An example of Bitcoin's proof of work concept in Ruby

proof_of_work.rb

# https://en.bitcoin.it/wiki/Proof_of_work

# usage: ProofOfWork.new("Hello World!", 3).nonce_and_hash

# In this example think of the "Hello World!" as a a collection of Bitcoin transactions waiting to be verified.
# 3 is the arbitary number that makes it harder to verify the transactions. Over time 3 goes up and up and the 
# difficulty is exponential.

require 'digest'

class ProofOfWork

  attr :input, :accuracy

  def initialize(input, accuracy=1)
    @input = input
    @accuracy = accuracy
  end

  def nonce_and_hash
    hash = ""
    nonce = 0

    until prefix_for(hash) == target_prefix
      hash = Digest::SHA256.hexdigest("#{input}#{nonce}")
      nonce += 1
    end

    [nonce, hash]
  end

  private

  def prefix_for(hash)
    hash[0..accuracy-1]
  end

  def target_prefix
    @_target_prefix ||= Array.new(accuracy, "0").join("")
  end

end



if __FILE__ == $0
  require 'minitest/autorun'
  require 'minitest/unit'
  require 'minitest/benchmark'

  class TestProofOfWork < MiniTest::Unit::TestCase
    def test_hello_world
      nonce, hash = ProofOfWork.new("Hello World!", 2).nonce_and_hash

      assert_equal 30, nonce
      assert_equal "0063c24653b3f0a21d126fc7bb754bb70b0b97782cf989191ebc420a4dc6208f", hash
    end

    # sensible accuracies to attempt benchmark...
    def self.bench_range
      [1, 2, 3, 4, 5]
    end

    def test_benchmark
      assert_performance_exponential 0.7 do |n|
        ProofOfWork.new("Hello World!", n).nonce_and_hash
      end
    end
  end
end