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

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