#!/usr/bin/env ruby
#
# youtube-snarf-audio - Extract audio from YouTube videos
#
# Downloads YouTube videos and extracts audio to m4a format.
# Accepts YouTube URLs or just video IDs as arguments.
# Skips audio extraction if output file already exists.
#
# Usage: youtube-snarf-audio <youtube-url> [<youtube-url> ...]
#
# Examples:
#   youtube-snarf-audio https://youtube.com/watch?v=abc123
#   youtube-snarf-audio abc123
#
# Requires: youtube-dl, ffmpeg

require 'cgi'
require 'faraday'
require 'rack'
require 'uri'

def main
  while youtube_url = ARGV.shift
    puts ">>> Snarfing audio from #{youtube_url}..."
    if YouTubeSnarfer.snarf(youtube_url)
      sleep 60
    end
  end
end

class YouTubeSnarfer

  attr_reader :url, :video_filename

  def self.snarf(url)
    new(url).snarf
  end

  def initialize(url)
    @url = url
    unless @url =~ /^http/
      @url = "http://youtube.com/watch?v=#{@url}"
    end
  end

  def snarf
    encode_audio
  end

  def title
    video_filename.sub(/\s*-\w+.mp4\s*$/, '')
  end

  def audio_filename
    "#{title}.m4a"
  end

  def encode_audio
    save_video
    if File.exists?(audio_filename)
      puts ">>> Skipping, #{audio_filename} already exists. Delete it if you want to re-extract the audio."
      return false
    end
    cmd = "ffmpeg -i '#{video_filename}' -vn -acodec copy '#{audio_filename}'"
    IO.popen(cmd) do |io|
      io.each_line do |line|
        puts line
      end
    end
    true
  end

  def save_video
    IO.popen("youtube-dl #{url}") do |io|
      io.each_line do |line|
        puts line

        if line =~ /^\[download\] /
          @video_filename =
            if line =~ /Destination:/
              line.split(': ').last
            else
              line.sub('[download] ', '').sub(' has already been downloaded', '')
            end.sub(/[\s\r\n]*$/, '')
        end
      end
    end
  end

end

main if $0 == __FILE__