Bitcoin concepts in Ruby

bin.rb

# Taken from: https://blockexplorer.com/api/status?q=getDifficulty
DIFFICULTY = 595921917085.416

digest = Bitcoin::Digest.new
proof = Bitcoin::ProofOfWork.new(digest)

input = "Hello, world!"

nonce = proof.prove(input, DIFFICULTY)  # => some int
hash = digest.hexdigest(input, nonce)   # => some hash string

digest.rb

require "digest"

module Bitcoin
  class Digest
    def initialize(digest: ::Digest::SHA256)
      self.digest = digest
    end

    def hexdigest(string, nonce)
      digest.hexdigest("#{string}#{nonce}")
    end

    private

    attr_accessor :digest
  end
end

proof_of_work.rb

module Bitcoin
  class ProofOfWork
    attr_reader :digest

    def initialize(digest = Digest.new)
      self.digest = digest
    end

    def prove(string, target)
      nonce = 0

      loop do
        value = calculate_value(string, nonce)
        return nonce if valid?(value, target)
        nonce += 1
      end
    end

    private

    attr_writer :digest

    def calculate_value(string, nonce)
      digest.hexdigest(string, nonce).to_i(16)
    end

    def valid?(value, target)
      value <= target
    end
  end
end

The current Bitcoin proof doesn't actually check the number of leading zeros, it checks that the calculated hash is less than or equal to some target 256-bit value.

I believe the nonce is all that's needed to be returned here as well, not the calculated hash value.