parallel_hash.rb

require 'thread'
require 'digest/md5'

# For motivation on why you might want to do all this hashing, see:
# https://en.wikipedia.org/wiki/Hashcash

# Thread::abort_on_exception = true

t1 = Time.now

MAXIMUM = 2 ** 128 - 1
DIFFICULTY = 1_000_000.0
GOAL = (MAXIMUM / DIFFICULTY)
NUM_HASHES = 5

NUM_THREADS = 8

def start_thread(thread_idx, queue)
  thread = Thread.new do
    # inside the block is the code the thread runs. Then the thread quits.
    string = "#{thread_idx}"
    while true
      hash = Digest::MD5.hexdigest(string).to_i(16)
      if hash < GOAL
        queue << [string, hash]
      end

      string = hash.to_s
    end
  end
  
  thread
end

# "thread safe" queue
queue = Queue.new
threads = []
NUM_THREADS.times { |thread_idx| threads << start_thread(thread_idx, queue) }

NUM_HASHES.times do
  result = queue.shift
  p result
end

threads.each { |thread| thread.kill }

t2 = Time.now
puts t2 - t1