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

module Rscons
  # The Cache class keeps track of file checksums, build target commands and
  # dependencies in a YAML file which persists from one invocation to the next.
  # Example cache:
  #   {
  #     version: '1.2.3',
  #     targets: {
  #       'program' => {
  #         'checksum' => 'A1B2C3D4',
  #         'command' => ['gcc', '-o', 'program', 'program.o'],
  #         'deps' => [
  #           {
  #             'fname' => 'program.o',
  #             'checksum' => '87654321',
  #           }
  #         ],
  #       }
  #       'program.o' => {
  #         'checksum' => '87654321',
  #         'command' => ['gcc', '-c', '-o', 'program.o', 'program.c'],
  #         'deps' => [
  #           {
  #             'fname' => 'program.c',
  #             'checksum' => '456789ABC',
  #           },
  #           {
  #             'fname' => 'program.h',
  #             'checksum' => '7979764643',
  #           }
  #         ]
  #       }
  #     }
  #   }
  class Cache
    #### Constants

    # Name of the file to store cache information in
    CACHE_FILE = '.rsconscache'

    #### Class Methods

    # Remove the cache file
    def self.clear
      FileUtils.rm_f(CACHE_FILE)
    end

    #### Instance Methods

    # Create a Cache object and load in the previous contents from the cache
    # file.
    def initialize
      @cache = YAML.load(File.read(CACHE_FILE)) rescue {
        targets: {},
        version: VERSION,
      }
      @lookup_checksums = {}
    end

    # Write the cache to disk to be loaded next time.
    def write
      File.open(CACHE_FILE, 'w') do |fh|
        fh.puts(YAML.dump(@cache))
      end
    end

    # Check if a target is up to date
    # @param target [String] The name of the target file.
    # @param command [Array] The command used to build the target.
    # @param deps [Array] List of the target's dependency files.
    # @param options [Hash] Optional options. Can contain the following keys:
    #   :strict_deps::
    #     Only consider a target up to date if its list of dependencies is
    #     exactly equal (including order) to the cached list of dependencies
    # @return true value if the target is up to date, meaning that:
    #   - the target exists on disk
    #   - the cache has information for the target
    #   - the command used to build the target is the same as last time
    #   - all dependencies listed are also listed in the cache, or, if
    #     :strict_deps was given in options, the list of dependencies is
    #     exactly equal to those cached
    #   - each cached dependency file's current checksum matches the checksum
    #     stored in the cache file
    def up_to_date?(target, command, deps, options = {})
      # target file must exist on disk
      return false unless File.exists?(target)

      # target must be registered in the cache
      return false unless @cache[:targets].has_key?(target)

      # command used to build target must be identical
      return false unless @cache[:targets][target][:command] == command

      cached_deps = @cache[:targets][target][:deps].map { |dc| dc[:fname] }
      if options[:strict_deps]
        # depedencies passed in must exactly equal those in the cache
        return false unless deps == cached_deps
      else
        # all dependencies passed in must exist in cache (but cache may have more)
        return false unless (Set.new(deps) - Set.new(cached_deps)).empty?
      end

      # all cached dependencies must have their checksums match
      @cache[:targets][target][:deps].map do |dep_cache|
        dep_cache[:checksum] == lookup_checksum(dep_cache[:fname])
      end.all?
    end

    # Store cache information about a target built by a builder
    # @param target [String] The name of the target.
    # @param command [Array] The command used to build the target.
    # @param deps [Array] List of dependencies for the target.
    def register_build(target, command, deps)
      @cache[:targets][target] = {
        command: command,
        checksum: calculate_checksum(target),
        deps: deps.map do |dep|
          {
            fname: dep,
            checksum: lookup_checksum(dep),
          }
        end
      }
    end

    # Private Instance Methods
    private

    # Return a file's checksum, or the previously calculated checksum for
    # the same file
    # @param file [String] The file name.
    def lookup_checksum(file)
      @lookup_checksums[file] || calculate_checksum(file)
    end

    # Calculate and return a file's checksum
    # @param file [String] The file name.
    def calculate_checksum(file)
      @lookup_checksums[file] = Digest::MD5.hexdigest(File.read(file)).encode(__ENCODING__) rescue ''
    end
  end
end