# Liquidsoap

## audio stream generation

### Introduction

You can use external programs in liquidsoap to decode audio files. The program must be able to output WAV data to its standard output (`stdout`) and, posssibly, read encoded data from its standard input.

Please note that this feature is not available under Windows.

### Basic operators

External decoders are registered using the `add_decoder` and `add_oblivious_decoder` operators. They are invoked the following way:

#### add_decoder

    add_decoder(name="my_decoder",description="My custom decoder",
                test,decoder)@, where:

`add_decoder` is used for external decoders that can read the encoded data from their standard input (stdin) and write the decoded data as WAV to their standard output (stdout). This operator is recommended because its estimation of the remaining time is better than the estimation done by the decoders registered using `add_oblivious_decoder`. The important parameters are:

- `test` is a function used to determine if the file should be decoded by the decoder. Returned values are:
  - `0`: no decodable audio,
  - `-1`: decodable audio but number of audio channels unknown,
  - `x`: fixed number of decodable audio channels.
- `decoder` is the string containing the shell command to run to execute the decoding process.

#### add_oblivious_decoder

`add_oblivious_decoder` is very similar to `add_decoder`. The main difference is that the decoding program reads encoded data directly from the local files and not its standard input. Decoders registered using this operator do not have a reliable estimation of the remaining time. You should use `add_oblivious_decoder` only if your decoding program is not able to read the encoded data from its standard input.

    add_oblivious_decoder(name="my_decoder",description="My custom decoder",
                          buffer=5., test,decoder)@, where:

`add_decoder` is used for external decoders that can read the encoded data from their standard input (stdin) and write the decoded data as WAV to their standard output (stdout). This operator is recommended because its estimation of the remaining time is better than the estimation done by the decoders registered using `add_oblivious_decoder`. The important parameters are:

- `test` is a function used to determine if the file should be decoded by the decoder. Returned values are:
  - `0`: no decodable audio,
  - `-1`: decodable audio but number of audio channels unknown,
  - `x`: fixed number of decodable audio channels.
- `decoder` is a function that receives the name of the file that should be decoded and returns a string containing the shell command to run to execute the decoding process.

#### add_metadata_resolver

You may also register new metadata resolvers using the `add_metadata_resolver` operator. It is invoked the following way: `add_metadata_resolver(format,resolver)`, where:

- `format` is the name of the resolved format. It is only informative.
- `resolver` is a function `f` that returns a list of metadata of the form: `(label, value)`. It is invoked the following way: `f(format=name,file)`, where:
  - `format` contains the name of the format, as returned by the decoder that accepted to decode the file. `f` may return immediately if this is not an expected value.
  - `file` is the name of the file to decode.

### Wrappers

On top of the basic operators, wrappers have been written for some common decoders. This includes the `flac` and `faad` decoders, by default. All the operators are defined in `externals.liq`.

#### The FLAC decoder

The flac decoder uses the `flac` command line. It is enabled if the binary can be found in the current `$PATH`.

Its code is the following:

      def test_flac(file) =
        if test_process("which metaflac") then
          channels = list.hd(get_process_lines("metaflac \
                                                --show-channels #{quote(file)} \
                                                2>/dev/null"))
          # If the value is not an int, this returns 0 and we are ok :)
          int_of_string(channels)
        else
          # Try to detect using mime test..
          mime = get_mime(file)
          if string.match(pattern="flac",file) then
            # We do not know the number of audio channels
            # so setting to -1
            (-1)
          else
            # All tests failed: no audio decodable using flac..
            0
          end
        end
      end
      add_decoder(name="FLAC",description="Decode files using the flac \
                  decoder binary.", test=test_flac,flac_p)

Additionaly, a metadata resolver is registered when the `metaflac` command can be found in the `$PATH`:

    if test_process("which metaflac") then
      log(level=3,"Found metaflac binary: \
                   enabling flac external metadata resolver.")
      def flac_meta(file)
        ret = get_process_lines("metaflac --export-tags-to=- \
                                #{quote(file)} 2>/dev/null")
        ret = list.map(string.split(separator="="),ret)
        # Could be made better..
        def f(l',l)=
          if list.length(l) >= 2 then
            list.append([(list.hd(l),list.nth(l,1))],l')
          else
            if list.length(l) >= 1 then
              list.append([(list.hd(l),"")],l')
            else
              l'
            end
          end
        end
      list.fold(f,[],ret)
      end
      add_metadata_resolver("FLAC",flac_meta)
    end

#### The faad decoder

The faad decoder uses the `faad` program, if found in the `$PATH`. It can decode AAC and AAC+ audio files. This program does not support reading encoded data from its standard input so the decoder is registered using `add_oblivious_decoder`.

Its code is the following:

      aac_mimes = ["audio/aac", "audio/aacp", "audio/3gpp", "audio/3gpp2", "audio/mp4",
                   "audio/MP4A-LATM", "audio/mpeg4-generic", "audio/x-hx-aac-adts"]
      aac_filexts = ["m4a", "m4b", "m4p", "m4v",
                     "m4r", "3gp", "mp4", "aac"]

      # Faad is not very selective so
      # We are checking only file that
      # end with a known extension or mime type
      def faad_test(file) =
        # Get the file's mime
        mime = get_mime(file)
        # Test mime
        if list.mem(mime,aac_mimes) then
          true
        else
          # Otherwise test file extension
          ret = string.extract(pattern='\.(.+)$',file)
            if list.length(ret) != 0 then
              ext = ret["1"]
              list.mem(ext,aac_filexts)
            else
              false
            end
        end
      end

      if test_process("which faad") then
        log(level=3,"Found faad binary: enabling external faad decoder and \
                     metadata resolver.")
        faad_p = (fun (f) -> "faad -w #{quote(f)} 2>/dev/null")
        def test_faad(file) =
          if faad_test(file) then
            channels = list.hd(get_process_lines("faad -i #{quote(file)} 2>&1 | \
                                                  grep 'ch,'"))
            ret = string.extract(pattern=", (\d) ch,",channels)
            ret =
              if list.length(ret) == 0 then
              # If we pass the faad_test, chances are
              # high that the file will contain aac audio data..
                "-1"
              else
                ret["1"]
              end
            int_of_string(default=(-1),ret)
          else
            0
          end
        end
        add_oblivious_decoder(name="FAAD",description="Decode files using \
                              the faad binary.", test=test_faad, faad_p)
        def faad_meta(file) =
          if faad_test(file) then
            ret = get_process_lines("faad -i \
                         #{quote(file)} 2>&1")
            # Yea, this is tuff programming (again) !
            def get_meta(l,s)=
              ret = string.extract(pattern="^(\w+):\s(.+)$",s)
              if list.length(ret) > 0 then
                list.append([(ret["1"],ret["2"])],l)
              else
                l
              end
            end
            list.fold(get_meta,[],ret)
          else
            []
          end
        end
        add_metadata_resolver("FAAD",faad_meta)
      end