blocks.cr

struct Int
  def palindrome?
    self.to_s == self.to_s.reverse
  end
end

def nums
  (100..9999).each do |i|
    (i..9999).each do |j|
      yield i*j
    end
  end
end

result = 0
nums do |product|
  if product > result && product.palindrome?
    result = product
  end
end
puts result

nums.enum_for.lazy.select(&.palindrome?).max

channels.cr

struct Int
  def palindrome?
    self.to_s == self.to_s.reverse
  end
end

chan = Channel(Int32 | Iterator::Stop).new(1)
spawn do
  (100..999).each do |i|
    (i..999).each do |j|
      chan.send i * j
    end
  end
  chan.send Iterator::Stop::INSTANCE
end

class Products
  include Iterator(Int32)
  def initialize(chan : Channel(Int32 | Iterator::Stop))
    @chan = chan
  end

  def next
    @chan.receive
  end
end

products = Products.new(chan)
puts products.select(&.palindrome?).max

loops.cr

struct Int
  def palindrome?
    self.to_s == self.to_s.reverse
  end
end

result = 0
(100..999).each do |i|
  (i..999).each do |j|
    if i*j > result && (i*j).palindrome?
      result = i*j
    end
  end
end

puts result

# What I'd like to write is...

nums = Iterator.new do |yielder|
  (100..999).each do |i|
    (i..999).each do |j|
      yielder << i * j
    end
  end
end

nums.select(&.palindrome?).max

# As this would separate the work I'm doing from the things I'm doing the work on.