Code Kata #4: Cache

cache.rb

class Cache
  attr_accessor :data

  def initialize(data: {})
    @data = data
  end

  def get(key)
    return if data.fetch(key, {}).fetch(:expires_at, nil) <= now

    value_for(key)
  end

  def set(key, value, expires_at: default_expires_at)
    data[key] = { value: value, expires_at: expires_at }
  end

  def delete(key)
    data.delete(key)
  end

  def fetch(key, &block)
    value = value_for(key)
    data[key] = { value: yield, expires_at: default_expires_at } if value.nil?
    value_for(key)
  end

  private

  def now
    # timezones? UTC?
    Time.now
  end

  def default_expires_at
    now + 100
  end

  def value_for(key)
    data.fetch(key, {}).fetch(:value, nil)
  end
end

run.rb

require './cache.rb'

cache = Cache.new

tests = [
  # 0
  Proc.new do |cache|
    cache.get(:foo).nil?
  end,

  # 1
  Proc.new do |cache|
    cache.set(:foo, 'FOO') == 'FOO'
  end,

  # 2
  Proc.new do |cache|
    cache.set(:foo, 'FOO')
    cache.get(:foo) == 'FOO'
  end,

  # 3
  Proc.new do |cache|
    cache.set(:foo, 'FOO')
    cache.delete(:foo) == 'FOO'
  end,

  # 4
  Proc.new do |cache|
    cache.set(:foo, 'FOO')
    cache.delete(:foo)
    cache.get(:foo).nil?
  end,

  # 5
  Proc.new do |cache|
    num_calls   = 0
    calculation = -> { num_calls += 1 }

    cache.fetch(:foo, &calculation) == 1 && cache.fetch(:foo, &calculation) == 1
  end,

  # 6
  Proc.new do |cache|
    now         = Time.now
    expires_at  = now + 100

    cache.set(:foo, 'FOO', expires_at: expires_at)
    cache.get(:foo) == 'FOO'
  end,

  # 7
  Proc.new do |cache|
    now         = Time.now
    expires_at  = now - 1

    cache.set(:foo, 'FOO', expires_at: expires_at)
    cache.get(:foo).nil?
  end,

  # 8
  Proc.new do |cache|
    now         = Time.now
    expires_at  = now + 1

    cache.set(:foo, 'FOO', expires_at: expires_at)
    sleep(1.2)
    cache.get(:foo).nil?
  end,
]

tests.each_with_index do |test, i|
  cache = Cache.new

  print "Test #{i}: "

  begin
    if test.call(cache)
      puts 'success'
    else
      puts 'fail'
    end
  rescue StandardError => exception
    puts "error: #{exception.inspect}"
  end
end

test_status.md

$ ruby run.rb
Test 0: error: #<NoMethodError: undefined method `<=' for nil:NilClass>
Test 1: fail
Test 2: success
Test 3: fail
Test 4: error: #<NoMethodError: undefined method `<=' for nil:NilClass>
Test 5: success
Test 6: success
Test 7: success
Test 8: success