Class: Shazamio::DecodedMessage

Inherits:
Object
  • Object
show all
Defined in:
lib/shazamio/signature.rb

Overview

Shazam's raw binary "signature" format: a 48-byte header (magic numbers, CRC32, sample count) followed by a TLV list of frequency-band peaks. Ported field-for-field from signature.py's RawSignatureHeader / DecodedMessage, replacing ctypes.Structure with Array#pack/#unpack.

Constant Summary collapse

HEADER_FORMAT =

12 x uint32 little-endian = 48 bytes

"V12"
MAGIC1 = 
0xCAFE2580
MAGIC2 = 
0x94119C00

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ DecodedMessage

Returns a new instance of DecodedMessage.



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# File 'lib/shazamio/signature.rb', line 47

def initialize
  @frequency_band_to_sound_peaks = {}
end

Instance Attribute Details

#frequency_band_to_sound_peaks ⇒ Object

Returns the value of attribute frequency_band_to_sound_peaks.



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# File 'lib/shazamio/signature.rb', line 45

def frequency_band_to_sound_peaks
  @frequency_band_to_sound_peaks
end

#number_samples ⇒ Object

Returns the value of attribute number_samples.



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# File 'lib/shazamio/signature.rb', line 45

def number_samples
  @number_samples
end

#sample_rate_hz ⇒ Object

Returns the value of attribute sample_rate_hz.



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# File 'lib/shazamio/signature.rb', line 45

def sample_rate_hz
  @sample_rate_hz
end

Class Method Details

.decode_from_binary(data) ⇒ Object 



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# File 'lib/shazamio/signature.rb', line 51

def self.decode_from_binary(data)
  msg = new
  buf = StringIO.new(data)
  buf.binmode

  buf.seek(8)
  check_summable_data = buf.read
  buf.seek(0)

  header_bytes = buf.read(48)
  fields = header_bytes.unpack(HEADER_FORMAT)
  magic1, crc32, size_minus_header, magic2, _v1a, _v1b, _v1c,
    shifted_sample_rate_id, _v2a, _v2b, number_samples_plus_divided_sample_rate, _fixed_value = fields

  raise "bad magic1" unless magic1 == MAGIC1
  raise "bad size" unless size_minus_header == data.bytesize - 48
  raise "bad crc32" unless (Zlib.crc32(check_summable_data) & 0xFFFFFFFF) == crc32
  raise "bad magic2" unless magic2 == MAGIC2

  msg.sample_rate_hz = SampleRate.hz_for(shifted_sample_rate_id >> 27)
  msg.number_samples = (number_samples_plus_divided_sample_rate - msg.sample_rate_hz * 0.24).to_i

  raise "bad tlv preamble" unless buf.read(4).unpack1("V") == 0x40000000
  raise "bad tlv size" unless buf.read(4).unpack1("V") == data.bytesize - 48

  msg.frequency_band_to_sound_peaks = {}

  until buf.eof?
    tlv_header = buf.read(8)
    break if tlv_header.nil? || tlv_header.empty?

    frequency_band_id, frequency_peaks_size = tlv_header.unpack("VV")
    padding = (-frequency_peaks_size) % 4

    peaks_buf = StringIO.new(buf.read(frequency_peaks_size))
    buf.read(padding)

    band = frequency_band_id - 0x60030040
    fft_pass_number = 0
    msg.frequency_band_to_sound_peaks[band] = []

    until peaks_buf.eof?
      raw_offset = peaks_buf.read(1)
      break if raw_offset.nil? || raw_offset.empty?

      offset = raw_offset.unpack1("C")
      if offset == 0xFF
        fft_pass_number = peaks_buf.read(4).unpack1("V")
        next
      else
        fft_pass_number += offset
      end

      peak_magnitude = peaks_buf.read(2).unpack1("v")
      corrected_peak_frequency_bin = peaks_buf.read(2).unpack1("v")

      msg.frequency_band_to_sound_peaks[band] << FrequencyPeak.new(
        fft_pass_number, peak_magnitude, corrected_peak_frequency_bin, msg.sample_rate_hz
      )
    end
  end

  msg
end

Instance Method Details

#encode_to_binary ⇒ Object 



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# File 'lib/shazamio/signature.rb', line 116

def encode_to_binary
  contents_buf = StringIO.new
  contents_buf.binmode

  frequency_band_to_sound_peaks.sort.each do |frequency_band, frequency_peaks|
    peaks_buf = StringIO.new
    peaks_buf.binmode
    fft_pass_number = 0

    frequency_peaks.each do |peak|
      raise "peaks must be sorted by fft_pass_number" if peak.fft_pass_number < fft_pass_number

      if peak.fft_pass_number - fft_pass_number >= 255
        peaks_buf.write([0xFF].pack("C"))
        peaks_buf.write([peak.fft_pass_number].pack("V"))
        fft_pass_number = peak.fft_pass_number
      end

      peaks_buf.write([peak.fft_pass_number - fft_pass_number].pack("C"))
      peaks_buf.write([peak.peak_magnitude].pack("v"))
      peaks_buf.write([peak.corrected_peak_frequency_bin].pack("v"))

      fft_pass_number = peak.fft_pass_number
    end

    peaks_bytes = peaks_buf.string
    contents_buf.write([0x60030040 + frequency_band].pack("V"))
    contents_buf.write([peaks_bytes.bytesize].pack("V"))
    contents_buf.write(peaks_bytes)
    contents_buf.write("\x00" * ((-peaks_bytes.bytesize) % 4))
  end

  contents = contents_buf.string
  size_minus_header = contents.bytesize + 8
  shifted_sample_rate_id = SampleRate.value_for_hz(sample_rate_hz) << 27
  fixed_value = (15 << 19) + 0x40000
  number_samples_plus_divided_sample_rate = (number_samples + sample_rate_hz * 0.24).to_i

  # First pass: header with crc32 = 0, to compute the real crc32 over it.
  header_fields = [
    MAGIC1, 0, size_minus_header, MAGIC2,
    0, 0, 0,
    shifted_sample_rate_id,
    0, 0,
    number_samples_plus_divided_sample_rate,
    fixed_value
  ]

  body = StringIO.new
  body.binmode
  body.write([0x40000000].pack("V"))
  body.write([contents.bytesize + 8].pack("V"))
  body.write(contents)

  crc = Zlib.crc32(header_fields.pack(HEADER_FORMAT)[8..] + body.string) & 0xFFFFFFFF
  header_fields[1] = crc

  header_fields.pack(HEADER_FORMAT) + body.string
end

#encode_to_uri ⇒ Object 



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# File 'lib/shazamio/signature.rb', line 176

def encode_to_uri
  DATA_URI_PREFIX + Base64.strict_encode64(encode_to_binary)
end