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games/midi: fix desyncs after tempo changes in multitrack files

same as 89c60481afd8307f7825281bd561995f83a7bfb2.

--- a/sys/src/games/midi.c

+++ b/sys/src/games/midi.c

@@ -6,7 +6,8 @@

 struct Tracker {

 	uchar *data;

 	char ended;

-	uvlong t;

+	vlong t;

+	vlong Δ;

 	uchar notes[16][128];

 	int cmd;

 } *tr;

@@ -13,8 +14,8 @@

 typedef struct Tracker Tracker;

+int debug;

 int fd, ofd, div, tempo = 500000, ntrack;

-uvlong T;

 int freq[128];

 uchar out[8192], *outp = out;

@@ -30,6 +31,20 @@

 	return v;

 }

+void

+dprint(char *fmt, ...)

+{

+	char s[256];

+	va_list arg;

+

+	if(!debug)

+		return;

+	va_start(arg, fmt);

+	vseprint(s, s+sizeof s, fmt, arg);

+	va_end(arg);

+	fprint(2, "%s", s);

+}

+

 int

 get8(Tracker *src)

 {

@@ -40,6 +55,7 @@

 			sysfatal("unexpected eof");

 		return c;

 	}

+	dprint("%#p:%02ux", src, *src->data);

 	return *src->data++;

 }

@@ -64,7 +80,7 @@

 getvar(Tracker *src)

 {

 	int k, x;

-	

+

 	x = get8(src);

 	k = x & 0x7F;

 	while(x & 0x80){

@@ -95,10 +111,10 @@

 	while(--x);

 }

-uvlong

+double

 tconv(int n)

 {

-	uvlong v;

+	double v;

 	v = n;

 	v *= tempo;

@@ -109,14 +125,20 @@

 }

 void

-run(uvlong n)

+run(double n)

 {

-	int samp, j, k, no[128];

+	int j, k, no[128];

 	int t, f;

 	short u;

+	uvlong samp;

+	double Δ;

 	Tracker *x;

-	

-	samp = n - T;

+	static double T, ε;

+	static uvlong τ;

+

+	Δ = tconv(n) + ε;

+	samp = Δ;

+	ε = Δ - samp;

 	if(samp <= 0)

 		return;

 	memset(no, 0, sizeof no);

@@ -130,7 +152,7 @@

 	while(samp--){

 		t = 0;

 		for(k = 0; k < 128; k++){

-			f = (T % freq[k]) >= freq[k]/2 ? 1 : 0;

+			f = (τ % freq[k]) >= freq[k]/2 ? 1 : 0;

 			t += f * no[k];

 		}

 		u = t*10;

@@ -141,7 +163,7 @@

 			write(ofd, out, sizeof out);

 			outp = out;

 		}

-		T++;

+		τ++;

 	}

 }

@@ -148,11 +170,9 @@

 void

 readevent(Tracker *src)

 {

-	uvlong l;

 	int n,t;

-	

-	l = tconv(getvar(src));

-	run(src->t += l);

+

+	dprint(" [%zd] ", src - tr);

 	t = get8(src);

 	if((t & 0x80) == 0){

 		src->data--;

@@ -161,6 +181,7 @@

 			sysfatal("invalid midi");

 	}else

 		src->cmd = t;

+	dprint("(%02ux) ", t >> 4);

 	switch(t >> 4){

 	case 0x8:

 		n = get8(src);

@@ -171,12 +192,12 @@

 		n = get8(src);

 		src->notes[t & 15][n] = get8(src);

 		break;

-	case 0xB:

-		get16(src);

-		break;

+	case 0xA:

+	case 0xD:

 	case 0xC:

 		get8(src);

 		break;

+	case 0xB:

 	case 0xE:

 		get16(src);

 		break;

@@ -201,7 +222,7 @@

 			skip(src, n);

 			break;

 		default:

-			fprint(2, "unknown meta event type %.2x\n", t);

+			dprint("unknown meta event type %.2x\n", t);

 		case 3: case 1: case 2: case 0x58: case 0x59: case 0x21:

 			skip(src, n);

 		}

@@ -209,16 +230,20 @@

 	default:

 		sysfatal("unknown event type %x", t>>4);

 	}

+	dprint("\n");

 }

 void

 main(int argc, char **argv)

 {

-	int i, size;

-	uvlong t, mint;

-	Tracker *x, *minx;

+	int i, size, end;

+	uvlong z;

+	Tracker *x;

 	ARGBEGIN{

+	case 'D':

+		debug = 1;

+		break;

 	case 'c':

 		ofd = 1;

 		break;

@@ -235,31 +260,36 @@

 	ntrack = get16(nil);

 	div = get16(nil);

 	tr = emallocz(ntrack * sizeof(*tr));

-	for(i = 0; i < ntrack; i++){

+	for(x=tr, z=-1UL; x<tr+ntrack; x++){

 		if(get32(nil) != 0x4D54726B)

 			sysfatal("invalid track header");

 		size = get32(nil);

-		tr[i].data = emallocz(size);

-		readn(fd, tr[i].data, size);

+		x->data = emallocz(size);

+		readn(fd, x->data, size);

+		x->Δ = getvar(x);	/* prearm */

+		if(x->Δ < z)

+			z = x->Δ;

 	}

+	for(x=tr; x<tr+ntrack; x++)

+		x->Δ -= z;

 	for(i = 0; i < 128; i++)

 		freq[i] = SAMPLE / (440 * pow(1.05946, i - 69));

-	for(;;){

-		minx = nil;

-		mint = 0;

-		for(x = tr; x < tr + ntrack; x++){

+	for(end=0; !end;){

+		end = 1;

+		for(x=tr; x<tr+ntrack; x++){

 			if(x->ended)

 				continue;

-			t = tconv(peekvar(x)) + x->t;

-			if(t < mint || minx == nil){

-				mint = t;

-				minx = x;

+			end = 0;

+			x->Δ--;

+			x->t += tconv(1);

+			while(x->Δ <= 0){

+				readevent(x);

+				if(x->ended)

+					break;

+				x->Δ = getvar(x);

 			}

 		}

-		if(minx == nil){

-			write(ofd, out, outp - out);

-			exits(nil);

-		}

-		readevent(minx);

+		run(1);

 	}

+	exits(nil);

 }