d, want 5", got)
#     }
# }
#
# func BenchmarkAdd(b *testing.B) {
#     for i := 0; i < b.N; i++ {
#         Add(2, 3)
#     }
# }

# go fmt: format code (the ONE true Go style)
go fmt ./...              # format all files
gofmt -d main.go          # show diff without changing
gofmt -w main.go          # write changes in place

# go vet: static analysis for common mistakes
go vet ./...              # check all packages
# Catches: printf format mismatches, unreachable code,
# struct tag errors, lock copies, shadowed variables

# golangci-lint: meta-linter (runs many linters)
# Install: go install github.com/golangci/golangci-lint/cmd/golangci-lint@latest
golangci-lint run         # run all enabled linters
golangci-lint run --enable=gosec,goconst  # enable specific linters

# Common linters:
# errcheck    - check unchecked errors
# gosec       - security issues
# govet       - go vet checks
# staticcheck - advanced static analysis
# ineffassign - detect ineffectual assignments
# unused      - find unused code

# goimports: fmt + auto-manage imports
# go install golang.org/x/tools/cmd/goimports@latest
goimports -w main.go

# CPU profiling
go test -cpuprofile=cpu.prof -bench=.
go tool pprof cpu.prof
# Interactive commands: top, list FunctionName, web (graphviz)

# Memory profiling
go test -memprofile=mem.prof -bench=.
go tool pprof mem.prof

# Built-in HTTP pprof endpoint (for production servers)
import _ "net/http/pprof"
# Then visit: http://localhost:8080/debug/pprof/
# Capture a 30-second CPU profile:
# go tool pprof http://localhost:8080/debug/pprof/profile?seconds=30

# In code:
import "runtime/pprof"
f, _ := os.Create("cpu.prof")
pprof.StartCPUProfile(f)
defer pprof.StopCPUProfile()
// ... code to profile ...

# Trace (execution tracer)
go test -trace=trace.out -bench=.
go tool trace trace.out  # opens in browser

import "database/sql"
import _ "github.com/lib/pq"
db, err := sql.Open("postgres", "host=localhost dbname=mydb")
if err != nil { log.Fatal(err) }
defer db.Close()
err = db.Ping()

rows, err := db.Query("SELECT id, name FROM users WHERE age > $1", 18)
if err != nil { log.Fatal(err) }
defer rows.Close()
for rows.Next() {
    var id int; var name string
    if err := rows.Scan(&id, &name); err != nil { log.Fatal(err) }
    fmt.Printf("%d: %s\n", id, name)
}

var name string
err := db.QueryRow("SELECT name FROM users WHERE id = $1", 1).Scan(&name)
if err != nil {
    if err == sql.ErrNoRows { fmt.Println("Not found") } else { log.Fatal(err) }
}

stmt, err := db.Prepare("INSERT INTO users(name) VALUES($1)")
if err != nil { log.Fatal(err) }
defer stmt.Close()
_, err = stmt.Exec("Alice")
_, err = stmt.Exec("Bob")

tx, err := db.Begin()
if err != nil { log.Fatal(err) }
defer tx.Rollback()  // Safe to call after commit
_, err = tx.Exec("UPDATE accounts SET bal = bal - 100 WHERE id = 1")
_, err = tx.Exec("UPDATE accounts SET bal = bal + 100 WHERE id = 2")
if err != nil { log.Fatal(err) }
err = tx.Commit()

func TestAdd(t *testing.T) {
    got := Add(2, 3)
    want := 5
    if got != want {
        t.Errorf("Add(2,3) = %d; want %d", got, want)
    }
}

func TestAdd(t *testing.T) {
    tests := []struct {
        a, b, want int
    }{
        {1, 2, 3}, {-1, 1, 0}, {0, 0, 0},
    }
    for _, tt := range tests {
        got := Add(tt.a, tt.b)
        if got != tt.want {
            t.Errorf("Add(%d,%d)=%d; want %d", tt.a, tt.b, got, tt.want)
        }
    }
}

func TestAdd(t *testing.T) {
    t.Run("positive", func(t *testing.T) {
        if Add(1, 2) != 3 { t.Error("failed") }
    })
    t.Run("negative", func(t *testing.T) {
        if Add(-1, -2) != -3 { t.Error("failed") }
    })
}
// Run specific: go test -run TestAdd/negative

func TestMain(m *testing.M) {
    setup()
    code := m.Run()
    teardown()
    os.Exit(code)
}
func setup() { /* initialize DB, etc. */ }
func teardown() { /* cleanup */ }

type Store interface { Get(id int) (string, error) }
type MockStore struct { data map[int]string }
func (m *MockStore) Get(id int) (string, error) {
    if v, ok := m.data[id]; ok { return v, nil }
    return "", errors.New("not found")
}
// Use in tests by injecting MockStore

func BenchmarkAdd(b *testing.B) {
    for i := 0; i < b.N; i++ {
        Add(2, 3)
    }
}
// Run: go test -bench=.
// Output: BenchmarkAdd-8    1000000000    0.3 ns/op

func BenchmarkSort(b *testing.B) {
    sizes := []int{100, 1000, 10000}
    for _, size := range sizes {
        b.Run(fmt.Sprintf("size-%d", size), func(b *testing.B) {
            data := generateData(size)
            b.ResetTimer()
            for i := 0; i < b.N; i++ {
                Sort(data)
            }
        })
    }
}

func BenchmarkConcat(b *testing.B) {
    b.ReportAllocs()
    for i := 0; i < b.N; i++ {
        s := "a" + "b" + "c"
        _ = s
    }
}
// Output includes allocs/op and B/op

func BenchmarkParallel(b *testing.B) {
    b.RunParallel(func(pb *testing.PB) {
        for pb.Next() {
            Add(2, 3)
        }
    })
}

# Run with memory stats
benchstat old.txt new.txt
# Compare two runs
# name      old time/op  new time/op  delta
# Add-8     2.3ns        1.8ns        -21.7%

import "runtime/pprof"
f, _ := os.Create("cpu.prof")
pprof.StartCPUProfile(f)
defer pprof.StopCPUProfile()
// Run your code here
// Analyze: go tool pprof cpu.prof

// Add to code
runtime.GC()
f, _ := os.Create("mem.prof")
pprof.WriteHeapProfile(f)
f.Close()
// Or use runtime.MemProfileRate = 1 for all allocations

import _ "net/http/pprof"
go func() {
    log.Println(http.ListenAndServe("localhost:6060", nil))
}()
// Analyze live:
// go tool pprof http://localhost:6060/debug/pprof/profile

# Start interactive pprof
go tool pprof cpu.prof
(pprof) top 10        # Top functions by CPU
(pprof) list Add      # Show source with timings
(pprof) web           # Open graph in browser
(pprof) tree          # Call tree
(pprof) png > out.png # Save graph as PNG

import "runtime/trace"
f, _ := os.Create("trace.out")
trace.Start(f)
defer trace.Stop()
// Run code
// View: go tool trace trace.out

//go:build linux
// +build linux

package main
// This file only compiles on Linux

//go:build linux && amd64
// +build linux,amd64

//go:build linux || darwin
// +build linux darwin

//go:build !windows
// +build !windows

//go:build debug
// +build debug

package main
const DebugMode = true
// Build with: go build -tags debug

// file: utils_linux.go    -> Linux only
// file: utils_darwin.go   -> macOS only
// file: utils_windows.go  -> Windows only
// file: utils_amd64.go    -> amd64 only
// file: utils_debug.go    -> requires -tags debug

# Build with debug tag
go build -tags debug
# Multiple tags
go build -tags "debug verbose"
# List available tags
go list -tags ./...
# Conditional in code
//go:build go1.18  // Requires Go 1.18+

/*
#include <stdio.h>
void hello() {
    printf("Hello from C!\n");
}
*/
import "C"
func main() {
    C.hello()
}

/*
#include <string.h>
int len(const char* s) { return strlen(s); }
*/
import "C"
import "unsafe"
s := "Hello"cs := C.CString(s)
defer C.free(unsafe.Pointer(cs))
length := C.len(cs)

/*
#cgo LDFLAGS: -lm
#include <math.h>
*/
import "C"
result := float64(C.sqrt(16.0))
fmt.Println(result)  // 4

/*
extern void goCallback(int);
void callGo(int n) { goCallback(n); }
*/
import "C"
//export goCallback
func goCallback(n C.int) {
    fmt.Printf("Called with %d\n", n)
}

// CGO calls have overhead (~100ns)
// Avoid in hot paths
// Batch C calls to amortize cost
// CGO disables inlining and some optimizations
// Build: CGO_ENABLED=1 go build

http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello, %s!", r.URL.Path[1:])
})
http.HandleFunc("/api", apiHandler)
log.Fatal(http.ListenAndServe(":8080", nil))

import "github.com/gin-gonic/gin"
r := gin.Default()
r.GET("/users/:id", func(c *gin.Context) {
    id := c.Param("id")
    c.JSON(200, gin.H{"user": id})
})
r.Run(":8080")

func Logger() gin.HandlerFunc {
    return func(c *gin.Context) {
        start := time.Now()
        c.Next()
        fmt.Printf("%s %s %v\n", c.Request.Method, c.URL.Path, time.Since(start))
    }
}
r := gin.New()
r.Use(Logger(), gin.Recovery())

import "github.com/labstack/echo/v4"
e := echo.New()
e.GET("/users/:id", func(c echo.Context) error {
    id := c.Param("id")
    return c.JSON(200, map[string]string{"user": id})
})
e.Start(":8080")

// net/http
fs := http.FileServer(http.Dir("./static"))
http.Handle("/static/", http.StripPrefix("/static/", fs))
// Gin
r.Static("/assets", "./assets")
r.StaticFile("/favicon.ico", "./favicon.ico")

syntax = "proto3";
package greet;
service Greeter {
    rpc SayHello (HelloRequest) returns (HelloReply) {}
}
message HelloRequest { string name = 1; }
message HelloReply { string message = 1; }

# Install protoc and plugins
protoc --go_out=. --go-grpc_out=. \
    greet.proto
# This generates greet.pb.go and greet_grpc.pb.go
# Containing message types and service interfaces

type server struct { greet.UnimplementedGreeterServer }
func (s *server) SayHello(ctx context.Context, in *greet.HelloRequest) (*greet.HelloReply, error) {
    return &greet.HelloReply{Message: "Hello " + in.Name}, nil
}
lis, _ := net.Listen("tcp", ":50051")
grpc.NewServer().Serve(lis)

conn, _ := grpc.Dial("localhost:50051", grpc.WithInsecure())
client := greet.NewGreeterClient(conn)
resp, _ := client.SayHello(context.Background(), &greet.HelloRequest{Name: "Alice"})
fmt.Println(resp.Message)

// Server streaming
rpc LotsOfReplies(HelloRequest) returns (stream HelloReply);
// Client streaming
rpc LotsOfGreetings(stream HelloRequest) returns (HelloReply);
// Bidirectional
rpc BidiHello(stream HelloRequest) returns (stream HelloReply);

if err != nil {
    return fmt.Errorf("failed to open config: %w", err)
}

err := someOperation()
if errors.Is(err, sql.ErrNoRows) {
    // Handle not found
}
var pathErr *fs.PathError
if errors.As(err, &pathErr) {
    fmt.Println("Path:", pathErr.Path)
}

type ValidationError struct {
    Field string
    Msg   string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("%s: %s", e.Field, e.Msg)
}
func validate(s string) error {
    if s == "" { return &ValidationError{Field: "name", Msg: "required"} }
    return nil
}

var ErrNotFound = errors.New("not found")
func Find(id int) (*User, error) {
    if id > 100 { return nil, ErrNotFound }
    return &User{}, nil
}
// Check with errors.Is
if errors.Is(err, ErrNotFound) { /* ... */ }

// Immediate handling
if err := doSomething(); err != nil {
    return fmt.Errorf("operation failed: %w", err)
}
// Deferred error checking (e.g., Close)
func readFile() (err error) {
    f, e := os.Open("file.txt")
    if e != nil { return e }
    defer func() {
        if cerr := f.Close(); err == nil { err = cerr }
    }()
    // ...
}

// BUG: goroutine leaks if receiver stops
func send(ch chan int) {
    go func() { ch <- 1 }()  // Blocks forever
}
// FIX: use context or buffered channel
func send(ctx context.Context, ch chan int) {
    go func() {
        select {
        case ch <- 1:
        case <-ctx.Done():
        }
    }()
}

// Only the sender should close a channel
// Closing from receiver causes panic
ch := make(chan int)
go func() {
    defer close(ch)  // Sender closes
    for i := 0; i < 5; i++ { ch <- i }
}()
for v := range ch { fmt.Println(v) }

// BUG (Go < 1.22): all goroutines see last value
for i := 0; i < 3; i++ {
    go func() { fmt.Println(i) }()  // Prints 3,3,3
}
// FIX: pass as parameter
for i := 0; i < 3; i++ {
    go func(i int) { fmt.Println(i) }(i)
}

// BUG: concurrent map writes panic
m := map[int]int{}
go func() { m[1] = 1 }()
go func() { m[2] = 2 }()
// FIX: use sync.Map or mutex
var mu sync.Mutex
mu.Lock(); m[1] = 1; mu.Unlock()

// BUG: nil check fails
type MyError struct{}
func (e *MyError) Error() string { return "err" }
func doSomething() error {
    var err *MyError = nil
    return err  // Non-nil interface!
}
// FIX: return nil explicitly
func doSomething() error {
    return nil
}