blockchain.rb

require 'digest'

class Block
  attr_reader :index, :hash

  def initialize(index:, timestamp: Time.now, data:, previous_hash:)
    @data          = data
    @index         = index
    @timestamp     = timestamp
    @previous_hash = previous_hash

    str = [index, timestamp, data, previous_hash].map(&:to_s).to_s

    @hash = Digest::SHA256.hexdigest(str)
  end
end

b0 = Block.new(index: 0,            data: 'Zero',   previous_hash: '0')
b1 = Block.new(index: b0.index + 1, data: 'First',  previous_hash: b0.hash)
b2 = Block.new(index: b1.index + 1, data: 'Second', previous_hash: b1.hash)

[b0, b1, b2].each do |block| 
  p block
end