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m4a: add replay gain support

binary files a/input/noise.m4a b/input/noise.m4a differ

--- a/m4a.c

+++ b/m4a.c

@@ -10,6 +10,7 @@

 	uint64_t duration;

 	uint x;

 	uint8_t *d;

+	char *k, *v;

 	int sz, type, dtype, i, skip, n;

 	d = (uint8_t*)ctx->buf;

@@ -44,7 +45,8 @@

 			memcmp(d, "minf", 4) == 0 ||

 			memcmp(d, "moov", 4) == 0 ||

 			memcmp(d, "trak", 4) == 0 ||

-			memcmp(d, "stbl", 4) == 0){

+			memcmp(d, "stbl", 4) == 0 ||

+			memcmp(d, "----", 4) == 0){

 			sz = 0;

 			continue;

 		}else if(memcmp(d, "stsd", 4) == 0){

@@ -118,6 +120,35 @@

 					duration = ((uint64_t)beuint(&d[20])<<32 | beuint(&d[24])) / (uint64_t)x;

 			}

 			ctx->duration = duration * 1000;

+			continue;

+		}else if(memcmp(d, "name", 4) == 0){

+			if(sz <= 16 || sz >= ctx->bufsz-1) /* 1 for '\0' */

+				continue;

+			if(ctx->read(ctx, d, sz) != sz)

+				return -1;

+			// FIXME - do first 4 bytes mean anything?

+			k = (char*)d + 4;

+			d[sz] = 0;

+			n = sz + 1;

+			v = (char*)d + n;

+

+			if(ctx->read(ctx, d, 4) != 4) /* size */

+				return -1;

+			sz = beuint(d);

+			if(sz <= 16 || 12 > ctx->bufsz-n || sz-16 >= ctx->bufsz-n-1){

+				sz -= 4;

+				continue;

+			}

+			sz -= 4+4+4+4;

+			if(ctx->read(ctx, v, 12) != 12) /* "data", data type (should be text) and 4 more bytes */

+				return -1;

+			if(memcmp(v, "data", 4) != 0 || beuint(v+4) != 1)

+				continue;

+			if(ctx->read(ctx, v, sz) != sz)

+				return -1;

+			v[sz] = 0;

+			cbvorbiscomment(ctx, k, v);

+			sz = 0;

 			continue;

 		}