Lua WAV file reader (chunk parser)

wav-parser.lua

-- http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/WAVE.html
-- usage example at the end

local WavReader = {}
WavReader.__index = WavReader

function WavReader.new(fn)
  local self = setmetatable({}, WavReader)

  self.file, err = io.open(fn)
  if not self.file then error(err) end

  -- is this a WAV file?
  self.bytesToEOC = 12
  local ckID, cksize, WAVEID = self:read(nil, '<c4 I c4')
  if ckID ~= 'RIFF' or WAVEID ~= "WAVE" then
    error(string.format('%s: not a WAV file (got ckID=%s, WAVEID=%s)', fn, ckID, WAVEID))
  end

  return self
end

function WavReader.nextChunk(self)
  if self.bytesToEOC > 0 then
    self:skip(self.bytesToEOC)
  end

  self.bytesToEOC = 8
  self.chunkID, self.bytesToEOC = self:read(nil, '<c4 I')

  return self.chunkID, self.bytesToEOC
end

function WavReader.skip(self, size)
  assert(size > 0, 'negative or null skip size')
  assert(size <= self.bytesToEOC, 'skip would exceed chunk boundary')

  self.bytesToEOC = self.bytesToEOC - size
  return self.file:seek('cur', size)
end

function WavReader.read(self, size, fmt)
  if not size then
    size = self.bytesToEOC
  end

  assert(size > 0, 'negative or null read size')
  assert(size <= self.bytesToEOC, 'read would exceed chunk boundary')

  self.bytesToEOC = self.bytesToEOC - size

  local data = self.file:read(size)

  if data:len() < size then
    return false -- EOF
  elseif fmt then
    return string.unpack(fmt, data)
  else
    return data
  end
end

function WavReader.close(self)
  self.file:close()
end

-- usage example
function getChanCountAndBitDepth(fn)
  local wav = WavReader.new(fn)

  while wav:nextChunk() do
    if wav.chunkID == "fmt " then
      local fmtData = {wav:read(16, '<H H I I H H')}
      wav:close()
      return fmtData[2], fmtData[6]
    end

  end

  wav:close()
end