Ruby Cheatsheet

name = "Alice"    # string
age = 30           # integer
pi = 3.14          # float
is_dev = true      # boolean
nums = [1, 2, 3]   # array
puts name.class    # String
puts age.is_a?(Integer)  # true

status = :active
puts status.class        # Symbol
puts status.to_s         # "active"
# Symbols are immutable, reusable strings
hash = { name: "Alice", status: :active }
puts hash[:status]       # active

x = nil
puts x.nil?        # true
puts x || "default"  # default
# Only nil and false are falsy; 0 and "" are truthy!
puts 0 ? "truthy" : "falsy"  # truthy
puts "" ? "truthy" : "falsy" # truthy
puts nil ? "truthy" : "falsy" # falsy

puts "42".to_i      # 42
puts 42.to_s        # "42"
puts "3.14".to_f    # 3.14
puts 3.14.to_i      # 3
puts "abc".to_i     # 0 (no error)
puts Integer("42")  # 42 (raises if invalid)

name = "Alice"
age = 30
puts "Name: #{name}, Age: #{age}"
puts "5 + 3 = #{5 + 3}"
puts "Upper: #{name.upcase}"
# Single quotes don't interpolate
puts 'No #{name} here'  # No #{name} here

s = "Hello, World"
puts s.length         # 12
puts s.upcase         # HELLO, WORLD
puts s.downcase       # hello, world
puts s.reverse        # dlroW ,olleH
puts s.split(", ")    # ["Hello", "World"]
puts s.gsub("o", "0") # Hell0, W0rld

s = "hello"
s.upcase!    # s is now "HELLO"
s.gsub!(/L/, "1")  # s is now "HE11O"
puts s       # HE11O
# Bang methods modify in-place; use with caution
arr = [3, 1, 2]
arr.sort!    # arr is now [1, 2, 3]

text = <<~HEREDOC
  Hello,
  World!
  Indentation is stripped.
HEREDOC
puts text
# <<~ strips leading whitespace (Ruby 2.3+)
# <<HEREDOC preserves indentation

puts sprintf("%s is %d", "Alice", 30)
puts "%-10s|%5d" % ["Name", 42]
puts format("%.2f", 3.14159)  # 3.14
puts "%08.2f" % 3.14          # 00003.14
# %s string, %d integer, %f float

s1 = "Hello" + ", " + "World"
s2 = ["a", "b", "c"].join("-")  # a-b-c
s3 = "Hello"
s3 << " " << "World"  # in-place append
s4 = "x" * 3          # xxx (repetition)
puts s1, s2, s3, s4

nums = [1, 2, 3, 4, 5]
nums.push(6)          # [1,2,3,4,5,6]
nums << 7             # same as push
nums[0] = 0
puts nums.first       # 0
puts nums.length      # 7
puts nums.sum         # 28
puts nums.include?(3) # true

user = { name: "Alice", age: 30 }
user[:email] = "[email protected]"
puts user[:name]          # Alice
puts user.key?(:name)     # true
user.each { |k, v| puts "#{k}: #{v}" }
puts user.values          # ["Alice", 30, "[email protected]"]
puts user.transform_values(&:to_s)

(1..5).each { |n| puts n }      # 1 2 3 4 5
puts (1...5).to_a                # [1, 2, 3, 4]
puts (1..10).select(&:even?)     # [2, 4, 6, 8, 10]
puts ('a'..'e').include?('c')    # true
puts (1..5).map { |n| n ** 2 }   # [1, 4, 9, 16, 25]

nums = [1, 2, 3, 4, 5]
puts nums.map { |n| n * 2 }.inspect    # [2,4,6,8,10]
puts nums.select(&:even?).inspect      # [2, 4]
puts nums.reject(&:odd?).inspect       # [2, 4]
puts nums.reduce(0) { |sum, n| sum + n } # 15
puts nums.find { |n| n > 3 }           # 4
puts nums.group_by(&:even?).inspect    # {false=>[1,3,5], true=>[2,4]}

require 'set'
a = Set.new([1, 2, 3])
b = Set.new([3, 4, 5])
puts a.union(b).to_a.inspect        # [1,2,3,4,5]
puts a.intersection(b).to_a.inspect # [3]
puts a.subtract([1]).to_a.inspect   # [2, 3]
puts a.subset?(Set.new([1,2,3,4]))  # true
a.add(6)

score = 85
if score >= 90
  puts "A"
elsif score >= 80
  puts "B"
else
  puts "C"
end
# Modifier form
puts "Pass" if score >= 60
puts "Fail" unless score >= 60

grade = "B"
case grade
when "A" then puts "Excellent"
when "B", "C" then puts "Good"
when "D".."F" then puts "Poor"
else puts "Unknown"
end
# Case without value = multi-condition if
case
when score > 90 then puts "Top"
when score > 60 then puts "Pass"
end

count = 0
while count < 3
  puts count
  count += 1
end
# until = opposite of while
n = 3
until n == 0
  puts n
  n -= 1
end
# Infinite loop with break
loop do
  puts "forever"
  break if rand > 0.8
end

3.times { |i| puts i }          # 0 1 2
1.upto(3) { |n| puts n }        # 1 2 3
3.downto(1) { |n| puts n }      # 3 2 1
[1,2,3].each_with_index do |n, i|
  puts "#{i}: #{n}"
end
(1..3).step(1) { |n| puts n }   # 1 2 3

[1, 2, 3, 4, 5].each do |n|
  next if n.even?    # skip even
  break if n > 4     # stop at 5
  puts n             # prints 1, 3
end
# break value returns from the block
result = [1,2,3].each { |n| break n * 10 if n == 2 }
puts result  # 20

def greet(name, greeting: "Hello")
  "#{greeting}, #{name}!"
end
puts greet("Alice")               # Hello, Alice!
puts greet("Bob", greeting: "Hi") # Hi, Bob!
def add(*nums)  # splat (variadic)
  nums.sum
end
puts add(1, 2, 3, 4)  # 10

def repeat(n)
  n.times { yield }
end
repeat(3) { puts "hi" }  # prints hi 3 times
def with_result
  yield(5)
end
puts with_result { |x| x * 2 }  # 10

square = proc { |x| x * x }
puts square.call(5)   # 25
puts square.(5)       # 25 (shortcut)
double = lambda { |x| x * 2 }
puts double.call(5)   # 10
# Lambda checks arity; Proc doesn't
# Lambda returns from itself; Proc returns from enclosing method

nums = ["1", "2", "3"]
ints = nums.map(&:to_i)  # [1, 2, 3]
puts ints.inspect
# &:to_i is shorthand for { |s| s.to_i }
names = ["alice", "bob"]
up = names.map(&:upcase)  # ["ALICE", "BOB"]
puts up.inspect

def status(ok)
  return "error" unless ok
  "ok"  # implicit return of last expression
end
puts status(true)   # ok
puts status(false)  # error
# Multiple assignment
def coords
  return 1, 2  # returns [1, 2]
end
x, y = coords

class Person
  attr_accessor :name, :age
  attr_reader :id
  @@count = 0  # class variable
  def initialize(name, age)
    @name = name  # instance variable
    @age = age
    @@count += 1
  end
  def self.count; @@count; end
end
p = Person.new("Alice", 30)
puts p.name, Person.count

class Animal
  def initialize(name); @name = name; end
  def speak; "..."; end
end
class Dog < Animal
  def speak; "#{@name}: Woof!"; end
end
class Puppy < Dog
  def speak; "#{super} (small)"; end
end
puts Puppy.new("Rex").speak  # Rex: Woof! (small)

module Walkable
  def walk; "#{@name} is walking"; end
end
module Swimmable
  def swim; "#{@name} is swimming"; end
end
class Duck
  include Walkable    # instance methods
  extend Swimmable    # class methods
  def initialize(n); @name = n; end
end
puts Duck.new("Donald").walk  # Donald is walking

class BankAccount
  def initialize(bal); @balance = bal; end
  def deposit(amt); @balance += amt; end
  def balance; @balance; end
  private
  def audit; "auditing..."; end
  protected
  def compare(other); @balance > other.balance; end
end
a = BankAccount.new(100)
puts a.balance  # 100
# a.audit  # Error: private method

class Counter
  @@total = 0
  def initialize; @@total += 1; end
  def self.total; @@total; end
  def self.reset!; @@total = 0; end
  def instance_method; "I'm an instance"; end
  alias_method :count, :instance_method
end
Counter.new; Counter.new
puts Counter.total  # 2

begin
  result = 10 / 0
rescue ZeroDivisionError => e
  puts "Caught: #{e.message}"
rescue => e
  puts "Other error: #{e.class}"
ensure
  puts "Always runs (cleanup)"
end
# ensure runs regardless of success/failure

class InvalidAgeError < StandardError; end
def set_age(age)
  raise InvalidAgeError, "Age cannot be negative" if age < 0
  raise ArgumentError, "Must be Integer" unless age.is_a?(Integer)
  @age = age
end
begin
  set_age(-5)
rescue InvalidAgeError => e
  puts "Custom: #{e.message}"
end

attempts = 0
begin
  attempts += 1
  fetch_data  # might fail
rescue NetworkError
  retry if attempts < 3
  puts "Failed after 3 attempts"
end
# retry re-runs the begin block from the top

result = risky_operation rescue "default"
puts result
# Same as:
# result = begin; risky_operation; rescue; "default"; end
# Catches StandardError only
json = JSON.parse(str) rescue nil
puts "Invalid JSON" if json.nil?

catch(:done) do
  [1, 2, 3, 4, 5].each do |n|
    throw :done, n if n > 3
    puts n
  end
end
# Prints 1, 2, 3 and returns 4
# Not for errors—use for early exit from nested loops

# Write
File.write("test.txt", "Hello, File!")
# Read
content = File.read("test.txt")
puts content  # Hello, File!
# Append
File.open("log.txt", "a") { |f| f.puts "new line" }
# Read line by line
File.foreach("test.txt") { |line| puts line.chomp }

File.open("data.txt", "w") do |f|
  f.puts "Line 1"
  f.puts "Line 2"
  f.write("No newline")
end  # file auto-closed here
File.open("data.txt", "r") do |f|
  f.each_line.with_index { |line, i| puts "#{i}: #{line}" }
end

puts File.exist?("test.txt")  # true
puts File.file?("test.txt")   # true (regular file)
puts File.directory?(".")     # true
puts File.size("test.txt")    # bytes
puts File.mtime("test.txt")   # modification time
File.rename("old.txt", "new.txt")
File.delete("new.txt") if File.exist?("new.txt")

Dir.mkdir("test_dir") unless Dir.exist?("test_dir")
Dir.chdir("test_dir") do
  File.write("a.txt", "a")
  puts Dir.pwd  # current path
end
Dir.glob("*.txt") { |f| puts f }  # list .txt files
Dir.glob("**/*.rb") { |f| puts f } # recursive

require 'csv'
require 'json'
CSV.write("data.csv", [["a", 1], ["b", 2]])
CSV.foreach("data.csv") { |row| puts row.inspect }
data = { name: "Alice", age: 30 }
File.write("data.json", data.to_json)
parsed = JSON.parse(File.read("data.json"))
puts parsed["name"]  # Alice

require 'time'
now = Time.now
puts now                    # 2024-01-15 14:30:00 +0800
puts now.strftime("%Y-%m-%d %H:%M")  # 2024-01-15 14:30
tomorrow = now + 86400      # +1 day (seconds)
puts tomorrow.strftime("%A") # weekday name
date = Date.today
puts date.next_day          # tomorrow

require 'date'
d = Date.parse("2024-01-15")
puts d.year   # 2024
puts d + 7    # 2024-01-22 (+7 days)
puts d.next_month  # 2024-02-15
diff = (Date.today - d).to_i
puts "#{diff} days since"
puts Date.today.upto(Date.today + 6).map(&:wday)

s = "Phone: 123-4567"
if s =~ /(\d+)-(\d+)/
  puts "Area: #{$1}, Number: #{$2}"
end
m = /\w+@(\w+)/.match("[email protected]")
puts m[1]  # example
puts "abc123" =~ /\d/ ? "has digit" : "no digit"

s = "Hello, World"
puts s.gsub(/o/, "0")       # Hell0, W0rld
puts s.gsub(/\w+/) { |w| w.capitalize }
puts s.scan(/\w+/).inspect  # ["Hello", "World"]
puts "2024-01-15".gsub(/(\d+)-(\d+)-(\d+)/, '\3/\2/\1')
# => 15/01/2024

puts /hello/i =~ "HELLO"  # 0 (case-insensitive)
puts /line/m =~ "a\nb"    # 0 (multiline: . matches \n)
puts /x.y/x =~ "x y"       # 0 (extended: ignore whitespace)
# Common patterns
email = /[\w.]+@[\w]+\.[a-z]+/
url = %r{https?://[\w./]+}
puts email.match("[email protected]") ? "valid" : "invalid"

class Dynamic
  [:foo, :bar, :baz].each do |name|
    define_method(name) { puts "Called #{name}" }
  end
end
d = Dynamic.new
d.foo  # Called foo
d.bar  # Called bar
# Useful for generating similar methods

class Proxy
  def method_missing(name, *args)
    puts "Called: #{name} with #{args.inspect}"
  end
  def respond_to_missing?(name, include_private = false)
    true
  end
end
p = Proxy.new
p.anything(1, 2)  # Called: anything with [1, 2]

threads = [1, 2, 3].map do |n|
  Thread.new { puts "Thread #{n}: #{n * n}" }
end
threads.each(&:join)  # wait for all
# MRI has GIL—threads don't run truly parallel for CPU work
# Use for I/O concurrency (network, file)

fiber = Fiber.new do
  Fiber.yield 1
  Fiber.yield 2
  3
end
puts fiber.resume  # 1
puts fiber.resume  # 2
puts fiber.resume  # 3
# Fibers yield control cooperatively, not preemptively

class Obj
  def secret; "hidden"; end
end
o = Obj.new
puts o.send(:secret)  # hidden (calls private too)
puts o.public_send(:secret)  # hidden (respects visibility)
x = 5
result = eval("x * 2")
puts result  # 10
# eval runs a string as code—avoid with untrusted input!

# Block: anonymous chunk of code passed to a method
[1, 2, 3].each { |n| puts n }          # do-end for multi-line
[1, 2, 3].each do |n|
  puts n * 2
end

# yield: call the block from a method
def greet
  puts "before"
  yield  # invokes the block
  puts "after"
end
greet { puts "in block" }
# before / in block / after

# yield with arguments
def compute
  result = yield(10, 20)
  puts "got: #{result}"
end
compute { |a, b| a + b }  # got: 30

# block_given? checks if a block was passed
def maybe_yield
  return "no block" unless block_given?
  yield
end

# Proc: a block stored as an object
p = Proc.new { |x| puts x * 2 }
p.call(5)      # 10
p.(5)          # 10 (shorthand)
p[5]           # 10 (another shorthand)

# Lambda: stricter Proc
l = lambda { |x| puts x * 2 }
l = ->(x) { puts x * 2 }  # stabby lambda syntax
l.call(5)

# Key differences:
# 1. Argument checking
p = Proc.new { |a, b| puts a }
p.call(1)        # 1 (b is nil, no error)
l = ->(a, b) { puts a }
l.call(1)        # ArgumentError (wrong number of args)

# 2. return behavior
def proc_test
  p = Proc.new { return 1 }
  p.call
  return 2  # never reached—Proc return exits method
end
proc_test  # 1

def lambda_test
  l = lambda { return 1 }
  l.call  # returns from lambda, not method
  return 2
end
lambda_test  # 2

class Calculator
  def add(a, b) a + b end
end

calc = Calculator.new
# method(:name) gets a Method object
m = calc.method(:add)
m.call(2, 3)  # 5
m.(2, 3)      # 5
m[2, 3]       # 5

# Convert method to Proc
p = m.to_proc
p.call(4, 5)  # 9

# &:method shorthand (common in map, each)
["a", "b", "c"].map(&:upcase)  # ["A", "B", "C"]
# Equivalent to: .map { |s| s.upcase }

# Symbol#to_proc
:upcase.to_proc.call("hello")  # "HELLOW"

# Useful for passing methods as blocks
[1, 2, 3].each(&method(:puts))  # prints 1, 2, 3

# Closures capture surrounding variables
counter = 0
increment = lambda { counter += 1 }
increment.call  # 1
increment.call  # 2
puts counter    # 2 (closure modified it)

# Multiple closures share the same variables
x = 10
add = lambda { |n| x += n }
get = lambda { x }
add.call(5)
get.call  # 15

# Proc#binding: access the closure's environment
def make_counter
  count = 0
  lambda { count += 1 }
end
c = make_counter
c.call  # 1
# c.binding.eval("count")  # 1 (peek at captured var)

# Binding for eval in a specific context
b = binding
x = 42
eval("x", b)  # 42

# Define your own iterator using yield
class LinkedList
  include Enumerable  # gets map, select, etc. for free

  def each
    node = @head
    while node
      yield node.value
      node = node.next
    end
  end
end

# each_with_object (memo pattern)
result = [1, 2, 3].each_with_object({}) do |n, hash|
  hash[n] = n * n
end
# {1=>1, 2=>4, 3=>9}

# tap (for debugging chains)
[1, 2, 3].map { |n| n * 2 }
         .tap { |arr| puts "after map: #{arr}" }
         .select { |n| n > 2 }

# Custom method with block
def with_timing
  start = Time.now
  yield
  puts "took #{Time.now - start}s"
end
with_timing { sleep(1) }

nums = [1, 2, 3, 4, 5, 6]

# map/collect: transform
nums.map { |n| n * 2 }        # [2, 4, 6, 8, 10, 12]

# select/filter: keep matching
nums.select { |n| n.even? }   # [2, 4, 6]
nums.reject { |n| n.even? }   # [1, 3, 5]

# reduce/inject: accumulate
nums.reduce(0) { |sum, n| sum + n }  # 21
nums.reduce(:+)                        # 21 (symbol shorthand)
nums.reduce(:*)                        # 720

# find/detect: first match
nums.find { |n| n > 3 }       # 4

# group_by: cluster
nums.group_by { |n| n.even? } # {false=>[1,3,5], true=>[2,4,6]}

# partition: split into two
nums.partition { |n| n.even? } # [[2,4,6], [1,3,5]]

# chunk: consecutive grouping
[1,1,2,2,3].chunk { |n| n }.to_a

# Lazy: evaluate on demand (infinite sequences possible)
require 'prime'

# Without lazy: infinite loop!
# primes = (1..Float::INFINITY).select(&:prime?).first(5)

# With lazy: works!
primes = (1..Float::INFINITY).lazy.select(&:prime?).first(5)
# [2, 3, 5, 7, 11]

# Chain without intermediate arrays
(1..1_000_000).lazy
  .map { |n| n * 2 }
  .select { |n| n > 1_000_000 }
  .first(10)  # only computes what's needed

# Build a lazy enumerator
enum = Enumerator::Lazy.new(1..Float::INFINITY) do |yielder, n|
  yielder << n if n.prime?
end
enum.first(5)  # [2, 3, 5, 7, 11]

# Enumerator: an iterator object you can control
enum = [10, 20, 30].each
enum.next  # 10
enum.next  # 20
enum.next  # 30
enum.next  # StopIteration

# External iteration with loop (handles StopIteration)
enum = [1, 2, 3].each
loop do
  puts enum.next
end

# Create custom Enumerator
fib = Enumerator.new do |yielder|
  a, b = 0, 1
  loop do
    yielder << a
    a, b = b, a + b
  end
end
fib.first(10)  # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

# Convert to array
fib.take(10).to_a

arr = [3, 1, 4, 1, 5, 9, 2, 6]

# sort: returns new array
arr.sort                    # [1, 1, 2, 3, 4, 5, 6, 9]
arr.sort { |a, b| b <=> a } # descending (9, 6, 5, ...)

# sort_by: more efficient (Schwartzian transform)
arr.sort_by { |n| -n }      # descending
words = ["banana", "apple", "cherry"]
words.sort_by { |w| w.length }  # ["apple", "banana", "cherry"]

# min/max
arr.min        # 1
arr.max        # 9
arr.minmax     # [1, 9]
arr.min_by { |n| n.abs }  # 1

# min/max with block
people.max_by { |p| p[:age] }

# <=> (spaceship): -1, 0, 1
1 <=> 2   # -1
2 <=> 2   # 0
3 <=> 2   # 1

hash = { a: 1, b: 2, c: 3 }

# Iterate
hash.each { |k, v| puts "#{k}=#{v}" }
hash.each_key { |k| puts k }
hash.each_value { |v| puts v }
hash.each_pair { |k, v| puts "#{k}: #{v}" }

# Transform keys/values
hash.transform_keys { |k| k.to_s }  # {"a"=>1, "b"=>2, "c"=>3}
hash.transform_values { |v| v * 10 } # {a:10, b:20, c:30}

# Filter
hash.select { |k, v| v > 1 }  # {b:2, c:3}
hash.reject { |k, v| v > 1 }  # {a:1}

# Merge
{ a: 1 }.merge({ b: 2 })  # {a:1, b:2}
{ a: 1 }.merge({ a: 2 }) { |k, old, new| old + new }  # {a:3}

# Invert (values become keys)
{ a: 1, b: 2 }.invert  # {1=>:a, 2=>:b}

# Group arrays into hash
%w[apple apricot banana].group_by { |w| w[0] }
# {"a"=>["apple", "apricot"], "b"=>["banana"]}

# Install a gem
$ gem install rails
$ gem install rails -v 7.0.0
$ gem install rails --pre  # pre-release

# List installed gems
$ gem list
$ gem list rails

# Uninstall
$ gem uninstall rails

# Gemfile: project dependencies
# Gemfile
source 'https://rubygems.org'
gem 'rails', '7.0.0'
gem 'pg', '~> 1.4'  # pessimistic: >= 1.4, < 2.0
gem 'puma', '>= 5.0'  # optimistic: >= 5.0
gem 'rspec', group: :test  # group-specific
gem 'pry', require: false  # don't auto-require

# Groups
group :development, :test do
  gem 'rspec-rails'
  gem 'factory_bot_rails'
end

group :production do
  gem 'newrelic_rpm'
end

# Install all gems from Gemfile
$ bundle install
$ bundle install --without production  # skip group

# Update gems
$ bundle update              # update all
$ bundle update rails        # update specific gem
$ bundle outdated            # show outdated gems

# Run commands in bundle context
$ bundle exec rails server
$ bundle exec rspec
$ bundle exec rake db:migrate

# Check for dependency issues
$ bundle check
$ bundle doctor

# Lock file: Gemfile.lock
# Records exact versions installed (for reproducibility)
# Commit Gemfile.lock to version control!

# Add/remove gems
$ bundle add rspec
$ bundle remove rspec

# Clean old gems
$ bundle clean

# my_gem.gemspec
Gem::Specification.new do |spec|
  spec.name = "my_gem"
  spec.version = "0.1.0"
  spec.summary = "A useful Ruby gem"
  spec.description = "Longer description..."
  spec.authors = ["Alice"]
  spec.email = ["[email protected]"]
  spec.homepage = "https://github.com/alice/my_gem"
  spec.license = "MIT"

  spec.files = Dir["lib/**/*.rb"] + ["README.md"]
  spec.require_paths = ["lib"]

  spec.required_ruby_version = ">= 3.0"

  spec.add_dependency "httparty", "~> 0.21"
  spec.add_development_dependency "rspec", "~> 3.12"
end

# Directory structure:
# my_gem/
#   lib/my_gem.rb        (main file)
#   lib/my_gem/version.rb
#   my_gem.gemspec
#   Gemfile
#   spec/                (tests)

# Build and install
$ gem build my_gem.gemspec  # creates my_gem-0.1.0.gem
$ gem install ./my_gem-0.1.0.gem

# Rakefile
require 'rspec/core/rake_task'

RSpec::Core::RakeTask.new(:spec)

task default: :spec

# Custom task
task :greet, [:name] do |t, args|
  puts "Hello, #{args.name}!"
end
# $ rake greet[Alice]

# Namespace
namespace :db do
  task :migrate do
    puts "migrating..."
  end
  task :seed do
    puts "seeding..."
  end
end
# $ rake db:migrate

# File tasks (build dependencies)
file 'output.txt' => 'input.txt' do |t|
  sh "cp #{t.source} #{t.name}"
end

# Multi-line task
task :deploy do
  sh 'git push heroku main'
  sh 'heroku run rails db:migrate'
end

# rbenv: lightweight Ruby version manager
$ rbenv install 3.2.0        # install a version
$ rbenv global 3.2.0         # set global version
$ rbenv local 3.1.0          # set per-project (.ruby-version)
$ rbenv versions             # list installed

# .ruby-version file (committed to project)
# 3.2.0

# RVM: alternative version manager
$ rvm install 3.2.0
$ rvm use 3.2.0
$ rvm gemset create myapp    # isolated gem sets
$ rvm use 3.2.0@myapp

# Bundler config
$ bundle config set path 'vendor/bundle'  # install locally
$ bundle config set without 'development test'  # for production

# Check versions
$ ruby -v
$ gem -v
$ bundle -v

# Rails follows Model-View-Controller
# app/
#   models/    (ActiveRecord: data + business logic)
#   controllers/ (ActionController: handle requests)
#   views/     (ActionView: render responses)
#   helpers/   (view helpers)
# config/routes.rb (URL routing)

# Route → Controller → Model → View
# GET /users → UsersController#index → User.all → index.html.erb

# config/routes.rb
Rails.application.routes.draw do
  resources :users  # generates 7 RESTful routes
  # GET    /users          → index
  # GET    /users/new      → new
  # POST   /users          → create
  # GET    /users/:id      → show
  # GET    /users/:id/edit → edit
  # PATCH  /users/:id      → update
  # DELETE /users/:id      → destroy

  root 'pages#home'  # root path
  get 'about', to: 'pages#about'  # custom route
end

# app/models/user.rb
class User < ApplicationRecord
  # Validations
  validates :name, presence: true, length: { maximum: 50 }
  validates :email, presence: true, uniqueness: true,
            format: { with: URI::MailTo::EMAIL_REGEXP }

  # Associations
  has_many :posts, dependent: :destroy
  has_one :profile
  belongs_to :company
  has_and_belongs_to_many :tags

  # Callbacks
  before_save :normalize_email
  after_create :send_welcome_email

  # Scopes (reusable queries)
  scope :active, -> { where(active: true) }
  scope :recent, -> { order(created_at: :desc).limit(10) }

  private
  def normalize_email
    self.email = email.downcase
  end
end

# Usage
User.create(name: "Alice", email: "[email protected]")
User.active.recent
User.find(1)
User.where("age > ?", 18)

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  before_action :set_user, only: %i[show edit update destroy]

  def index
    @users = User.all
  end

  def show; end  # @user set by before_action

  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user, notice: 'User created'
    else
      render :new, status: :unprocessable_entity
    end
  end

  private

  def set_user
    @user = User.find(params[:id])
  end

  # Strong parameters: whitelist allowed fields
  def user_params
    params.require(:user).permit(:name, :email, :age)
  end
end

<!-- app/views/users/index.html.erb -->
<h1>Users</h1>
<%= link_to "New User", new_user_path %>
<ul>
  <% @users.each do |user| %>
    <li>
      <%= link_to user.name, user_path(user) %>
      (<%= user.email %>)
      <%= link_to "Edit", edit_user_path(user) %>
      <%= button_to "Delete", user, method: :delete %>
    </li>
  <% end %>
</ul>

<!-- Partials: reusable view fragments -->
<!-- app/views/users/_form.html.erb -->
<%= form_with model: user do |form| %>
  <% if user.errors.any? %>
    <div class="errors">
      <% user.errors.full_messages.each do |msg| %>
        <p><%= msg %></p>
      <% end %>
    </div>
  <% end %>
  <%= form.text_field :name %>
  <%= form.email_field :email %>
  <%= form.submit %>
<% end %>

<!-- Render partial -->
<%= render "form", user: @user %>

# Generate a migration
$ rails generate migration CreateUsers name:string email:string

# db/migrate/20240101_create_users.rb
class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users do |t|
      t.string :name, null: false
      t.string :email, null: false, index: { unique: true }
      t.integer :age, default: 0
      t.text :bio
      t.references :company, foreign_key: true

      t.timestamps  # created_at, updated_at
    end
  end
end

# Run migrations
$ rails db:migrate
$ rails db:rollback      # undo last migration
$ rails db:seed          # load seed data
$ rails db:reset         # drop + create + migrate + seed

# Schema file (don't edit manually)
# db/schema.rb reflects current database structure

# Add a column later
$ rails generate migration AddAgeToUsers age:integer
# creates: add_column :users, :age, :integer

# spec/models/user_spec.rb
require 'rails_helper'

RSpec.describe User, type: :model do
  # Setup with let (lazy, memoized)
  let(:user) { User.new(name: "Alice", email: "[email protected]") }

  describe '#name' do
    it 'returns the name' do
      expect(user.name).to eq("Alice")
    end

    # Multiple expectations with context
    context 'when name is blank' do
      let(:user) { User.new(name: "") }
      it 'is invalid' do
        expect(user).not_to be_valid
      end
    end
  end

  # One-liner syntax
  it { should validate_presence_of(:email) }
  it { should have_many(:posts) }
end

RSpec.describe PaymentService do
  let(:user) { create(:user) }

  it 'charges the card' do
    # Stub: replace a method's return value
    allow(user).to receive(:premium?).and_return(true)

    # Mock: expect a method to be called
    expect(Stripe::Charge).to receive(:create).with(
      amount: 1000, currency: 'usd'
    ).and_return(double(id: 'ch_123'))

    service = PaymentService.new(user)
    result = service.charge(10)
    expect(result).to eq('ch_123')
  end

  it 'handles failure' do
    # Stub to raise an error
    allow(Stripe::Charge).to receive(:create)
      .and_raise(Stripe::StripeError.new("card declined"))

    service = PaymentService.new(user)
    expect { service.charge(10) }.to raise_error(PaymentError)
  end
end

# Double: test stand-in for an object
fake_card = double('Card', last4: '4242', brand: 'Visa')

# spec/factories/users.rb
FactoryBot.define do
  factory :user do
    name { "Alice" }
    email { "[email protected]" }
    age { 30 }
    active { true }

    # Traits: variations
    trait :admin do
      role { "admin" }
    end

    trait :inactive do
      active { false }
    end

    # Associations
    company { association(:company) }

    # Sequences for unique values
    sequence(:email) { |n| "user#{n}@example.com" }
  end
end

# Usage in specs
let(:user) { create(:user) }           # saved to DB
let(:admin) { create(:user, :admin) }  # with trait
let(:user) { build(:user) }            # not saved
let(:users) { create_list(:user, 5) }  # 5 users

# Override attributes
let(:alice) { create(:user, name: "Alice") }

RSpec.describe User do
  before(:all) do
    # Runs once before all tests in this group
    @shared_data = load_expensive_data
  end

  before(:each) do  # or just 'before'
    # Runs before each test
    @user = User.create(name: "test")
  end

  after(:each) do
    # Runs after each test (cleanup)
    User.delete_all
  end

  after(:all) do
    # Runs once after all tests
    @shared_data = nil
  end

  # Database cleaner for transactional tests
  config.before(:suite) do
    DatabaseCleaner.strategy = :transaction
    DatabaseCleaner.clean_with(:truncation)
  end
end

# Use before for setup, after for cleanup
# Prefer let over before(:each) for lazy evaluation

# spec/requests/users_spec.rb (integration)
require 'rails_helper'

RSpec.describe 'Users API', type: :request do
  describe 'GET /users' do
    before { create_list(:user, 3) }

    it 'returns all users' do
      get '/users'
      expect(response).to have_http_status(200)
      expect(JSON.parse(response.body).size).to eq(3)
    end
  end

  describe 'POST /users' do
    it 'creates a user' do
      post '/users', params: { user: { name: 'Bob' } }
      expect(response).to have_http_status(:created)
    end
  end
end

# spec/system/login_spec.rb (browser tests)
require 'rails_helper'

RSpec.describe 'Login', type: :system do
  it 'logs in a user' do
    user = create(:user, password: 'secret')
    visit login_path
    fill_in 'Email', with: user.email
    fill_in 'Password', with: 'secret'
    click_button 'Log in'
    expect(page).to have_content('Welcome')
  end
end

# Read entire file
content = File.read("data.txt")

# Read line by line (memory efficient)
File.foreach("large.txt") do |line|
  puts line.chomp  # chomp removes trailing newline
end

# Read all lines into array
lines = File.readlines("data.txt")

# With a block (auto-closes)
File.open("data.txt") do |f|
  f.each_line { |line| puts line }
end

# Read with options
File.read("data.txt", encoding: "utf-8")
File.binread("image.png")  # binary mode

# Check existence
File.exist?("data.txt")
File.file?("data.txt")   # is it a regular file?
File.directory?("path")  # is it a directory?

# Write (overwrites)
File.write("output.txt", "Hello, World!")

# Append
File.write("output.txt", "More text\n", mode: "a")

# With a block (buffered, auto-closes)
File.open("log.txt", "w") do |f|
  f.puts "Line 1"
  f.puts "Line 2"
  f.write("No newline")
  f << "appended"  # << is an alias for write
end

# Binary write
File.binwrite("data.bin", binary_data)

# Modes: r (read), w (write/truncate), a (append),
#        r+ (read/write), w+ (read/write/truncate),
#        b (binary, Windows)

# Flush buffer
f = File.open("log.txt", "w")
f.write("data")
f.flush  # write without closing
f.close

require 'fileutils'

# List directory contents
Dir.entries(".")  # [".", "..", "file.txt", ...]
Dir.glob("*.rb")  # ["script.rb", "test.rb"]
Dir.glob("**/*.rb")  # recursive
Dir.glob("src/**/*.{rb,erb}")  # multiple extensions

# Create directories
Dir.mkdir("new_dir")
Dir.mkdir("nested/path") rescue Errno::ENOENT  # fails if parent missing
FileUtils.mkdir_p("nested/path/deep")  # creates all parents

# Remove
Dir.rmdir("empty_dir")  # only works if empty
FileUtils.rm_rf("dir")  # recursive force (dangerous!)

# Copy/Move
FileUtils.cp("a.txt", "b.txt")
FileUtils.mv("old.txt", "new.txt")
FileUtils.cp_r("src_dir", "dest_dir")  # recursive

# Change directory
Dir.chdir("/tmp") { puts Dir.pwd }  # temporarily
Dir.chdir("/tmp")  # permanently for process

# Check if directory
File.directory?("path")
File.exist?("path")

require 'pathname'
require 'tempfile'

# Pathname: object-oriented path manipulation
path = Pathname.new("/home/user/docs/file.txt")
path.dirname   # #<Pathname:/home/user/docs>
path.basename  # #<Pathname:file.txt>
path.extname   # ".txt"
path.parent    # #<Pathname:/home/user/docs>
path.join("sub", "file.rb")  # /home/user/docs/sub/file.rb

path.exist?
path.directory?
path.file?
path.readable?

# Tempfile: auto-deleted file
Tempfile.create("prefix") do |f|
  f.write("temporary data")
  f.rewind
  puts f.read
end  # file deleted after block

# Tempfile without block (must close/unlink manually)
tf = Tempfile.new("prefix")
tf.write("data")
tf.close
tf.unlink  # delete

# Dir.mktmpdir for temp directories
Dir.mktmpdir do |dir|
  # work in dir
end  # dir deleted after

require 'csv'
require 'json'

# CSV reading
CSV.foreach("data.csv", headers: true) do |row|
  puts row['name']  # access by header
  puts row[0]       # access by index
end

# CSV reading all at once
rows = CSV.read("data.csv", headers: true)
rows.first['name']  # first row's name

# CSV writing
CSV.open("output.csv", "w") do |csv|
  csv << ["name", "age"]  # header
  csv << ["Alice", 30]
  csv << ["Bob", 25]
end

# JSON
data = { name: "Alice", age: 30 }
json = data.to_json  # '{"name":"Alice","age":30}'
parsed = JSON.parse(json)  # {"name"=>"Alice", "age"=>30}

# JSON with symbols
JSON.parse(json, symbolize_names: true)  # {name: "Alice", age: 30}

# Read/write JSON files
File.write("data.json", data.to_json)
loaded = JSON.parse(File.read("data.json"))

# Ruby strings have an encoding
"hello".encoding  # #<Encoding:UTF-8>
"café".encoding   # #<Encoding:UTF-8>

# Default external encoding (for file I/O)
Encoding.default_external  # #<Encoding:UTF-8>

# Force an encoding (reinterprets bytes, doesn't convert)
bytes = "café".bytes  # [99, 97, 102, 195, 169]
latin1 = bytes.pack("C*").force_encoding("ISO-8859-1")
# "caf" + é (as single byte)

# Convert encoding (transcodes)
utf8 = "café"
latin1 = utf8.encode("ISO-8859-1")  # converts
back = latin1.encode("UTF-8")        # converts back

# Check if valid
"abc".valid_encoding?  # true
"\xff".valid_encoding?  # false (invalid byte)

# Common encodings: UTF-8, ASCII, ISO-8859-1, Windows-1252
# Always use UTF-8 unless you have a specific reason

# Read a file with specific encoding
content = File.read("data.txt", encoding: "ISO-8859-1")
# content.encoding is ISO-8859-1

# Read and convert to UTF-8
content = File.read("data.txt", encoding: "ISO-8859-1:UTF-8")
# The "source:target" syntax converts while reading

# Write with specific encoding
File.write("output.txt", "café", encoding: "UTF-8")

# Open with encoding
File.open("data.txt", "r:ISO-8859-1") do |f|
  f.read  # ISO-8859-1 encoded string
end

# Convert while opening
File.open("data.txt", "r:ISO-8859-1:UTF-8") do |f|
  f.read  # UTF-8 encoded string
end

# Set default encodings
Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8

s = "Hello, World"

# Inspection
s.length    # 12
s.empty?    # false
s.include?("World")  # true
s.start_with?("Hello")  # true
s.end_with?("World")  # true

# Transformation (return new string)
s.upcase     # "HELLO, WORLD"
s.downcase   # "hello, world"
s.capitalize # "Hello, world"
s.swapcase   # "hELLO, wORLD"
s.reverse    # "dlroW ,olleH"
s.strip      # remove leading/trailing whitespace
s.chomp(",") # remove trailing substring
s.chop       # remove last char

# Substrings
s[0, 5]      # "Hello" (start, length)
s[7..11]     # "World" (range)
s[-5..]      # "World" (negative index)

# Replace
s.sub("World", "Ruby")    # first match
s.gsub("o", "0")          # all matches
s.gsub(/\w+/) { |w| w.upcase }  # with block

# Split/Join
"a,b,c".split(",")  # ["a", "b", "c"]
["a", "b"].join("-")  # "a-b"

# frozen_string_literal: true  (at top of file)
# Makes all string literals in the file frozen (immutable)

# Mutable vs frozen
s = "hello"
s << " world"  # "hello world" (mutates)

f = "hello".freeze
# f << " world"  # Error: can't modify frozen String

# Why freeze? Performance: frozen strings share memory
# "abc".freeze is the same object everywhere
"a".freeze.equal?("a".freeze)  # true (same object)

# Building strings efficiently
# BAD: creates many intermediate strings
result = ""
items.each { |i| result += i.to_s }  # O(n²)

# GOOD: use << or join
result = ""
items.each { |i| result << i.to_s }  # O(n)
result = items.map(&:to_s).join  # cleanest

# StringIO for buffered building
require 'stringio'
io = StringIO.new
io << "line 1\n"
io << "line 2\n"
result = io.string

# Symbol: immutable, reusable identifier
:hello
:world
status = :active

# String: mutable, can have many instances
"hello"
"world"
status = "active"

# Key difference: identity
"hello".equal?("hello")  # false (different objects)
:hello.equal?(:hello)    # true (same object, singleton)

# Memory: symbols are singletons
1000.times { :foo }  # one :foo object
1000.times { "foo" }  # 1000 "foo" objects (unless frozen)

# Use symbols for:
# - Hash keys (faster comparison)
# - Method names (send(:method_name))
# - Enum-like values (:active, :pending, :closed)
# - Identifiers (not text data)

# Use strings for:
# - Text data (names, content)
# - Things that change

# Conversion
:hello.to_s  # "hello"
"hello".to_sym  # :hello
"hello world".to_sym  # :"hello world" (valid but ugly)

# Module: a namespace + collection of methods
module MathUtils
  PI = 3.14159

  def self.circle_area(radius)  # module method
    PI * radius ** 2
  end

  def square(x)  # instance method (for mixins)
    x * x
  end
end

# Access constants and module methods
MathUtils::PI  # 3.14159
MathUtils.circle_area(5)  # 78.54

# Namespacing classes
module MyApp
  class User
    # MyApp::User
  end
end

# Prevent instantiation (modules can't be instantiated)
# MathUtils.new  # NoMethodError

module Greetable
  def greet
    "Hello from #{self.class}"
  end
end

class User
  include Greetable  # adds as instance methods
end
User.new.greet  # "Hello from User"

class Service
  extend Greetable  # adds as class methods
end
Service.greet  # "Hello from Service"

# Method lookup order:
# prepend → class → include → super
module Logging
  def save
    puts "before save"
    super  # calls the original save
    puts "after save"
  end
end

class Record
  prepend Logging  # Logging#save runs first
  def save; puts "saving"; end
end
Record.new.save
# before save / saving / after save

# Include Enumerable + define #each = 50+ methods free
class Playlist
  include Enumerable

  def initialize(songs)
    @songs = songs
  end

  def each
    @songs.each { |song| yield song }
  end
end

playlist = Playlist.new(["Song A", "Song B", "Song C"])
playlist.map { |s| s.upcase }   # ["SONG A", "SONG B", "SONG C"]
playlist.select { |s| s.include?("A") }  # ["Song A"]
playlist.reduce(:+)            # "Song ASong BSong C"
playlist.sort                  # ["Song A", "Song B", "Song C"]
playlist.first(2)              # ["Song A", "Song B"]
playlist.include?("Song B")    # true
playlist.count                 # 3

# Comparable: define <=> for natural ordering
class Temperature
  include Comparable
  attr_reader :celsius

  def initialize(c) @celsius = c end
  def <=>(other) celsius <=> other.celsius end
end

t1 = Temperature.new(20)
t2 = Temperature.new(30)
t1 < t2   # true (uses <=>)
t1 == t2  # false

# Singleton method: defined on one object only
str = "hello"
def str.shout
  upcase + "!"
end
str.shout  # "HELLO!"
# "world".shout  # NoMethodError (only str has it)

# Class methods are singleton methods on the class
class Calculator
  def self.add(a, b)  # singleton method on Calculator
    a + b
  end
end
Calculator.add(1, 2)  # 3

# Singleton class (eigenclass): where singleton methods live
class Calculator
  class << self  # opens the singleton class
    def multiply(a, b) a * b end
    def divide(a, b) a / b end
  end
end
Calculator.multiply(2, 3)  # 6

# Singleton pattern
class Logger
  @instance = Logger.new
  class << self
    attr_reader :instance
  end
end
Logger.instance.equal?(Logger.instance)  # true

# Refinement: limited monkey patching
module StringRefinements
  refine String do
    def shout
      upcase + "!!!"
    end

    def palindrome?
      downcase == downcase.reverse
    end
  end
end

# Without using: shout is undefined
# "hello".shout  # NoMethodError

# With using: refinement is active in this scope
using StringRefinements
"hello".shout        # "HELLO!!!"
"racecar".palindrome?  # true

# Scoped to file or module
class App
  using StringRefinements
  def greet
    "hi".shout  # works here
  end
end
# "hi".shout  # NoMethodError (outside App)

# Safer than global monkey patching
# Refinements only affect code that opts in

# Block: anonymous chunk of code passed to a method
[1, 2, 3].each { |n| puts n }      # Single-line
[1, 2, 3].each do |n|              # Multi-line
  puts n
end

# yield invokes the block
def greet
  print "Hello, "
  yield          # Calls the block
  puts "!"
end

greet { print "World" }   # Hello, World!

# yield with arguments
def calculate(a, b)
  yield(a + b)
  yield(a * b)
end

calculate(3, 4) { |result| puts "Result: #{result}" }

# Proc: object wrapping a block
my_proc = Proc.new { |x| puts x * 2 }
my_proc.call(5)         # 10
my_proc.(5)             # 10 (shorthand)
my_proc[5]              # 10 (another shorthand)

# Lambda: stricter Proc
my_lambda = lambda { |x| puts x * 2 }
my_lambda = ->(x) { puts x * 2 }   # Stabby lambda

# Difference 1: argument checking
my_proc.call(1, 2, 3)    # OK (ignores extra args)
my_lambda.call(1, 2, 3)  # ArgumentError (wrong number of args)

# Difference 2: return behavior
def proc_test
  p = Proc.new { return 1 }
  p.call
  return 2  # Never reached
end

def lambda_test
  l = lambda { return 1 }
  l.call    # Returns 1 to the lambda, not the method
  return 2  # Reached
end

# Convert method to Proc with method()
class Calculator
  def add(a, b) a + b end
  def multiply(a, b) a * b end
end

calc = Calculator.new
add_proc = calc.method(:add)
puts add_proc.call(2, 3)   # 5

# Pass method as block with & operator
def apply_op(a, b, op)
  op.call(a, b)
end

puts apply_op(2, 3, calc.method(:add))       # 5
puts apply_op(2, 3, calc.method(:multiply))  # 6

# Symbol#to_proc
words = ["hello", "world"]
upcased = words.map(&:upcase)   # ["HELLO", "WORLD"]
# Equivalent to: words.map { |w| w.upcase }

# Common with &:map(&:to_i), select(&:positive?), sort_by(&:length)

# Blocks/Procs/Lambdas capture variables (closures)
counter = 0
increment = lambda { counter += 1 }

increment.call
increment.call
puts counter   # 2

# Multiple closures sharing state
def make_counters
  count = 0
  [
    lambda { count += 1 },
    lambda { count },
    lambda { count = 0 }
  ]
end

inc, get, reset = make_counters
inc.call; inc.call
puts get.call   # 2
reset.call
puts get.call   # 0

# Block-local variables (shadow outer)
x = 10
[1, 2, 3].each do |y; x|   # x is block-local
  x = y * 2
end
puts x   # 10 (unchanged)

class Tree
  include Enumerable

  def initialize(value, children = [])
    @value = value
    @children = children
  end

  def each(&block)
    block.call(@value)
    @children.each { |child| child.each(&block) }
  end
end

tree = Tree.new(1, [
  Tree.new(2, [Tree.new(4), Tree.new(5)]),
  Tree.new(3)
])

tree.each { |v| puts v }            # 1 2 4 5 3
puts tree.map { |v| v * 2 }.inspect # [2, 4, 8, 10, 6]
puts tree.select(&:even?).inspect   # [2, 4]
puts tree.reduce(:+)                # 15

# Including Enumerable gives you map, select, reduce, etc.

class Person
  attr_accessor :name, :age

  def initialize(name, age)
    @name = name
    @age = age
  end

  # Method missing: catch undefined method calls
  def method_missing(name, *args, &block)
    if name.to_s =~ /^(.*)_with_prefix$/
      attr_name = $1
      "PREFIX_#{send(attr_name)}"
    else
      super
    end
  end

  def respond_to_missing?(name, include_private = false)
    name.to_s =~ /^(.*)_with_prefix$/ || super
  end
end

p = Person.new("Alice", 30)
puts p.name_with_prefix   # PREFIX_Alice

# Define methods dynamically
class Person
  [:home, :work, :mobile].each do |type|
    define_method("#{type}_phone") do
      instance_variable_get("@#{type}_phone")
    end

    define_method("#{type}_phone=") do |value|
      instance_variable_set("@#{type}_phone", value)
    end
  end
end

# Reopen existing classes (monkey patching)
class String
  def shout
    upcase + "!"
  end

  def word_count
    split.size
  end
end

puts "hello world".shout       # HELLO WORLD!
puts "one two three".word_count # 3

# Refinements: scoped monkey patches
module ShoutRefinement
  refine String do
    def shout
      upcase + "!"
    end
  end
end

class MyClass
  using ShoutRefinement

  def greet(name)
    "hello #{name}".shout   # Works here
  end
end

# "test".shout  # NoMethodError (refinement not active)

class Observable
  # Hook: called when class is subclassed
  def self.inherited(subclass)
    puts "#{subclass} inherits from #{self}"
  end

  # Hook: called when a module is included
  def self.included(base)
    puts "#{self} included in #{base}"
  end

  # Hook: called when a method is added
  def method_added(name)
    puts "Added method: #{name}"
  end

  # Hook: called when method is undefined
  def method_removed(name)
    puts "Removed method: #{name}"
  end
end

class Child < Observable
  def my_method; end
end
# Output:
# Child inherits from Observable
# Added method: my_method

# Other hooks: method_undefined, extended, prepended

# eval: execute a string as Ruby code
result = eval("1 + 2 * 3")
puts result   # 7

# eval with binding (context)
def eval_in_context(code)
  x = 10
  binding   # Returns a Binding object capturing local variables
end

b = eval_in_context("binding")
puts eval("x * 2", b)   # 20

# class_eval: execute code in class context
String.class_eval do
  def palindrome?
    self == reverse
  end
end

puts "racecar".palindrome?   # true

# instance_eval: execute in instance context
"hello".instance_eval do
  puts length   # 5 (self is the string)
end

# NEVER eval user input (security risk)

class User
  attr_accessor :name, :email

  def initialize(name, email)
    @name = name
    @email = email
  end

  def save; end
  def valid?; true; end
end

u = User.new("Alice", "[email protected]")

# Inspect methods
puts u.methods - Object.methods    # [:name, :email, :save, :valid?, ...]
puts u.public_methods(false)       # Only this class's methods
puts User.instance_methods(false)  # [:save, :valid?, ...]

# Inspect variables
puts u.instance_variables.inspect   # [:@name, :@email]
puts u.instance_variable_get(:@name) # Alice

# Inspect class
puts User.ancestors.inspect
puts User.instance_method(:save).parameters

# Check if responds to method
puts u.respond_to?(:save)   # true
puts u.is_a?(User)          # true

# spec/spec_helper.rb
require 'rspec'
require_relative '../lib/my_app'

# spec/models/user_spec.rb
require 'spec_helper'

RSpec.describe User do
  # Setup with let (lazy, memoized)
  let(:user) { User.new(name: 'Alice', email: '[email protected]') }

  describe '#name' do
    it 'returns the name' do
      expect(user.name).to eq('Alice')
    end
  end

  describe '#valid?' do
    context 'with valid attributes' do
      it 'is valid' do
        expect(user).to be_valid
      end
    end

    context 'without name' do
      let(:user) { User.new(email: '[email protected]') }
      it 'is invalid' do
        expect(user).not_to be_valid
        expect(user.errors[:name]).to include("can't be blank")
      end
    end
  end

  # Multiple examples
  describe '#age' do
    subject { user.age }
    it { is_expected.to be >= 0 }
  end
end

# Run: rspec spec/models/user_spec.rb
# Run all: rspec

RSpec.describe PaymentProcessor do
  let(:gateway) { double('PaymentGateway') }
  let(:processor) { PaymentProcessor.new(gateway) }

  describe '#charge' do
    it 'charges the gateway' do
      # Stub: replace method return value
      allow(gateway).to receive(:charge).and_return(success: true)

      result = processor.charge(100)
      expect(result[:success]).to be true
    end

    it 'raises on gateway error' do
      allow(gateway).to receive(:charge)
        .and_raise(GatewayError, 'Network failure')

      expect { processor.charge(100) }
        .to raise_error(GatewayError, /Network/)
    end

    it 'verifies the gateway was called' do
      # Mock: expect method to be called
      expect(gateway).to receive(:charge).with(100)
      processor.charge(100)
    end

    it 'receives multiple calls' do
      allow(gateway).to receive(:charge).and_return(true, false, true)
      expect(gateway.charge(1)).to be true
      expect(gateway.charge(2)).to be false
    end
  end
end

# test/test_helper.rb
require 'minitest/autorun'
require_relative '../lib/my_app'

# test/models/user_test.rb
require 'test_helper'

class UserTest < Minitest::Test
  def setup
    @user = User.new(name: 'Alice', email: '[email protected]')
  end

  def test_name
    assert_equal 'Alice', @user.name
  end

  def test_valid_with_attributes
    assert @user.valid?
  end

  def test_invalid_without_name
    @user.name = nil
    refute @user.valid?
    assert_includes @user.errors[:name], "can't be blank"
  end

  def test_raises_on_error
    assert_raises(ArgumentError) { User.new! }
  end
end

# Spec-style Minitest
class UserSpec < Minitest::Spec
  let(:user) { User.new(name: 'Alice') }

  it 'has a name' do
    _(user.name).must_equal 'Alice'
  end
end

# Run: rake test
# Run specific: ruby -Itest test/models/user_test.rb

# FactoryBot (gem 'factory_bot')
# spec/factories/users.rb
FactoryBot.define do
  factory :user do
    name { 'Alice' }
    email { '[email protected]' }
    age { 30 }

    trait :admin do
      role { 'admin' }
    end

    trait :with_posts do
      after(:create) do |user|
        create_list(:post, 3, user: user)
      end
    end

    factory :admin_user, traits: [:admin]
    factory :user_with_posts, traits: [:with_posts]
  end
end

# Usage in specs
let(:user) { create(:user) }            # Saved to DB
let(:admin) { create(:admin_user) }
let(:user_with_posts) { create(:user_with_posts) }
let(:unsaved_user) { build(:user) }     # Not saved

# Fixtures (alternative)
# test/fixtures/users.yml
# alice:
#   name: Alice
#   email: [email protected]

# In tests: users(:alice)

# spec/requests/users_spec.rb (integration)
require 'spec_helper'

RSpec.describe 'Users API', type: :request do
  describe 'GET /users' do
    before { create_list(:user, 3) }

    it 'returns all users' do
      get '/users', as: :json
      expect(response).to have_http_status(200)
      expect(JSON.parse(response.body).size).to eq(3)
    end
  end

  describe 'POST /users' do
    it 'creates a user' do
      post '/users', params: { user: { name: 'Bob', email: '[email protected]' } }
      expect(response).to have_http_status(201)
      expect(User.last.name).to eq('Bob')
    end
  end
end

# System tests (browser automation)
# spec/system/login_spec.rb
require 'spec_helper'

RSpec.describe 'Login', type: :system do
  it 'logs in a user' do
    user = create(:user, password: 'password123')
    visit login_path
    fill_in 'Email', with: user.email
    fill_in 'Password', with: 'password123'
    click_button 'Log In'
    expect(page).to have_content('Welcome')
  end
end

# Create and run threads
threads = (1..3).map do |i|
  Thread.new(i) do |n|
    puts "Thread #{n} started"
    sleep(1)
    puts "Thread #{n} finished"
  end
end

# Wait for all threads
threads.each(&:join)
puts "All done"

# Thread with return value
thread = Thread.new { 1 + 2 }
result = thread.value   # 3 (waits for completion)

# Thread status
thread = Thread.new { sleep(1) }
puts thread.status   # 'run', 'sleep', false (finished), nil (error)
thread.join

# Current thread
Thread.current
Thread.list   # All threads
Thread.main   # Main thread

# Mutex: mutual exclusion
counter = 0
mutex = Mutex.new

threads = 10.times.map do
  Thread.new do
    1000.times do
      mutex.synchronize { counter += 1 }
    end
  end
end
threads.each(&:join)
puts counter   # 10000 (without mutex, would be less)

# Queue: thread-safe FIFO
require 'thread'
queue = Queue.new

producer = Thread.new do
  5.times { |i| queue << "item #{i}" }
  queue << :done
end

consumer = Thread.new do
  loop do
    item = queue.pop
    break if item == :done
    puts "Processed: #{item}"
  end
end

[producer, consumer].each(&:join)

# ConditionVariable: signal between threads
mutex = Mutex.new
cv = ConditionVariable.new

# Fiber: cooperative concurrency (manual scheduling)
fiber = Fiber.new do
  puts "Fiber started"
  Fiber.yield "first yield"
  puts "Fiber resumed"
  Fiber.yield "second yield"
  puts "Fiber ending"
  "fiber done"
end

puts fiber.resume   # "Fiber started", returns "first yield"
puts fiber.resume   # "Fiber resumed", returns "second yield"
puts fiber.resume   # "Fiber ending", returns "fiber done"

# Generator pattern with Fiber
def fibonacci
  Fiber.new do
    a, b = 0, 1
    loop do
      Fiber.yield a
      a, b = b, a + b
    end
  end
end

fib = fibonacci
10.times { print fib.resume, " " }
# 0 1 1 2 3 5 8 13 21 34

# Async gem for modern async I/O
# gem install async
require 'async'
require 'async/http/internet'

Async do
  internet = Async::HTTP::Internet.new

  # Run requests concurrently
  tasks = 3.times.map do |i|
    Async do
      response = internet.get("https://httpbin.org/delay/#{i}")
      puts "Request #{i}: #{response.status}"
    end
  end

  tasks.each(&:wait)
end

# Concurrent Ruby (gem 'concurrent-ruby')
require 'concurrent'

# Future: async computation
future = Concurrent::Future.execute { sleep(1); 42 }
puts future.value   # 42 (blocks until ready)

# Promise chain
Concurrent::Promise.execute { 1 }
  .then { |v| v + 1 }
  .then { |v| v * 2 }
  .then { |v| puts v }   # 4

# Thread pool
pool = Concurrent::FixedThreadPool.new(4)
10.times do |i|
  pool.post { puts "Task #{i} on #{Thread.current.object_id}" }
end
pool.shutdown

# Ractor: true parallelism (no GIL)
# Ruby 3.0+ feature for thread-safe parallel execution

ractor = Ractor.new do
  value = receive    # Receive from main
  value * 2
end

ractor.send(21)
puts ractor.take     # 42

# Parallel computation
ractors = (1..4).map do |i|
  Ractor.new(i) do |n|
    sleep(1)
    n * n
  end
end

results = Ractor.select(*ractors)
# Or: ractors.map(&:take)  # [1, 4, 9, 16]

# Ractor restrictions:
# - Cannot share mutable objects
# - Communication only via messages
# - Copy or move semantics for sending

# Shared frozen objects are OK
SHARED = Ractor.new { [1, 2, 3].freeze }
# Multiple ractors can read SHARED safely

# Ruby strings have encodings
s = "hello"
puts s.encoding              # <Encoding:UTF-8>
puts s.bytesize              # 5
puts s.length               # 5

# Multibyte characters
japanese = "こんにちは"
puts japanese.encoding       # UTF-8
puts japanese.bytesize       # 15 (3 bytes per char)
puts japanese.length         # 5 (5 characters)

# Force encoding (interpret bytes differently)
bytes = "caf\u00e9".force_encoding('ASCII-8BIT')
puts bytes.encoding          # ASCII-8BIT

# Encode (convert to different encoding)
utf8 = "café"
latin1 = utf8.encode('ISO-8859-1')
puts latin1.encoding         # ISO-8859-1

# Invalid bytes
bad = "\xFF\xFE".force_encoding('UTF-8')
puts bad.valid_encoding?     # false
fixed = bad.encode('UTF-8', invalid: :replace, replace: '?')

# Read entire file
content = File.read('data.txt')

# Read line by line (memory efficient)
File.foreach('large.log') do |line|
  puts line.chomp
end

# Write to file
File.write('output.txt', 'Hello, World!')

# Append
File.open('log.txt', 'a') do |f|
  f.puts "New log entry"
end

# Read/Write modes
File.open('file.txt', 'r+') do |f|
  content = f.read
  f.rewind
  f.write("Updated: " + content)
end

# Binary mode
File.open('image.png', 'rb') do |f|
  bytes = f.read
  puts bytes.bytesize
end

# File operations
File.exist?('file.txt')
File.size('file.txt')
File.mtime('file.txt')
File.delete('file.txt')
File.rename('old.txt', 'new.txt')
FileUtils.cp('a.txt', 'b.txt')   # require 'fileutils'

require 'stringio'
require 'tempfile'

# StringIO: in-memory file-like object
io = StringIO.new
io.puts "Hello"
io.puts "World"
io.rewind
puts io.read   # "Hello\nWorld\n"

# Use like a file
io = StringIO.new("line1\nline2\nline3")
io.each_line { |line| puts line.chomp }

# Tempfile: auto-deleted temporary file
Tempfile.create('myapp') do |f|
  f.write('temporary data')
  f.rewind
  puts f.read
end   # File automatically deleted

# Tempfile with explicit cleanup
temp = Tempfile.new('myapp')
begin
  temp.write('data')
  temp.rewind
  # Use temp.path to pass to external programs
ensure
  temp.close
  temp.unlink
end

require 'socket'
require 'net/http'
require 'uri'

# TCP Client
TCPSocket.open('example.com', 80) do |socket|
  socket.write("GET / HTTP/1.0\r\n\r\n")
  puts socket.read
end

# TCP Server
server = TCPServer.new(2000)
loop do
  client = server.accept
  client.puts "Hello from server"
  client.close
end

# HTTP Client
uri = URI('https://api.example.com/data')
response = Net::HTTP.get_response(uri)
puts response.code          # 200
puts response.body

# HTTP POST
response = Net::HTTP.post_form(uri, key: 'value')

# HTTP with custom headers
http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true
request = Net::HTTP::Get.new(uri)
request['Authorization'] = 'Bearer token'
response = http.request(request)

# Standard IO
$stdout.puts "To stdout"
$stderr.puts "To stderr"
input = $stdin.gets   # Read from stdin

# Redirect IO
$stdout = File.open('log.txt', 'w')
puts "This goes to file"
$stdout = $stdout   # Restore (or use STDOUT constant)

# Capture output
captured = StringIO.new
original = $stdout
$stdout = captured
puts "Captured"
$stdout = original
puts captured.string   # "Captured\n"

# Open3: run commands with full IO control
require 'open3'

Open3.popen3('grep hello') do |stdin, stdout, stderr, wait|
  stdin.puts "hello world"
  stdin.puts "goodbye"
  stdin.close
  puts stdout.read      # "hello world"
  puts wait.value.exitstatus
end

# Pipeline
output = Open3.pipeline('ls', 'grep .rb', 'wc -l')

# in: pattern matching (Ruby 2.7+, stable in 3.0)
case [1, 2, 3]
in [1, *rest]
  puts "Starts with 1, rest: #{rest}"
end

case {name: 'Alice', age: 30}
in {name: String => name, age: Integer => age}
  puts "#{name} is #{age}"
end

# Multiple patterns with |
case status
in :success | :ok
  puts "Good"
else
  puts "Other"
end

# Array patterns
case [1, 2, 3, 4]
in [_, _, *rest]
  puts "Rest: #{rest}"   # [3, 4]
end

# Hash patterns (partial match by default)
case {a: 1, b: 2, c: 3}
in {a: Integer}
  puts "Has a"   # Matches (ignores b, c)
end

user = {name: 'Alice', age: 30, role: :admin}

case user
in {name: String => name, age: Integer => age, role: :admin} if age > 18
  puts "#{name} is an adult admin"
end

# Pin operator (^): match against variable value
expected = 'Alice'
case user
in {name: ^expected}
  puts "Matched expected name"
end

# Array destructuring with binding
case [1, 2, 3]
in [first, *middle, last]
  puts "First: #{first}, Middle: #{middle}, Last: #{last}"
end

# Find pattern (search within array)
case [1, 2, 3, 4, 5]
in [*, 3 => three, *]
  puts "Found 3: #{three}"
end

# Match by class
case value
in Integer
  puts "Integer"
in String
  puts "String"
in Array
  puts "Array"
end

# Class with destructuring
class Point
  attr_reader :x, :y
  def initialize(x, y) @x, @y = x, y end
  def deconstruct = [@x, @y]              # Array pattern
  def deconstruct_keys(keys) = {x: @x, y: @y}  # Hash pattern
end

p = Point.new(1, 2)
case p
in Point[x, y]
  puts "Point at (#{x}, #{y})"
end

case p
in Point(x:, y:)
  puts "X: #{x}, Y: #{y}"
end

# Built-in classes support destructuring
case Time.now
in Time(hour: h) if h < 12
  puts "Morning"
end

# One-line form (Ruby 3.0+)
{status: :ok, data: 42} in {status: :ok, data: Integer => data}
puts data   # 42

# Returns true/false (does not raise)
[1, 2, 3] in [Integer, Integer, Integer]   # true
[1, 2, 3] in [String, *, *]                # false

# Useful for conditionals
if user in {role: :admin}
  puts "Admin access"
end

# Guard clauses
def process(data)
  return unless data in {type: String, value: Integer}
  # ...
end

# Extracting from JSON
json = JSON.parse('{"user":{"name":"Alice","age":30}}')
json in {"user" => {"name" => String => name, "age" => Integer => age}}
puts "#{name}, #{age}"   # Alice, 30

# 1. Parse JSON API responses
def handle_response(response)
  case JSON.parse(response)
  in {"status" => "success", "data" => Array => items}
    items.each { |item| process(item) }
  in {"status" => "error", "message" => String => msg}
    raise "API error: #{msg}"
  in {"status" => "error", "code" => Integer => code} if code >= 500
    retry_request
  else
    raise "Unknown response format"
  end
end

# 2. State machines
case state
in :idle, event: :start
  transition_to :running
in :running, event: :pause
  transition_to :paused
in :paused, event: :resume
  transition_to :running
in :running | :paused, event: :stop
  transition_to :idle
end

# 3. AST traversal
def evaluate(node)
  case node
  in {type: 'number', value: n}
    n
  in {type: 'add', left:, right:}
    evaluate(left) + evaluate(right)
  in {type: 'mul', left:, right:}
    evaluate(left) * evaluate(right)
  end
end

class Foo
  define_method(:greet) do |name|
    "Hello, #{name}"
  end
end
Foo.new.greet("Alice")  # "Hello, Alice"
# Dynamic method call
Foo.new.send(:greet, "Bob")  # "Hello, Bob"
Foo.new.public_send(:greet, "Bob")  # Respects visibility

class DynamicHash
  def method_missing(name, *args)
    if name.to_s.end_with?('=')
      self[name.to_s.chomp('=')] = args.first
    else
      self[name.to_s]
    end
  end
  def respond_to_missing?(name, include_private = false)
    true
  end
end

# Execute a string as Ruby code
result = eval("1 + 2 * 3")  # 7
# Dynamic method definition
eval("def dynamic_method; 42; end")
dynamic_method  # 42
# WARNING: never eval untrusted input!

class String
  def shout
    upcase + "!"
  end
end
"hello".shout  # "HELLO!"
# Monkey patching: modifying existing classes
# Use sparingly, can cause conflicts

class ActiveRecord::Base
  def self.has_many(name)
    define_method(name) { [] }
  end
end
class Post < ActiveRecord::Base
  has_many :comments
end
Post.new.comments  # []