gistfile1.rb

require 'mp3info'
require 'yaml'

rule %r{^dst/.*\.mp3$} => [proc { |tn| tn.sub(/^dst\//, 'src/') }] do |t|
  puts "*** copying #{t.source}"

  # Copy
  FileUtils.mkdir_p(File.dirname(t.name))
  FileUtils.copy_file(t.source, t.name)
end

rule %r{^dst/.*\.yaml$} => [proc { |tn| tn.sub(/^dst\//, 'src/').sub(/\.yaml$/, '.mp3') }] do |t|
  puts "*** extracting metadata from #{t.source}"

  # Don’t overwrite already existing metadata
  if File.file?(t.name)
    puts "    ... skipping ..."
    FileUtils.touch(t.name)
    next
  end

  # Extract metadata
  metadata = {}
  Mp3Info.open(t.source, :encoding => 'utf-8') do |mp3|
    metadata['title']  = mp3.tag.title
    metadata['artist'] = mp3.tag.artist
    metadata['album']  = mp3.tag.album
  end

  # Write extracted metadata
  FileUtils.mkdir_p(File.dirname(t.name))
  File.open(t.name, 'w') { |io| io.write(YAML.dump(metadata)) }
end

# Cleanup: remove stray files in dst directory
task :cleanup do
  # For each file in the dst/ dir...
  Dir['dst/**/*.{mp3,yaml}'].each do |dst_filename|
    # Find a matching source file
    matching_src_filename = dst_filename.sub(/^dst\//, 'src/').sub(/\.yaml$/, '.mp3')

    # Remove dst file if there is no matching source file
    if !File.file?(matching_src_filename)
      puts "removing #{dst_filename}"
      FileUtils.rm(dst_filename)

      # Remove empty parent directories
      last_dirname = File.dirname(dst_filename)
      loop do
        # Stop if directory is not empty
        break if Dir.entries(last_dirname).any? { |e| e != '.' && e != '..' }

        # Remove
        puts "removing #{last_dirname}"
        FileUtils.rmdir(last_dirname)

        # Continue
        last_dirname = File.dirname(last_dirname)
        break if last_dirname == '.'
      end
    end
  end
end

# Default: copy MP3s and extra metadata
src_files = Dir['src/**/*.mp3']
dst_files_mp3  = src_files.map { |fn| fn.sub(/^src\//, 'dst/') }
dst_files_yaml = dst_files_mp3.map { |fn| fn.sub(/\.mp3$/, '.yaml') }
task :default => (dst_files_mp3 + dst_files_yaml)