An ID3 tag reader implemented in pure Lua. Supports ID3v1 tags and a subset of ID3v2 tags.

id3.lua

--- A simple Love2D module to read ID3 tags from MP3 files. 
-- Supports ID3v1 tags and a (meaningful) subset of ID3v2 tags.
-- @class module
-- @name id3
-- @author Michal Kottman and Love2D compatibility updates by Blake Wyatt
-- @copyright 2011, released under MIT license

local id3 = {}

local function textFrame(name)
	return function (reader, info, frameSize)
		local encoding = reader.readByte()
		info[name] =  reader.readStr(frameSize - 1)
	end
end

-- only decode these ID3v2 frames
local frameDecoders = {
	COMM = function (reader, info, frameSize)
		local encoding = reader.readByte()
		local language = reader.readStr(4)
		info.comment = reader.readStr(frameSize - 5)
	end,
	TALB = textFrame 'album',
	TBPM = textFrame 'bpm',
	TENC = textFrame 'encoder',
	TLEN = textFrame 'length',
	TIT2 = textFrame 'title',
	TPE1 = textFrame 'artist',
	TRCK = textFrame 'track',
	TYER = textFrame 'year',
}

local function unpad(str)
	return (str:gsub('[%s%z]+$', ''))
end

function isbitset(x, p)
	local b = 2 ^ (p - 1)
	return x % (b + b) >= b       
end

--- Read ID3 tags from MP3 file. First tries ID3v2 tags, then ID3v1 and returns those
-- which are found first. Returns the following tags (if they are contained in the file):
-- <ul><li>title</li><li>artist</li><li>album</li><li>year</li><li>comment</li></ul>
-- @name readtags
-- @param file Either string (filename) or a file object opened by io.open()
-- @return Table containing the metadata from ID3 tag, or nil.
function id3.readtags(file)
  file:open("r")
	local position = file:tell()
	
	local function decodeID3v2(reader)
		local info = {}
		local rb = reader.readByte
		local version = reader.readInt(2)
		local flags = rb()
		local size = reader.readInt(4, 128)
		
		if isbitset(flags, 7) then
			local mult = version >= 0x0400 and 128 or 256
			local extendedSize = reader.readInt(4, mult)
			local extendedFlags = reader.readInt(2)
			local padding = reader.readInt(4)
			reader.skip(extendedSize - 10)
		end
		while reader.position() < size + 3 do
			local frameID = reader.readStr(4)
			local frameSize = reader.readInt(4)
			local frameFlags = reader.readInt(2)
			if frameDecoders[frameID] then
				frameDecoders[frameID](reader, info, frameSize)
			else
				reader.skip(frameSize)
			end
		end
		file:seek(position)
		return info
	end
	
	local function decodeID3v1(reader)
		local info = {}
		info.title = reader.readStr(30)
		info.artist = reader.readStr(30)
		info.album = reader.readStr(30)
		info.year = reader.readStr(4)
		info.comment = reader.readStr(28)
		local zero = reader.readByte()
		local track = reader.readByte()
		local genre = reader.readByte()
		if zero == 0 then
			info.track = track
			info.genre = genre
		else
			info.comment = unpad(info.comment .. string.char(zero, track, genre))
		end
		
		file:seek(file:getSize() -128 - 227)
		local hdr = reader.readStr(4)
		if hdr == "TAG+" then
			info.title = unpad(info.title .. reader.readStr(60))
			info.artist = unpad(info.artist .. reader.readStr(60))
			info.album = unpad(info.album .. reader.readStr(60))
			-- some other tags omitted
		end
		
		file:seek(position)
		return info
	end
	
	local function readByte()
		local byte = assert(file:read(1), 'Could not read byte.')
		return string.byte(byte)
	end
	local reader = {
		readStr = function(len)
			local str = assert(file:read(len), 'Could not read '..len..'-byte string.')
			return unpad(str)
		end,
		readByte = readByte,
		readInt = function(size, mult)
			mult = mult or 256
			local n = readByte()
			for i=2, size do
				n = n*mult + readByte()
			end
			return n
		end,
		position = function() return file:tell() end,
		skip = function(offset) file:seek(file:tell()+offset) end
	}
	
	-- try ID3v2
	file:seek(0)
	local header = file:read(3)
	if header == "ID3" then
    		local temp = decodeID3v2(reader)
   		file:close()
		return temp
	end
	
	-- try ID3v1
	file:seek(file:getSize() -128)
	header = file:read(3)
	if header == "TAG" then
		local temp = decodeID3v1(reader)
    		file:close()
		return temp
	end
	
	file:close()
end

return id3

song.mp3