require 'yaml'
require 'fileutils'
require 'digest/md5'
require 'set'

module Rscons
  class Cache
    CACHE_FILE = '.rscons'

    private_class_method :new
    def initialize
      @cache = YAML.load(File.read(CACHE_FILE)) rescue {}
      @lookup_checksums = {}
      @stored_checksums = {}
    end

    def self.open
      @@cache ||= new
    end

    def clear
      FileUtils.rm_f(CACHE_FILE)
      initialize
    end

    def write
      File.open(CACHE_FILE, 'w') do |fh|
        fh.puts(YAML.dump(@cache))
      end
    end

    def up_to_date?(file, deps = nil)
      return false unless File.exists?(file)
      stored_md5 = @cache[:checksums][file] rescue nil
      return false unless stored_md5
      return false unless lookup_checksum(file) == stored_md5
      cached_deps = @cache[:deps][file] || [] rescue []
      return false if deps and Set.new(cached_deps) != Set.new(deps)
      cached_deps.map {|d| up_to_date?(d)}.all?
    end

    def register_build(target, deps)
      @cache[:deps] ||= {}
      @cache[:deps][target] = deps
      store_checksum(target)
      deps.each do |dep|
        store_checksum(dep)
      end
      # it is unfortunate to write the file here since many file writes may
      # occur before the final version, but I don't know where else to do so
      write
    end

    private

    def lookup_checksum(file)
      @lookup_checksums[file] ||= calculate_checksum(file)
    end

    def store_checksum(file)
      cs = calculate_checksum(file)
      if @stored_checksums[file] and @stored_checksums[file] != cs
        $stderr.puts "Warning: file #{file.inspect} changed after previously used by builder"
      end
      @cache[:checksums] ||= {}
      @cache[:checksums][file] = cs
      @lookup_checksums[file] = cs
    end

    def calculate_checksum(file)
      Digest::MD5.hexdigest(File.read(file))
    end
  end
end