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audio/mp3dec: add -s SECONDS option

--- a/sys/man/1/audio

+++ b/sys/man/1/audio

@@ -4,6 +4,9 @@

 .SH SYNOPSIS

 .B audio/mp3dec

 [

+.B -s

+.I seconds

+] [

 .B -d

 ]

 .br

--- a/sys/src/cmd/audio/mp3dec/main.c

+++ b/sys/src/cmd/audio/mp3dec/main.c

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

 /* Current input file */

 vlong offset;

+double seekto = 0.0, curpos = 0.0;

 int debug = 0;

 int ifd = -1;

@@ -30,6 +31,22 @@

 }

 static enum mad_flow

+header(void *, struct mad_header const* header)

+{

+	if(seekto > 0 && (header->duration.seconds > 0 || header->duration.fraction > 0)){

+		double dur = header->duration.seconds + (double)header->duration.fraction / MAD_TIMER_RESOLUTION;

+		seekto -= dur;

+		if(seekto > 0){

+			curpos += dur;

+			return MAD_FLOW_IGNORE;

+		}

+		fprint(2, "time: %g\n", curpos);

+		seekto = 0;

+	}

+	return MAD_FLOW_CONTINUE;

+}

+

+static enum mad_flow

 output(void *, struct mad_header const* header, struct mad_pcm *pcm)

 {

 	static int rate, chans;

@@ -39,6 +56,9 @@

 	int i, j, n;

 	uchar *p;

+	if(seekto > 0)

+		return MAD_FLOW_CONTINUE;

+

 	/* start converter if format changed */

 	if(rate != pcm->samplerate || chans != pcm->channels){

 		int pid, pfd[2];

@@ -145,11 +165,15 @@

 	case 'd':

 		debug++;

 		break;

+	case 's':

+		seekto = atof(EARGF(usage()));

+		if(seekto >= 0.0)

+			break;

 	default:

 		usage();

 	}ARGEND

-	mad_decoder_init(&decoder, nil, input, nil, nil, output, error, nil);

+	mad_decoder_init(&decoder, nil, input, header, nil, output, error, nil);

 	mad_decoder_run(&decoder, MAD_DECODER_MODE_SYNC);

 	mad_decoder_finish(&decoder);