michael@0: /* This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
michael@0:  * You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: /* MPEG format parsing */
michael@0: 
michael@0: #include "mp3sniff.h"
michael@0: 
michael@0: /* Maximum packet size is 320 kbits/s * 144 / 32 kHz + 1 padding byte */
michael@0: #define MP3_MAX_SIZE 1441
michael@0: 
michael@0: typedef struct {
michael@0:   int version;
michael@0:   int layer;
michael@0:   int errp;
michael@0:   int bitrate;
michael@0:   int freq;
michael@0:   int pad;
michael@0:   int priv;
michael@0:   int mode;
michael@0:   int modex;
michael@0:   int copyright;
michael@0:   int original;
michael@0:   int emphasis;
michael@0: } mp3_header;
michael@0: 
michael@0: /* Parse the 4-byte header in p and fill in the header struct. */
michael@0: static void mp3_parse(const uint8_t *p, mp3_header *header)
michael@0: {
michael@0:   const int bitrates[2][16] = {
michael@0:         /* MPEG version 1 layer 3 bitrates. */
michael@0: 	{0,  32000,  40000,  48000,  56000,  64000,  80000,  96000,
michael@0:          112000, 128000, 160000, 192000, 224000, 256000, 320000, 0},
michael@0:         /* MPEG Version 2 and 2.5 layer 3 bitrates */
michael@0:         {0, 8000, 16000, 24000, 32000, 40000, 48000, 56000, 64000,
michael@0:          80000, 96000, 112000, 128000, 144000, 160000, 0} };
michael@0:   const int samplerates[4] = {44100, 48000, 32000, 0};
michael@0: 
michael@0:   header->version = (p[1] & 0x18) >> 3;
michael@0:   header->layer = 4 - ((p[1] & 0x06) >> 1);
michael@0:   header->errp = (p[1] & 0x01);
michael@0: 
michael@0:   header->bitrate = bitrates[(header->version & 1) ? 0 : 1][(p[2] & 0xf0) >> 4];
michael@0:   header->freq = samplerates[(p[2] & 0x0c) >> 2];
michael@0:   if (header->version == 2) header->freq >>= 1;
michael@0:   else if (header->version == 0) header->freq >>= 2;
michael@0:   header->pad = (p[2] & 0x02) >> 1;
michael@0:   header->priv = (p[2] & 0x01);
michael@0: 
michael@0:   header->mode = (p[3] & 0xc0) >> 6;
michael@0:   header->modex = (p[3] & 0x30) >> 4;
michael@0:   header->copyright = (p[3] & 0x08) >> 3;
michael@0:   header->original = (p[3] & 0x04) >> 2;
michael@0:   header->emphasis = (p[3] & 0x03);
michael@0: }
michael@0: 
michael@0: /* calculate the size of an mp3 frame from its header */
michael@0: static int mp3_framesize(mp3_header *header)
michael@0: {
michael@0:   int size;
michael@0:   int scale;
michael@0: 
michael@0:   if ((header->version & 1) == 0) scale = 72;
michael@0:   else scale = 144;
michael@0:   size = header->bitrate * scale / header->freq;
michael@0:   if (header->pad) size += 1;
michael@0: 
michael@0:   return size;
michael@0: }
michael@0: 
michael@0: static int is_mp3(const uint8_t *p, long length) {
michael@0:   /* Do we have enough room to see a 4 byte header? */
michael@0:   if (length < 4) return 0;
michael@0:   /* Do we have a sync pattern? */
michael@0:   if (p[0] == 0xff && (p[1] & 0xe0) == 0xe0) {
michael@0:     /* Do we have any illegal field values? */
michael@0:     if (((p[1] & 0x06) >> 1) == 0) return 0;  /* No layer 4 */
michael@0:     if (((p[2] & 0xf0) >> 4) == 15) return 0; /* Bitrate can't be 1111 */
michael@0:     if (((p[2] & 0x0c) >> 2) == 3) return 0;  /* Samplerate can't be 11 */
michael@0:     /* Looks like a header. */
michael@0:     if ((4 - ((p[1] & 0x06) >> 1)) != 3) return 0; /* Only want level 3 */
michael@0:     return 1;
michael@0:   }
michael@0:   return 0;
michael@0: }
michael@0: 
michael@0: /* Identify an ID3 tag based on its header. */
michael@0: /* http://id3.org/id3v2.4.0-structure */
michael@0: static int is_id3(const uint8_t *p, long length) {
michael@0:   /* Do we have enough room to see the header? */
michael@0:   if (length < 10) return 0;
michael@0:   /* Do we have a sync pattern? */
michael@0:   if (p[0] == 'I' && p[1] == 'D' && p[2] == '3') {
michael@0:     if (p[3] == 0xff || p[4] == 0xff) return 0; /* Illegal version. */
michael@0:     if (p[6] & 0x80 || p[7] & 0x80 ||
michael@0:         p[8] & 0x80) return 0; /* Bad length encoding. */
michael@0:     /* Looks like an id3 header. */
michael@0:     return 1;
michael@0:   }
michael@0:   return 0;
michael@0: }
michael@0: 
michael@0: /* Calculate the size of an id3 tag structure from its header. */
michael@0: static int id3_framesize(const uint8_t *p, long length)
michael@0: {
michael@0:   int size;
michael@0: 
michael@0:   /* Header is 10 bytes. */
michael@0:   if (length < 10) {
michael@0:     return 0;
michael@0:   }
michael@0:   /* Frame is header plus declared size. */
michael@0:   size = 10 + (p[9] | (p[8] << 7) | (p[7] << 14) | (p[6] << 21));
michael@0: 
michael@0:   return size;
michael@0: }
michael@0: 
michael@0: int mp3_sniff(const uint8_t *buf, long length)
michael@0: {
michael@0:   mp3_header header;
michael@0:   const uint8_t *p, *q;
michael@0:   long skip;
michael@0:   long avail;
michael@0: 
michael@0:   p = buf;
michael@0:   q = p;
michael@0:   avail = length;
michael@0:   while (avail >= 4) {
michael@0:     if (is_id3(p, avail)) {
michael@0:       /* Skip over any id3 tags */
michael@0:       skip = id3_framesize(p, avail);
michael@0:       p += skip;
michael@0:       avail -= skip;
michael@0:     } else if (is_mp3(p, avail)) {
michael@0:       mp3_parse(p, &header);
michael@0:       skip = mp3_framesize(&header);
michael@0:       if (skip < 4 || skip + 4 >= avail) {
michael@0:         return 0;
michael@0:       }
michael@0:       p += skip;
michael@0:       avail -= skip;
michael@0:       /* Check for a second header at the expected offset. */
michael@0:       if (is_mp3(p, avail)) {
michael@0:         /* Looks like mp3. */
michael@0:         return 1;
michael@0:       } else {
michael@0:         /* No second header. Not mp3. */
michael@0:         return 0;
michael@0:       }
michael@0:     } else {
michael@0:       /* No id3 tag or mp3 header. Not mp3. */
michael@0:       return 0;
michael@0:     }
michael@0:   }
michael@0: 
michael@0:   return 0;
michael@0: }