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music_mad.c: better skipping of tags:
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Instead of doing it 'as-we-go' every time, skip the tags at file-open
time only once.

The former implementation had the chance of not having the full tag in
the frame, that includes a possibility of even not having the complete
tag magic in the frame leading to not identifying a tag.

The new implementation skips all the tags at file-start and file-end,
and does that only once. That requires some local file data-start and
file-end bookkeeping for SDL_RWops which are added.

Also added are detection and skipping of extended ID3v1 and APEv1 tags.
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@sezero
sezero committed Jul 20, 2019
1 parent 7dfe465 commit c9249fe
Showing 1 changed file with 165 additions and 44 deletions.
209 changes: 165 additions & 44 deletions music_mad.c
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Expand Up @@ -138,6 +138,7 @@ enum {
typedef struct {
int play_count;
SDL_RWops *src;
Sint64 start, length, pos;
int freesrc;
struct mad_stream stream;
struct mad_frame frame;
Expand All @@ -151,7 +152,38 @@ typedef struct {
} MAD_Music;


static size_t MAD_RWread(MAD_Music *music, void *ptr, size_t size, size_t maxnum) {
size_t remaining = (size_t)(music->length - music->pos);
size_t ret;
maxnum *= size;
if (maxnum > remaining) maxnum = remaining;
ret = SDL_RWread(music->src, ptr, 1, maxnum);
music->pos += (Sint64)ret;
return ret;
}

static Sint64 MAD_RWseek(MAD_Music *music, Sint64 offset, int whence) {
Sint64 ret;
switch (whence) { /* assumes a legal whence value */
case RW_SEEK_CUR:
offset += music->pos;
break;
case RW_SEEK_END:
offset = music->length + offset;
break;
}
if (offset < 0) return -1;
if (offset > music->length)
offset = music->length;
ret = SDL_RWseek(music->src, music->start + offset, RW_SEEK_SET);
if (ret < 0) return ret;
music->pos = offset;
return (music->pos - music->start);
}


static int MAD_Seek(void *context, double position);
static int skip_tags(MAD_Music *music);

static void *MAD_CreateFromRW(SDL_RWops *src, int freesrc)
{
Expand All @@ -165,6 +197,13 @@ static void *MAD_CreateFromRW(SDL_RWops *src, int freesrc)
music->src = src;
music->volume = MIX_MAX_VOLUME;

music->length = SDL_RWsize(src);
if (skip_tags(music) < 0) {
SDL_free(music);
Mix_SetError("music_mad: corrupt mp3 file.");
return NULL;
}

mad_stream_init(&music->stream);
mad_frame_init(&music->frame);
mad_synth_init(&music->synth);
Expand All @@ -191,15 +230,26 @@ static int MAD_Play(void *context, int play_count)

/*************************** TAG HANDLING: ******************************/

static SDL_INLINE SDL_bool is_id3v1(const Uint8 *data, size_t length)
static SDL_INLINE SDL_bool is_id3v1(const unsigned char *data, size_t length)
{
/* http://id3.org/ID3v1 : 3 bytes "TAG" identifier and 125 bytes tag data */
if (length < 3 || SDL_memcmp(data,"TAG",3) != 0) {
return SDL_FALSE;
}
return SDL_TRUE;
}
static SDL_INLINE SDL_bool is_id3v2(const Uint8 *data, size_t length)
static SDL_INLINE SDL_bool is_id3v1ext(const unsigned char *data, size_t length)
{
/* ID3v1 extended tag: just before ID3v1, always 227 bytes.
* https://www.getid3.org/phpBB3/viewtopic.php?t=1202
* https://en.wikipedia.org/wiki/ID3v1#Enhanced_tag
* Not an official standard, is only supported by few programs. */
if (length < 4 || SDL_memcmp(data,"TAG+",4) != 0) {
return SDL_FALSE;
}
return SDL_TRUE;
}
static SDL_INLINE SDL_bool is_id3v2(const unsigned char *data, size_t length)
{
/* ID3v2 header is 10 bytes: http://id3.org/id3v2.4.0-structure */
/* bytes 0-2: "ID3" identifier */
Expand All @@ -218,19 +268,35 @@ static SDL_INLINE SDL_bool is_id3v2(const Uint8 *data, size_t length)
}
return SDL_TRUE;
}
static SDL_INLINE SDL_bool is_apetag(const Uint8 *data, size_t length)
static SDL_INLINE long get_id3v2_len(const unsigned char *data, long length)
{
/* size is a 'synchsafe' integer (see above) */
long size = (long)((data[6]<<21) + (data[7]<<14) + (data[8]<<7) + data[9]);
size += 10; /* header size */
/* ID3v2 header[5] is flags (bits 4-7 only, 0-3 are zero).
* bit 4 set: footer is present (a copy of the header but
* with "3DI" as ident.) */
if (data[5] & 0x10) {
size += 10; /* footer size */
}
/* optional padding (always zeroes) */
while (size < length && data[size] == 0) {
++size;
}
return size;
}
static SDL_INLINE SDL_bool is_apetag(const unsigned char *data, size_t length)
{
/* http://wiki.hydrogenaud.io/index.php?title=APEv2_specification
* APEv2 header is 32 bytes: bytes 0-7 ident, bytes 8-11 version,
* bytes 12-17 size. bytes 24-31 are reserved: must be all zeroes.
* APEv1 has no header, so no luck. */
* Header/footer is 32 bytes: bytes 0-7 ident, bytes 8-11 version,
* bytes 12-17 size. bytes 24-31 are reserved: must be all zeroes. */
Uint32 v;

if (length < 32 || SDL_memcmp(data,"APETAGEX",8) != 0) {
return SDL_FALSE;
}
v = (data[11]<<24) | (data[10]<<16) | (data[9]<<8) | data[8]; /* version */
if (v != 2000U) {
if (v != 2000U && v != 1000U) {
return SDL_FALSE;
}
v = 0; /* reserved bits : */
Expand All @@ -239,48 +305,104 @@ static SDL_INLINE SDL_bool is_apetag(const Uint8 *data, size_t length)
}
return SDL_TRUE;
}

static size_t get_tagsize(const Uint8 *data, size_t length)
static SDL_INLINE long get_ape_len(const unsigned char *data, long datalen, Uint32 *version)
{
size_t size;
long size = (long)((data[15]<<24) | (data[14]<<16) | (data[13]<<8) | data[12]);
*version = (data[11]<<24) | (data[10]<<16) | (data[9]<<8) | data[8];
return size; /* caller will handle the additional v2 header length */
}

if (is_id3v1(data, length)) {
return 128; /* fixed length */
static int skip_tags(MAD_Music *music)
{
long len; size_t readsize;

readsize = MAD_RWread(music, music->input_buffer, 1, MAD_INPUT_BUFFER_SIZE);
if (!readsize) return -1;

/* ID3v2 tag is at the start */
if (is_id3v2(music->input_buffer, readsize)) {
len = get_id3v2_len(music->input_buffer, (long)readsize);
if (len >= music->length) return -1;
music->start += len;
music->length -= len;
MAD_RWseek(music, 0, RW_SEEK_SET);
}
/* APE tag _might_ be at the start: read the header */
else if (is_apetag(music->input_buffer, readsize)) {
Uint32 v;
len = get_ape_len(music->input_buffer, (long)readsize, &v);
len += 32; /* we're at top: have a header. */
if (len >= music->length) return -1;
music->start += len;
music->length -= len;
MAD_RWseek(music, 0, RW_SEEK_SET);
}
if (is_id3v2(data, length)) {
/* size is a 'synchsafe' integer (see above) */
size = (data[6]<<21) + (data[7]<<14) + (data[8]<<7) + data[9];
size += 10; /* header size */
/* ID3v2 header[5] is flags (bits 4-7 only, 0-3 are zero).
* bit 4 set: footer is present (a copy of the header but
* with "3DI" as ident.) */
if (data[5] & 0x10) {
size += 10; /* footer size */

/* ID3v1 tag is at the end */
if (music->length < 128) goto ape;
MAD_RWseek(music, -128, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 128);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 128) return -1;
if (is_id3v1(music->input_buffer, 128)) {
music->length -= 128;

/* APE tag may be before the ID3v1: read the footer */
if (music->length < 32) goto end;
MAD_RWseek(music, -32, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 32);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 32) return -1;
if (is_apetag(music->input_buffer, 32)) {
Uint32 v;
len = get_ape_len(music->input_buffer, (long)readsize, &v);
if (v == 2000U) len += 32; /* header */
if (len >= music->length) return -1;
if (v == 2000U) { /* verify header : */
MAD_RWseek(music, -len, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 32);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 32) return -1;
if (!is_apetag(music->input_buffer, 32)) return -1;
}
music->length -= len;
goto end;
}
/* optional padding (always zeroes) */
while (size < length && data[size] == 0) {
++size;
/* extended ID3v1 just before the ID3v1 tag? (unlikely) */
if (music->length < 227) goto end;
MAD_RWseek(music, -227, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 227);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 227) return -1;
if (is_id3v1ext(music->input_buffer, 227)) {
music->length -= 227;
goto end;
}
return size;
}
if (is_apetag(data, length)) {
size = (data[15]<<24) | (data[14]<<16) | (data[13]<<8) | data[12];
size += 32; /* header size */
return size;
ape: /* APE tag may be at the end: read the footer */
if (music->length >= 32) {
MAD_RWseek(music, -32, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 32);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 32) return -1;
if (is_apetag(music->input_buffer, 32)) {
Uint32 v;
len = get_ape_len(music->input_buffer, (long)readsize, &v);
if (v == 2000U) len += 32; /* header */
if (len >= music->length) return -1;
if (v == 2000U) { /* verify header : */
MAD_RWseek(music, -len, RW_SEEK_END);
readsize = MAD_RWread(music, music->input_buffer, 1, 32);
MAD_RWseek(music, 0, RW_SEEK_SET);
if (readsize != 32) return -1;
if (!is_apetag(music->input_buffer, 32)) return -1;
}
music->length -= len;
}
}
return 0;
}

static int consume_tag(struct mad_stream *stream)
{
/* FIXME: what if the buffer doesn't have the full tag ??? */
size_t remaining = stream->bufend - stream->next_frame;
size_t tagsize = get_tagsize(stream->this_frame, remaining);
if (tagsize != 0) {
mad_stream_skip(stream, tagsize);
return 0;
}
return -1;
end:
return (music->length > 0)? 0: -1;
}

/* Reads the next frame from the file.
Expand Down Expand Up @@ -310,7 +432,7 @@ static SDL_bool read_next_frame(MAD_Music *music)
}

/* Now read additional bytes from the input file. */
read_size = SDL_RWread(music->src, read_start, 1, read_size);
read_size = MAD_RWread(music, read_start, 1, read_size);

if (read_size == 0) {
if ((music->status & (MS_input_eof | MS_input_error)) == 0) {
Expand All @@ -334,7 +456,6 @@ static SDL_bool read_next_frame(MAD_Music *music)
its buffer. */
if (mad_frame_decode(&music->frame, &music->stream)) {
if (MAD_RECOVERABLE(music->stream.error)) {
consume_tag(&music->stream); /* consume any ID3 tags */
mad_stream_sync(&music->stream); /* to frame seek mode */
return SDL_FALSE;

Expand Down Expand Up @@ -485,7 +606,7 @@ static int MAD_Seek(void *context, double position)
mad_timer_reset(&music->next_frame_start);
music->status &= ~MS_error_flags;

SDL_RWseek(music->src, 0, RW_SEEK_SET);
MAD_RWseek(music, 0, RW_SEEK_SET);
}

/* Now we have to skip frames until we come to the right one.
Expand Down

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