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libtags: try other formats after id3v2 is found

Even though a valid id3v2 tag header may be present, the file format
itself might not be mp3. Instead, retry other format parsers on the
file after seeking right past the id3v2 header. This fixes FLACs that
are ID3v2-tagged (the reasoning behind that is left with no comment).

Also convert "TLEN" into duration while at it.

--- a/sys/src/cmd/audio/libtags/flac.c

+++ b/sys/src/cmd/audio/libtags/flac.c

@@ -25,7 +25,7 @@

 	ctx->duration = g * 1000 / ctx->samplerate;

 	/* skip the rest of the stream info */

-	if(ctx->seek(ctx, sz-18, 1) != 8+sz)

+	if(ctx->seek(ctx, sz-18, 1) != ctx->restart+8+sz)

 		return -1;

 	for(last = 0; !last;){

--- a/sys/src/cmd/audio/libtags/id3v2.c

+++ b/sys/src/cmd/audio/libtags/id3v2.c

@@ -27,6 +27,8 @@

 		txtcb(ctx, Tdate, k-1, v);

 	else if(strcmp(k, "RK") == 0 || strcmp(k, "RCK") == 0)

 		txtcb(ctx, Ttrack, k-1, v);

+	else if(strcmp(k, "LEN") == 0)

+		ctx->duration = atoi(v);

 	else if(strcmp(k, "CO") == 0 || strcmp(k, "CON") == 0){

 		for(; v[0]; v++){

 			if(v[0] == '(' && v[1] <= '9' && v[1] >= '0'){

@@ -387,6 +389,8 @@

 	if(ver == 2 && (d[5] & (1<<6)) != 0) /* compression */

 		return -1;

+

+	ctx->restart = sizeof(d)+sz;

 	if(ver > 2){

 		if((d[5] & (1<<4)) != 0) /* footer */

--- a/sys/src/cmd/audio/libtags/tags.c

+++ b/sys/src/cmd/audio/libtags/tags.c

@@ -65,6 +65,7 @@

 	ctx->channels = ctx->samplerate = ctx->bitrate = ctx->duration = 0;

 	ctx->found = 0;

 	ctx->format = Funknown;

+	ctx->restart = 0;

 	res = -1;

 	for(i = 0; i < nelem(g); i++){

 		ctx->num = 0;

@@ -72,7 +73,7 @@

 			ctx->format = g[i].format;

 			res = 0;

 		}

-		ctx->seek(ctx, 0, 0);

+		ctx->seek(ctx, ctx->restart, 0);

 	}

 	return res;

--- a/sys/src/cmd/audio/libtags/tags.h

+++ b/sys/src/cmd/audio/libtags/tags.h

@@ -81,6 +81,7 @@

 	/* Private, don't touch. */

 	int found;

 	int num;

+	int restart;

 };

 /* Parse the file using this function. Returns 0 on success. */