Extract the MD5 field from a FLAC file

flac_md5.go

package main

import (
	"encoding/binary"
	"encoding/hex"
	"log"
	"os"
)

const (
	// First 4 bytes is FLAC file header
	flacHeader = "fLaC"
	// Block header is also 4 bytes
	blockHeaderSize = 4
	// Ignore the first bit of a FLAC block header, which is set on the last metadata block.
	// STREAMINFO is the only required block, so the bit may or may not be set
	blockHeaderIdMask = 0x7F
	// STREAMINFO sets remaining 7 bits as 0
	streamInfoId = 0x00
	// STREAMINFO size is 34
	streamInfoBytes = 34
	// Overwrite a byte to 0 in buffer to read as a 4 byte uint32
	blockSizeBytes = 4
	// MD5 hash starts after flac header (4), STREAMINFO header(4),
	// and 18 bytes of other STREAMINFO data
	hashStart = 26
	// MD5 hashes are 128 bits = 16 bytes
	hashBytes = 16
)

func flacMd5(songFile *os.File) string {
	var buffer [hashBytes]byte
	// Read the flac header and first block header
	if _, err := songFile.ReadAt(buffer[:len(flacHeader)+blockHeaderSize], 0); err != nil {
		log.Println("FLAC read error", err)
		return ""
	}
	// check flac header
	header := string(buffer[:len(flacHeader)])
	if flacHeader != header {
		log.Printf("FLAC header is wrong, expected %x, got %x", flacHeader, header)
		return ""
	}
	// check first block id is STREAMINFO
	bufOffset := len(flacHeader)
	if streamInfoId != buffer[bufOffset]&blockHeaderIdMask {
		log.Println("FLAC first block id not STREAMINFO")
		return ""
	}
	// check block size is correct for STREAMINFO
	buffer[bufOffset] = 0
	size := binary.BigEndian.Uint32(buffer[bufOffset : bufOffset+blockSizeBytes])
	if streamInfoBytes != size {
		log.Println("FLAC STREAMINFO block size not 34")
		return ""
	}
	// read MD5
	if _, err := songFile.ReadAt(buffer[:], hashStart); err != nil {
		log.Println("FLAC read error", err)
		return ""
	}
	// ignore if all zero (not set)
	for _, b := range buffer {
		if b != 0 {
			hash := hex.EncodeToString(buffer[:])
			log.Printf("FLAC MD5=%s for file at %q", hash, songFile.Name())
			return hash
		}
	}
	log.Printf("FLAC file %q does not have an MD5 checksum set in its STREAMINFO block\n", songFile.Name())
	return ""
}

func main() {
	args := os.Args
	if len(args) != 2 {
		log.Println("flac file argument missing")
		return
	}
	if f, err := os.Open(args[1]); err == nil {
		defer func() {
			if err := f.Close(); err != nil {
				panic(err)
			}
		}()
		flacMd5(f)
	} else {
		panic(err)
	}
}

Thanks for sharing, Shawn. Just what I needed mate. 👍