require 'active_support/inflector'
require 'open3'
require 'sprockets'

module Palimpsest

  # Flexible asset pipeline using Sprockets.
  # Paths are loaded into a `Sprockets::Environment` (relative to {#directory} if given).
  # Asset tags are used in source code and replaced
  # with generated asset path or compiled source if `inline` is used.
  #
  # For example, if type is set to `:javascripts` the following replacements would be made:
  #
  #     [% javascript app %] -> app-9413c7f112033f0c6f2a8e8dd313399c18d93878.js
  #     [% javascript lib/jquery %] -> lib/jquery-e2a8cde3f5b3cdb011e38a673556c7a94729e0d1.js
  #     [% javascript inline tracking %] -> <compiled source of tracking.js asset>
  #
  class Assets

    # Default {#options}.
    DEFAULT_OPTIONS = {
      # default path to output all saved assets (relative to directory)
      output: '',

      # assume assets will be served under this url
      # e.g. `https://cdn.example.com/`
      cdn: '',

      # keyword to use in asset tag for inline assets
      inline: 'inline',

      # if true, also generate a gzipped asset
      gzip: false,

      # include hash in asset name
      hash: true,

      # opening and closing brackets for asset source tags
      src_pre: '[%',
      src_post: '%]',

      # allowed options for `Sprockets::Environment`
      sprockets_options: [ :js_compressor, :css_compressor ]
    }

    # @!attribute directory
    #   @return [String] directory which all paths will be relative to if set
    #
    # @!attribute paths
    #   @return [Array] paths to load into sprockets environment
    #
    # @!attribute type
    #   @return [Symbol] type of asset
    attr_accessor :directory, :paths, :type

    def initialize directory: '', options: {}, paths: {}
      self.options options
      self.directory = directory
      self.paths = paths
    end

    # Uses {DEFAULT_OPTIONS} as initial value.
    # @param options [Hash] merged with current options
    # @return [Hash] current options
    def options options={}
      @options ||= DEFAULT_OPTIONS
      @options = @options.merge options
    end

    # @return [Sprockets::Environment] the current sprockets environment
    def sprockets
      @sprockets ||= Sprockets::Environment.new
    end

    # Load options into the sprockets environment.
    # Values are loaded from {#options}.
    def load_options
      options[:sprockets_options].each do |opt|
        sprockets.send "#{opt}=".to_sym, options[opt] if options[opt]
      end
      return self
    end

    # Load paths into the sprockets environment.
    # Values are loaded from {#paths}.
    def load_paths
      paths.each do |path|
        sprockets.append_path "#{directory + '/' unless directory.empty?}#{path}"
      end
      return self
    end

    # @return [Sprockets::Environment] sprockets environment with {#options} and {#paths} loaded
    def assets
      unless @loaded
        load_options
        load_paths
      end
      @loaded = true
      sprockets
    end

    # Write a target asset to file with a hashed name.
    # @param target [String] logical path to asset
    # @param gzip [Boolean] if the asset should be gzipped
    # @param hash [Boolean] if the asset name should include the hash
    # @return [String, nil] the relative path to the written asset or `nil` if no such asset
    def write target, gzip: options[:gzip], hash: options[:hash]
      asset = assets[target]

      return if asset.nil?

      name = hash ? asset.digest_path : asset.logical_path.to_s
      name = "#{options[:output]}/#{name}" unless options[:output].empty?

      path = directory.empty? ? '' : "#{directory}/"
      path << name

      asset.write_to "#{path}.gz", compress: true if gzip
      asset.write_to path
      name
    end

    # (see #update_source)
    # @note this modifies the `source` `String` in place
    def update_source! source
      # e.g. /\[%\s+javascript\s+((\S+)\s?(\S+))\s+%\]/
      regex = /#{Regexp.escape options[:src_pre]}\s+#{type.to_s.singularize}\s+((\S+)\s?(\S+))\s+#{Regexp.escape options[:src_post]}/
      source.gsub! regex do
        if $2 == options[:inline]
          assets[$3].to_s
        else
          asset = write $1

          # @todo raise warning or error if asset not found
          p "asset not found: #{$1}" and next if asset.nil?

          options[:cdn].empty? ? asset : options[:cdn] + asset
        end
      end
      return true
    end

    # Replaces all asset tags in source string with asset path or asset source.
    # Writes any referenced assets to disk.
    # @param source [String] code to find and replace asset tags
    # @return [String] copy of `source` with asset tags replaced
    def update_source source
      s = source
      update_source! s
      s
    end

    # Scans all non-binary files under `path` ({#directory} by default) for asset tags.
    # Uses current asset {#type} (if set) and {#options}.
    # @param path [String] where to look for source files
    # @return [Array] files with asset tags
    def find_tags path: directory
      self.class.find_tags path, type, options
    end

    # Scans all non-binary files under `path` for asset tags.
    # @param path [String] where to look for source files
    # @param type [String, nil] only look for asset tags with this type (or any type if `nil`)
    # @param options [Hash] merged with {DEFAULT_OPTIONS}
    # (see #find_tags)
    def self.find_tags path, type=nil, options={}
      raise ArgumentError, 'path cannot be empty' if path.empty?

      options = DEFAULT_OPTIONS.merge options
      pre = Regexp.escape options[:src_pre]
      post= Regexp.escape options[:src_post]

      cmd = [ 'grep' ]
      cmd.concat [ '-l', '-I', '-r', '-E' ]
      cmd << \
        if type.nil?
          pre + '(.*?)' + post
        else
          pre + '\s+' + type.to_s + '\s+(.*?)' + post
        end
      cmd << path

      files = []
      Open3.capture2(*cmd).first.each_line { |l| files << l.chomp unless l.empty? }
      files
    end
  end
end