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games/dmid: fix tempo changes causing desyncs in multitrack files

if tempo changes while older events are in flight at different
timings in a multitrack file, and since there's no compensation
for it, tracks desyncronise, the order of events get jumbled up
and everything just falls apart.

rather than doing dances with the old code to figure out what
corrective factor to apply and where, just use the simplest and
most robust way, advance one tic at a time.

also add a tracing mode and don't always print useless and
annoying warnings.

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

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

@@ -77,14 +77,15 @@

 	u8int *s;

 	u8int *p;

 	u8int *e;

-	uvlong t;

+	vlong Δ;

+	vlong t;

 	int ev;

 };

 Trk *tr;

+int trace;

 double freq[128];

 int mfmt, ntrk, div = 1, tempo, opl2, stream;

-uvlong T;

 Biobuf *ib;

 void *

@@ -129,6 +130,20 @@

 		sysfatal("bread: short read");

 }

+void

+dprint(char *fmt, ...)

+{

+	char s[256];

+	va_list arg;

+

+	if(!trace)

+		return;

+	va_start(arg, fmt);

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

+	va_end(arg);

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

+}

+

 u8int

 get8(Trk *x)

 {

@@ -140,7 +155,9 @@

 	}

 	if(x->p >= x->e)

 		sysfatal("track overflow");

-	return *x->p++;

+	v = *x->p++;

+	dprint("%02ux", v);

+	return v;

 }

 u16int

@@ -326,10 +343,10 @@

 		}

 }

-uvlong

-tc(int n)

+double

+tc(double n)

 {

-	return ((uvlong)n * tempo * Rate / div) / 1000000;

+	return (n * tempo * Rate / div) / 1e6;

 }

 void

@@ -356,29 +373,19 @@

 	return v;

 }

-int

-peekvar(Trk *x)

-{

-	int v;

-	uchar *p;

-

-	p = x->p;

-	v = getvar(x);

-	x->p = p;

-	return v;

-}

-

 void

-samp(uvlong t´)

+samp(double n)

 {

-	int dt;

-	static uvlong t;

+	vlong t;

+	double Δ;

+	static double ε;

-	dt = t´ - t;

-	t += dt;

-	while(dt > 0){

-		putcmd(0, 0, dt > 0xffff ? 0xffff : dt);

-		dt -= 0xffff;

+	Δ = tc(n) + ε;

+	t = Δ;

+	ε = Δ - t;

+	while(t > 0){

+		putcmd(0, 0, t > 0xffff ? 0xffff : t);

+		t -= 0xffff;

 	}

 }

@@ -388,7 +395,7 @@

 	int e, n, m;

 	Chan *c;

-	samp(x->t += tc(getvar(x)));

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

 	e = get8(x);

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

 		x->p--;

@@ -398,6 +405,7 @@

 	}else

 		x->ev = e;

 	c = chan + (e & 15);

+	dprint("(%02ux) ", e);

 	n = get8(x);

 	switch(e >> 4){

 	case 0x8: noteoff(c, n, get8(x)); break;

@@ -408,7 +416,7 @@

 		case 0x00: case 0x01: case 0x20: break;

 		case 0x07: c->v = m; resetchan(c); break;

 		case 0x0a: c->pan = m < 32 ? 1<<4 : m > 96 ? 1<<5 : 3<<4; resetchan(c); break;

-		default: fprint(2, "unknown controller %d\n", n);

+		default: dprint("\nunknown controller %d", n);

 		}

 		break;

 	case 0xc: c->i = inst + n; break;

@@ -421,11 +429,11 @@

 		if((e & 0xf) == 0){

 			while(get8(x) != 0xf7)

 				;

-			return;

+			break;

 		}

 		m = get8(x);

 		switch(n){

-		case 0x2f: x->p = x->e; return;

+		case 0x2f: x->p = x->e; break;

 		case 0x51: tempo = get16(x) << 8; tempo |= get8(x); break;

 		default: skip(x, m);

 		}

@@ -434,6 +442,7 @@

 	case 0xd: get8(x); break;

 	default: sysfatal("invalid event %#ux\n", e >> 4);

 	}

+	dprint("\n");

 }

 void

@@ -489,7 +498,7 @@

 void

 readmid(char *file)

 {

-	u32int n;

+	u32int n, z;

 	uchar *s;

 	Trk *x;

@@ -506,7 +515,7 @@

 		sysfatal("unsupported format %d", mfmt);

 	div = get16(nil);

 	tr = emalloc(ntrk * sizeof *tr);

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

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

 		if(get32(nil) != 0x4d54726b)

 			sysfatal("invalid track");

 		n = get32(nil);

@@ -515,7 +524,12 @@

 		x->s = s;

 		x->p = s;

 		x->e = s + n;

+		x->Δ = getvar(x);	/* prearm */

+		if(x->Δ < z)

+			z = x->Δ;

 	}

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

+		x->Δ -= z;

 	Bterm(ib);

 }

@@ -522,7 +536,7 @@

 void

 usage(void)

 {

-	fprint(2, "usage: %s [-2s] [-i inst] [mid]\n", argv0);

+	fprint(2, "usage: %s [-2Ds] [-i inst] [mid]\n", argv0);

 	exits("usage");

 }

@@ -529,17 +543,19 @@

 void

 threadmain(int argc, char **argv)

 {

-	int n, t, mint;

+	int n, end, debug;

 	char *i;

 	double f;

 	uchar u[4];

 	Chan *c;

 	Opl *o;

-	Trk xs, *x, *minx;

+	Trk xs, *x;

 	i = "/mnt/wad/genmidi";

+	debug = 0;

 	ARGBEGIN{

 	case '2': opl2 = 1; ople = opl + 9; break;

+	case 'D': debug = 1; break;

 	case 'i': i = EARGF(usage()); break;

 	case 's': stream = 1; break;

 	default: usage();

@@ -560,6 +576,7 @@

 	tempo = 500000;

 	putcmd(Rwse, Mwse, 0);

 	putcmd(Rop3, 1, 0);

+	trace = debug;

 	if(stream){

 		if(proccreate(tproc, nil, mainstacksize) < 0)

 			sysfatal("proccreate: %r");

@@ -574,20 +591,22 @@

 		}

 		threadexitsall(n < 0 ? "read: %r" : nil);

 	}

-	for(;;){

-		minx = nil;

-		mint = 0;

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

+		end = 1;

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

 			if(x->p >= x->e)

 				continue;

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

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

-				mint = t;

-				minx = x;

+			end = 0;

+			x->Δ--;

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

+			while(x->Δ <= 0){

+				ev(x);

+				if(x->p >= x->e)

+					break;

+				x->Δ = getvar(x);

 			}

 		}

-		if(minx == nil)

-			exits(nil);

-		ev(minx);

+		samp(1);

 	}

+	exits(nil);

 }