# alternacache.rb

# =Notes:
# It is also possible to make the ChecksumCache class a singleton, as there's
# little reason to have more than one of these per-process and this might
# have important benefits if the backend store is a network daemon somewhere.
#
# Ruby's built-in singleton module also takes care of thread safety issues for
# you, whereas this example uses @cache_obj ||= ChecksumCache.new which is
# thread-UNsafe.

class Chef
  # if you want to cache other things in the future, extract superclass,
  # name it Cache and you're done. bam.
  class Cache
    include Singleton
    include Chef::Mixin::ConvertToClassName

    def initialize
      reset!
    end

    def reset!(backend=nil, backend_opts=nil)
      backend       ||= Chef::Config[:cache_type]
      backend_opts  ||= Chef::Config[:cache_options]
      # 08methodnotmissing can convert CamelCase to snake_case
      begin
        require "moneta/#{convert_to_snake_case(backend, Moneta)}"
      rescue LoadError => e
        Chef::Log.fatal "could not find moneta backend #{backend}"
        raise e
      end

      backend_class = Moneta.const_get(backend)
      @moneta = backend_class.new(backend_opts)
    end
  end

  class ChecksumCache < Cache

    def self.checksum_for_file(*args)
      instance.checksum_for_file(*args)
    end

    def checksum_for_file(file)
      fstat = File.stat(file)
      key = filename_to_keyname(file)
      find_checksum(key, fstat) || generate_checksum(key, fstat)
    end

    private

    def find_checksum(key, fstat)
      cached_value = @moneta.fetch(key)
      if cached_value && file_unchanged?(cached_value, fstat)
        cached_value["checksum"]
      else
        false
      end
    end

    def generate_checksum(file, key, fstat)
      digest = Digest::SHA256.new
      IO.foreach(file) {|line| digest.update(line) }
      checksum = digest.hexdigest
      @moneta.store(key, { "mtime" => fstat.mtime.to_f, "checksum" => checksum }
      checksum
    end

    def file_unchanged?(cached_value, stat)
      cached_value["mtime"] == stat.mtime.to_f
    end

    def filename_to_keyname(filename)
      "chef-file#{filename.gsub(/(#{File::SEPARATOR}|\.)/, '-')}"
    end

  end
end

class Chef
  module Mixin
    module Checksum

      def checksum(file)
        ChecksumCache.checksum_for_file(file)
      end

    end
  end
end