From: Jan Stary <hans@stare.cz>
Subject: Re: aucat: Add generic channel mapping in place of -j and -c options.
To: Alexandre Ratchov <alex@caoua.org>
Cc: tech@openbsd.org
Date: Tue, 19 Mar 2024 16:24:47 +0100

On Mar 18 18:12:46, alex@caoua.org wrote:
> On Mon, Mar 18, 2024 at 04:28:36PM +0100, Jan Stary wrote:
> > 
> > I like the producer/consumer nomenclature, as it is the closest
> > to how I think about a signal route - not in terms of files and devices.
> > 
> 
> Here's a better diff:
> - ranges in the producer/consumer order.
> - the mapping is reset after every -i and -o.

Thank you.

I have a few notes on the manpage, but they are not necessarily
-m related, so I will leave those till after this gets in.

Jan


> Index: aucat.1
> ===================================================================
> RCS file: /cvs/src/usr.bin/aucat/aucat.1,v
> diff -u -p -r1.119 aucat.1
> --- aucat.1	10 Jan 2023 20:48:34 -0000	1.119
> +++ aucat.1	18 Mar 2024 14:54:52 -0000
> @@ -24,13 +24,13 @@
>  .Nm aucat
>  .Op Fl dn
>  .Op Fl b Ar size
> -.Op Fl c Ar min : Ns Ar max
> +.Op Fl c Ar channels
>  .Op Fl e Ar enc
>  .Op Fl f Ar device
>  .Op Fl g Ar position
>  .Op Fl h Ar fmt
>  .Op Fl i Ar file
> -.Op Fl j Ar flag
> +.Op Fl m Ar min : Ns Ar max Ns / Ns Ar min : Ns Ar max
>  .Op Fl o Ar file
>  .Op Fl p Ar position
>  .Op Fl q Ar port
> @@ -78,11 +78,9 @@ The options are as follows:
>  .It Fl b Ar size
>  The buffer size of the audio device in frames.
>  Default is 7680.
> -.It Fl c Ar min : Ns Ar max
> -The range of audio file channel numbers.
> -The default is
> -.Cm 0:1 ,
> -i.e. stereo.
> +.It Fl c Ar channels
> +The audio file channels count.
> +The default is 2, i.e. stereo.
>  .It Fl d
>  Increase log verbosity.
>  .It Fl e Ar enc
> @@ -146,21 +144,9 @@ Play this audio file.
>  If the option argument is
>  .Sq -
>  then standard input will be used.
> -.It Fl j Ar flag
> -Control whether source channels are joined or expanded if
> -they don't match the destination number of channels.
> -If the flag is
> -.Cm off ,
> -then each source channel is routed to a single destination channel,
> -possibly discarding channels.
> -If the flag is
> -.Cm on ,
> -then a single source may be sent to multiple destinations
> -and multiple sources may be mixed into a single destination.
> -For instance, this feature could be used to convert
> -a stereo file into a mono file mixing left and right channels together.
> -The default is
> -.Cm off .
> +.It Fl m Ar min : Ns Ar max Ns / Ns Ar min : Ns Ar max
> +Map the given range of source channels into the given range of
> +destination channels.
>  .It Fl n
>  Off-line mode.
>  Read input files and store the result in the output files,
> @@ -198,7 +184,7 @@ The default is 127, i.e. no attenuation.
>  .Pp
>  On the command line,
>  per-file parameters
> -.Pq Fl cehjrv
> +.Pq Fl cehmrv
>  must precede the file definition
>  .Pq Fl io .
>  .Pp
> @@ -282,13 +268,13 @@ Record channels 2 and 3 into one stereo 
>  channels 6 and 7 into another stereo file using a 44.1kHz sampling
>  rate for both:
>  .Bd -literal -offset indent
> -$ aucat -r 44100 -c 2:3 -o file1.wav -c 6:7 -o file2.wav
> +$ aucat -r 44100 -m 2:3/0:1 -o file1.wav -m 6:7/0:1 -o file2.wav
>  .Ed
>  .Pp
>  Split a stereo file into two mono files:
>  .Bd -literal -offset indent
> -$ aucat -n -i stereo.wav -c 0:0 -o left.wav \e
> -	-c 1:1 -o right.wav
> +$ aucat -n -i stereo.wav -c 1 -m 0:0/0:0 -o left.wav \e
> +	-m 1:1/0:0 -o right.wav
>  .Ed
>  .Sh SEE ALSO
>  .Xr cdio 1 ,
> Index: aucat.c
> ===================================================================
> RCS file: /cvs/src/usr.bin/aucat/aucat.c,v
> diff -u -p -r1.179 aucat.c
> --- aucat.c	1 Feb 2024 05:28:54 -0000	1.179
> +++ aucat.c	18 Mar 2024 14:54:52 -0000
> @@ -75,14 +75,14 @@ struct slot {
>  	int volctl;			/* volume in the 0..127 range */
>  	struct abuf buf;		/* file i/o buffer */
>  	int bpf;			/* bytes per frame */
> -	int cmin, cmax;			/* file channel range */
> +	int imin, imax, omin, omax;	/* channel mapping ranges */
>  	struct cmap cmap;		/* channel mapper state */
>  	struct resamp resamp;		/* resampler state */
>  	struct conv conv;		/* format encoder state */
>  	int join;			/* channel join factor */
>  	int expand;			/* channel expand factor */
>  	void *resampbuf, *convbuf;	/* conversion tmp buffers */
> -	int dup;			/* mono-to-stereo and alike */
> +	int dup;			/* compat with legacy -j option */
>  	int round;			/* slot-side block size */
>  	int mode;			/* MODE_{PLAY,REC} */
>  #define SLOT_CFG	0		/* buffers not allocated yet */
> @@ -136,9 +136,9 @@ const unsigned int voice_len[] = { 3, 3,
>  const unsigned int common_len[] = { 0, 2, 3, 2, 0, 0, 1, 1 };
>  
>  char usagestr[] = "usage: aucat [-dn] [-b size] "
> -    "[-c min:max] [-e enc] [-f device] [-g position]\n\t"
> -    "[-h fmt] [-i file] [-j flag] [-o file] [-p position] [-q port]\n\t"
> -    "[-r rate] [-v volume]\n";
> +    "[-c channels] [-e enc] [-f device] [-g position]\n\t"
> +    "[-h fmt] [-i file] [-m min:max/min:max] [-o file] [-p position]\n\t"
> +    "[-q port] [-r rate] [-v volume]\n";
>  
>  static void *
>  allocbuf(int nfr, int nch)
> @@ -218,19 +218,22 @@ slot_fill(struct slot *s)
>  
>  static int
>  slot_new(char *path, int mode, struct aparams *par, int hdr,
> -    int cmin, int cmax, int rate, int dup, int vol, long long pos)
> +    int imin, int imax, int omin, int omax, int nch,
> +    int rate, int dup, int vol, long long pos)
>  {
>  	struct slot *s, **ps;
>  
>  	s = xmalloc(sizeof(struct slot));
>  	if (!afile_open(&s->afile, path, hdr,
>  		mode == SIO_PLAY ? AFILE_FREAD : AFILE_FWRITE,
> -		par, rate, cmax - cmin + 1)) {
> +		par, rate, nch)) {
>  		xfree(s);
>  		return 0;
>  	}
> -	s->cmin = cmin;
> -	s->cmax = cmin + s->afile.nch - 1;
> +	s->imin = (imin != -1) ? imin : 0;
> +	s->imax = (imax != -1) ? imax : s->imin + s->afile.nch - 1;
> +	s->omin = (omin != -1) ? omin : 0;
> +	s->omax = (omax != -1) ? omax : s->omin + s->afile.nch - 1;
>  	s->dup = dup;
>  	s->vol = MIDI_TO_ADATA(vol);
>  	s->mode = mode;
> @@ -240,11 +243,17 @@ slot_new(char *path, int mode, struct ap
>  		slot_log(s);
>  		log_puts(": ");
>  		log_puts(s->mode == SIO_PLAY ? "play" : "rec");
> -		log_puts(", chan ");
> -		log_putu(s->cmin);
> -		log_puts(":");
> -		log_putu(s->cmax);
>  		log_puts(", ");
> +		log_putu(s->afile.nch);
> +		log_puts("ch (");
> +		log_putu(s->imin);
> +		log_puts(":");
> +		log_putu(s->imax);
> +		log_puts("/");
> +		log_putu(s->omin);
> +		log_puts(":");
> +		log_putu(s->omax);
> +		log_puts("), ");
>  		log_putu(s->afile.rate);
>  		log_puts("Hz, ");
>  		switch (s->afile.fmt) {
> @@ -283,7 +292,7 @@ slot_new(char *path, int mode, struct ap
>  static void
>  slot_init(struct slot *s)
>  {
> -	unsigned int slot_nch, bufsz;
> +	unsigned int inch, onch, bufsz;
>  
>  #ifdef DEBUG
>  	if (s->pstate != SLOT_CFG) {
> @@ -292,7 +301,7 @@ slot_init(struct slot *s)
>  		panic();
>  	}
>  #endif
> -	s->bpf = s->afile.par.bps * (s->cmax - s->cmin + 1);
> +	s->bpf = s->afile.par.bps * s->afile.nch;
>  	s->round = ((long long)dev_round * s->afile.rate +
>  	    dev_rate - 1) / dev_rate;
>  
> @@ -310,54 +319,50 @@ slot_init(struct slot *s)
>  	}
>  #endif
>  
> -	slot_nch = s->cmax - s->cmin + 1;
>  	s->convbuf = NULL;
>  	s->resampbuf = NULL;
>  	s->join = 1;
>  	s->expand = 1;
> +	inch = s->imax - s->imin + 1;
> +	onch = s->omax - s->omin + 1;
> +	if (s->dup) {
> +		/* compat with legacy -j option */
> +		if (s->mode == SIO_PLAY)
> +			onch = dev_pchan;
> +		else
> +			inch = dev_rchan;
> +	}
> +	if (onch > inch)
> +		s->expand = onch / inch;
> +	else if (onch < inch)
> +		s->join = inch / onch;
>  	if (s->mode & SIO_PLAY) {
> -		if (s->dup) {
> -			if (dev_pchan > slot_nch)
> -				s->expand = dev_pchan / slot_nch;
> -			else if (dev_pchan < slot_nch)
> -				s->join = slot_nch / dev_pchan;
> -		}
>  		cmap_init(&s->cmap,
> -		    s->cmin, s->cmax,
> -		    s->cmin, s->cmax,
> -		    0, dev_pchan - 1,
> -		    0, dev_pchan - 1);
> +		    0, s->afile.nch - 1, s->imin, s->imax,
> +		    0, dev_pchan - 1, s->omin, s->omax);
>  		if (s->afile.fmt != AFILE_FMT_PCM ||
>  		    !aparams_native(&s->afile.par)) {
> -			dec_init(&s->conv, &s->afile.par, slot_nch);
> -			s->convbuf = allocbuf(s->round, slot_nch);
> +			dec_init(&s->conv, &s->afile.par, s->afile.nch);
> +			s->convbuf = allocbuf(s->round, s->afile.nch);
>  		}
>  		if (s->afile.rate != dev_rate) {
>  			resamp_init(&s->resamp, s->afile.rate, dev_rate,
> -			    slot_nch);
> -			s->resampbuf = allocbuf(dev_round, slot_nch);
> +			    s->afile.nch);
> +			s->resampbuf = allocbuf(dev_round, s->afile.nch);
>  		}
>  	}
>  	if (s->mode & SIO_REC) {
> -		if (s->dup) {
> -			if (dev_rchan > slot_nch)
> -				s->join = dev_rchan / slot_nch;
> -			else if (dev_rchan < slot_nch)
> -				s->expand = slot_nch / dev_rchan;
> -		}
>  		cmap_init(&s->cmap,
> -		    0, dev_rchan - 1,
> -		    0, dev_rchan - 1,
> -		    s->cmin, s->cmax,
> -		    s->cmin, s->cmax);
> +		    0, dev_rchan - 1, s->imin, s->imax,
> +		    0, s->afile.nch - 1, s->omin, s->omax);
>  		if (s->afile.rate != dev_rate) {
>  			resamp_init(&s->resamp, dev_rate, s->afile.rate,
> -			    slot_nch);
> -			s->resampbuf = allocbuf(dev_round, slot_nch);
> +			    s->afile.nch);
> +			s->resampbuf = allocbuf(dev_round, s->afile.nch);
>  		}
>  		if (!aparams_native(&s->afile.par)) {
> -			enc_init(&s->conv, &s->afile.par, slot_nch);
> -			s->convbuf = allocbuf(s->round, slot_nch);
> +			enc_init(&s->conv, &s->afile.par, s->afile.nch);
> +			s->convbuf = allocbuf(s->round, s->afile.nch);
>  		}
>  
>  		/*
> @@ -368,13 +373,13 @@ slot_init(struct slot *s)
>  	         */
>  		if (s->resampbuf) {
>  			memset(s->resampbuf, 0,
> -			    dev_round * slot_nch * sizeof(adata_t));
> +			    dev_round * s->afile.nch * sizeof(adata_t));
>  		} else if (s->convbuf) {
>  			memset(s->convbuf, 0,
> -			    s->round * slot_nch * sizeof(adata_t));
> +			    s->round * s->afile.nch * sizeof(adata_t));
>  		} else {
>  			memset(s->buf.data, 0,
> -			    bufsz * slot_nch * sizeof(adata_t));
> +			    bufsz * s->afile.nch * sizeof(adata_t));
>  		}
>  	}
>  	s->pstate = SLOT_INIT;
> @@ -487,7 +492,7 @@ slot_getcnt(struct slot *s, int *icnt, i
>  static void
>  play_filt_resamp(struct slot *s, void *res_in, void *out, int icnt, int ocnt)
>  {
> -	int i, offs, vol, nch;
> +	int i, offs, vol, inch, onch;
>  	void *in;
>  
>  	if (s->resampbuf) {
> @@ -496,18 +501,24 @@ play_filt_resamp(struct slot *s, void *r
>  	} else
>  		in = res_in;
>  
> -	nch = s->cmap.nch;
> +	inch = s->imax - s->imin + 1;
> +	onch = s->omax - s->omin + 1;
>  	vol = s->vol / s->join; /* XXX */
>  	cmap_add(&s->cmap, in, out, vol, ocnt);
>  
>  	offs = 0;
>  	for (i = s->join - 1; i > 0; i--) {
> -		offs += nch;
> +		offs += onch;
> +		if (offs + s->cmap.nch > s->afile.nch)
> +			break;
>  		cmap_add(&s->cmap, (adata_t *)in + offs, out, vol, ocnt);
>  	}
> +
>  	offs = 0;
>  	for (i = s->expand - 1; i > 0; i--) {
> -		offs += nch;
> +		offs += inch;
> +		if (offs + s->cmap.nch > dev_pchan)
> +			break;
>  		cmap_add(&s->cmap, in, (adata_t *)out + offs, vol, ocnt);
>  	}
>  }
> @@ -581,23 +592,28 @@ slot_mix_badd(struct slot *s, adata_t *o
>  static void
>  rec_filt_resamp(struct slot *s, void *in, void *res_out, int icnt, int ocnt)
>  {
> -	int i, vol, offs, nch;
> +	int i, vol, offs, inch, onch;
>  	void *out = res_out;
>  
>  	out = (s->resampbuf) ? s->resampbuf : res_out;
>  
> -	nch = s->cmap.nch;
> +	inch = s->imax - s->imin + 1;
> +	onch = s->omax - s->omin + 1;
>  	vol = ADATA_UNIT / s->join;
>  	cmap_copy(&s->cmap, in, out, vol, icnt);
>  
>  	offs = 0;
>  	for (i = s->join - 1; i > 0; i--) {
> -		offs += nch;
> +		offs += onch;
> +		if (offs + s->cmap.nch > dev_rchan)
> +			break;
>  		cmap_add(&s->cmap, (adata_t *)in + offs, out, vol, icnt);
>  	}
>  	offs = 0;
>  	for (i = s->expand - 1; i > 0; i--) {
> -		offs += nch;
> +		offs += inch;
> +		if (offs + s->cmap.nch > s->afile.nch)
> +			break;
>  		cmap_copy(&s->cmap, in, (adata_t *)out + offs, vol, icnt);
>  	}
>  	if (s->resampbuf)
> @@ -683,12 +699,12 @@ dev_open(char *dev, int mode, int bufsz,
>  		if (s->afile.rate > rate)
>  			rate = s->afile.rate;
>  		if (s->mode == SIO_PLAY) {
> -			if (s->cmax > pmax)
> -				pmax = s->cmax;
> +			if (s->omax > pmax)
> +				pmax = s->omax;
>  		}
>  		if (s->mode == SIO_REC) {
> -			if (s->cmax > rmax)
> -				rmax = s->cmax;
> +			if (s->imax > rmax)
> +				rmax = s->imax;
>  		}
>  	}
>  	sio_initpar(&par);
> @@ -1078,8 +1094,10 @@ offline(void)
>  	for (s = slot_list; s != NULL; s = s->next) {
>  		if (s->afile.rate > rate)
>  			rate = s->afile.rate;
> -		if (s->cmax > cmax)
> -			cmax = s->cmax;
> +		if (s->imax > cmax)
> +			cmax = s->imax;
> +		if (s->omax > cmax)
> +			cmax = s->omax;
>  	}
>  	dev_sh = NULL;
>  	dev_name = "offline";
> @@ -1323,26 +1341,78 @@ opt_hdr(char *s, int *hdr)
>  }
>  
>  static int
> -opt_ch(char *s, int *rcmin, int *rcmax)
> +opt_map(char *str, int *rimin, int *rimax, int *romin, int *romax)
>  {
> -	char *next, *end;
> -	long cmin, cmax;
> +	char *s, *next;
> +	long imin, imax, omin, omax;
>  
>  	errno = 0;
> -	cmin = strtol(s, &next, 10);
> +	s = str;
> +	imin = strtol(s, &next, 10);
> +	if (next == s || *next != ':')
> +		goto failed;
> +	s = next + 1;
> +	imax = strtol(s, &next, 10);
> +	if (next == s || *next != '/')
> +		goto failed;
> +	s = next + 1;
> +	omin = strtol(s, &next, 10);
>  	if (next == s || *next != ':')
>  		goto failed;
> -	cmax = strtol(++next, &end, 10);
> -	if (end == next || *end != '\0')
> +	s = next + 1;
> +	omax = strtol(s, &next, 10);
> +	if (next == s || *next != '\0')
>  		goto failed;
> -	if (cmin < 0 || cmax < cmin || cmax >= NCHAN_MAX)
> +	if (imin < 0 || imax < imin || imax >= NCHAN_MAX)
>  		goto failed;
> -	*rcmin = cmin;
> -	*rcmax = cmax;
> +	if (omin < 0 || omax < omin || omax >= NCHAN_MAX)
> +		goto failed;
> +	*rimin = imin;
> +	*rimax = imax;
> +	*romin = omin;
> +	*romax = omax;
>  	return 1;
>  failed:
> -	log_puts(s);
> -	log_puts(": channel range expected\n");
> +	log_puts(str);
> +	log_puts(": channel mapping expected\n");
> +	return 0;
> +}
> +
> +static int
> +opt_nch(char *str, int *rnch, int *roff)
> +{
> +	char *s, *next;
> +	long nch, off, cmin, cmax;
> +
> +	errno = 0;
> +	s = str;
> +	nch = strtol(s, &next, 10);
> +	if (next == s)
> +		goto failed;
> +	if (*next == ':') {
> +		/* compat with legacy -c syntax */
> +		s = next + 1;
> +		cmin = nch;
> +		cmax = strtol(s, &next, 10);
> +		if (next == s)
> +			goto failed;
> +		if (cmin < 0 || cmax < cmin || cmax >= NCHAN_MAX)
> +			goto failed;
> +		nch = cmax - cmin + 1;
> +		off = cmin;
> +	} else {
> +		off = 0;
> +		if (nch < 0 || nch >= NCHAN_MAX)
> +			goto failed;
> +	}
> +	if (*next != '\0')
> +		goto failed;
> +	*rnch = nch;
> +	*roff = off;
> +	return 1;
> +failed:
> +	log_puts(str);
> +	log_puts(": channel count expected\n");
>  	return 0;
>  }
>  
> @@ -1381,7 +1451,7 @@ opt_pos(char *s, long long *pos)
>  int
>  main(int argc, char **argv)
>  {
> -	int dup, cmin, cmax, rate, vol, bufsz, hdr, mode;
> +	int dup, imin, imax, omin, omax, nch, off, rate, vol, bufsz, hdr, mode;
>  	char *port, *dev;
>  	struct aparams par;
>  	int n_flag, c;
> @@ -1393,9 +1463,10 @@ main(int argc, char **argv)
>  	vol = 127;
>  	dup = 0;
>  	bufsz = 0;
> +	nch = 2;
> +	off = 0;
>  	rate = DEFAULT_RATE;
> -	cmin = 0;
> -	cmax = 1;
> +	imin = imax = omin = omax = -1;
>  	par.bits = ADATA_BITS;
>  	par.bps = APARAMS_BPS(par.bits);
>  	par.le = ADATA_LE;
> @@ -1409,14 +1480,14 @@ main(int argc, char **argv)
>  	pos = 0;
>  
>  	while ((c = getopt(argc, argv,
> -		"b:c:de:f:g:h:i:j:no:p:q:r:t:v:")) != -1) {
> +		"b:c:de:f:g:h:i:j:m:no:p:q:r:t:v:")) != -1) {
>  		switch (c) {
>  		case 'b':
>  			if (!opt_num(optarg, 1, RATE_MAX, &bufsz))
>  				return 1;
>  			break;
>  		case 'c':
> -			if (!opt_ch(optarg, &cmin, &cmax))
> +			if (!opt_nch(optarg, &nch, &off))
>  				return 1;
>  			break;
>  		case 'd':
> @@ -1438,22 +1509,41 @@ main(int argc, char **argv)
>  				return 1;
>  			break;
>  		case 'i':
> +			if (off > 0) {
> +				/* compat with legacy -c syntax */
> +				omin = off;
> +				omax = off + nch - 1;
> +			}
>  			if (!slot_new(optarg, SIO_PLAY,
> -				&par, hdr, cmin, cmax, rate, dup, vol, pos))
> +				&par, hdr, imin, imax, omin, omax,
> +				nch, rate, dup, vol, pos))
>  				return 1;
>  			mode |= SIO_PLAY;
> +			imin = imax = omin = omax = -1;
>  			break;
>  		case 'j':
> +			/* compat with legacy -j option */
>  			if (!opt_onoff(optarg, &dup))
>  				return 1;
>  			break;
> +		case 'm':
> +			if (!opt_map(optarg, &imin, &imax, &omin, &omax))
> +				return 1;
> +			break;
>  		case 'n':
>  			n_flag = 1;
>  			break;
>  		case 'o':
> +			if (off > 0) {
> +				/* compat with legacy -c syntax */
> +				imin = off;
> +				imax = off + nch - 1;
> +			}
>  			if (!slot_new(optarg, SIO_REC,
> -				&par, hdr, cmin, cmax, rate, dup, 0, pos))
> +				&par, hdr, imin, imax, omin, omax,
> +				nch, rate, dup, 0, pos))
>  				return 1;
> +			imin = imax = omin = omax = -1;
>  			mode |= SIO_REC;
>  			break;
>  		case 'p':
> Index: dsp.c
> ===================================================================
> RCS file: /cvs/src/usr.bin/aucat/dsp.c,v
> diff -u -p -r1.18 dsp.c
> --- dsp.c	26 Dec 2022 19:16:00 -0000	1.18
> +++ dsp.c	18 Mar 2024 14:54:53 -0000
> @@ -992,33 +992,35 @@ cmap_init(struct cmap *p,
>      int imin, int imax, int isubmin, int isubmax,
>      int omin, int omax, int osubmin, int osubmax)
>  {
> -	int cmin, cmax;
> +	int inch, onch, nch;
>  
> -	cmin = -NCHAN_MAX;
> -	if (osubmin > cmin)
> -		cmin = osubmin;
> -	if (omin > cmin)
> -		cmin = omin;
> -	if (isubmin > cmin)
> -		cmin = isubmin;
> -	if (imin > cmin)
> -		cmin = imin;
> +	/*
> +	 * Ignore channels outside of the available sets
> +	 */
> +	if (isubmin < imin)
> +		isubmin = imin;
> +	if (isubmax > imax)
> +		isubmax = imax;
> +	if (osubmin < omin)
> +		osubmin = omin;
> +	if (osubmax > omax)
> +		osubmax = omax;
>  
> -	cmax = NCHAN_MAX;
> -	if (osubmax < cmax)
> -		cmax = osubmax;
> -	if (omax < cmax)
> -		cmax = omax;
> -	if (isubmax < cmax)
> -		cmax = isubmax;
> -	if (imax < cmax)
> -		cmax = imax;
> +	/*
> +	 * Shrink the input or the output subset to make both subsets of
> +	 * the same size
> +	 */
> +	inch = isubmax - isubmin + 1;
> +	onch = osubmax - osubmin + 1;
> +	nch = (inch < onch) ? inch : onch;
> +	isubmax = isubmin + nch - 1;
> +	osubmax = osubmin + nch - 1;
>  
> -	p->ostart = cmin - omin;
> -	p->onext = omax - cmax;
> -	p->istart = cmin - imin;
> -	p->inext = imax - cmax;
> -	p->nch = cmax - cmin + 1;
> +	p->ostart = osubmin - omin;
> +	p->onext = omax - osubmax;
> +	p->istart = isubmin - imin;
> +	p->inext = imax - isubmax;
> +	p->nch = nch;
>  #ifdef DEBUG
>  	if (log_level >= 3) {
>  		log_puts("cmap: nch = ");
> 
>