A (probably incorrect) proof of work system based on my understanding of Hashcash. For instructional purposes only!

mini-proof.rb

require 'digest/sha1'

class Proof < Struct.new :data, :difficulty, :token
  def valid?
    hash = Digest::SHA1.hexdigest("#{data} #{difficulty} #{token}")
    pl = prefix_length(hash.to_i(16))
    difficulty <= pl
  end

  def increment
    Proof.new data, difficulty, token + 1
  end

  private

  def prefix_length number
    b = 0
    while (number & 1) > 0
      number = number >> 1
      b += 1
    end

    b
  end
end

def work data, difficulty
  proof = Proof.new data, difficulty, 0
  until proof.valid?
    proof = proof.increment
  end

  proof
end

p work "test", 21