i didn't double check if Runemax has made it into
libc.h.  i think it makes sense, even for 16-bit runes since
Bgetc returns an int.  and cases like this where a rune
value is calculated as an int to protect against overflow.
this patch depends on Runemax.

1.  fix output bug.  for character 0, the output was truncated
because %C formats a 0 character, terminating the string.
also defend against illegal utf values.

2.  add -b baseoff to correct the character values for subfonts
that don't start at character 0.  while this is a hack (we should
parse font(6) files instead so that each subfont could be
discontinuous with this previous), it does make viewing a single
subfont possible with much less work.

Reference: /n/sources/patch/maybe/tweaknil
Date: Sun Feb 14 17:16:43 CET 2010
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/tweak.c	Sun Feb 14 17:08:59 2010
+++ /sys/src/cmd/tweak.c	Sun Feb 14 17:08:56 2010
@@ -160,6 +160,7 @@
 Image		*values[256];
 Image		*greyvalues[256];
 uchar		data[8192];
+int		base;
 
 Thing*	tget(char*);
 void	mesg(char*, ...);
@@ -172,12 +173,29 @@
 void	tclose1(Thing*);
 
 void
+usage(void)
+{
+	fprint(2, "usage: tweak [-b base] [file] ...\n");
+	exits("usage");
+}
+
+void
 main(int argc, char *argv[])
 {
 	int i;
 	Event e;
 	Thing *t;
 
+	ARGBEGIN{
+	case 'b':
+		base = atoi(EARGF(usage()));
+		if((uint)base > Runemax)
+			usage();
+		break;
+	default:
+		usage();
+	}ARGEND
+
 	mag = Mag;
 	if(initdraw(error, 0, "tweak") < 0){
 		fprint(2, "tweak: initdraw failed: %r\n");
@@ -191,9 +209,8 @@
 	}
 	einit(Emouse|Ekeyboard);
 	eresized(0);
-	i = 1;
 	setjmp(err);
-	for(; i<argc; i++){
+	for(i = 0; i<argc; i++){
 		file = argv[i];
 		t = tget(argv[i]);
 		if(t)
@@ -334,8 +351,9 @@
 void
 stext(Thing *t, char *l0, char *l1)
 {
+	char buf[256], buf1[10];
+	int r;
 	Fontchar *fc;
-	char buf[256];
 
 	l1[0] = 0;
 	sprint(buf, "depth:%d r:%d %d  %d %d ", 
@@ -346,9 +364,16 @@
 	sprint(l0, "%s file: %s", buf, t->name);
 	if(t->c >= 0){
 		fc = &t->parent->s->info[t->c];
-		sprint(l1, "c(hex): %x c(char): %C x: %d "
+		r = base+t->c+t->parent->off;
+		if((uint)r > Runemax)
+			snprint(buf1, sizeof buf1, "<ill>");
+		else if(r == 0)
+			snprint(buf1, sizeof buf1, "<nil>");
+		else
+			snprint(buf1, sizeof buf1, "%C", (Rune)r);
+		sprint(l1, "c(hex): %x c(char): %s x: %d "
 			   "top: %d bottom: %d left: %d width: %d iwidth: %d",
-			(int)(t->c+t->parent->off), (int)(t->c+t->parent->off),
+			r, buf1,
 			fc->x, fc->top, fc->bottom, fc->left,
 			fc->width, Dx(t->b->r));
 	}else if(t->s)
--- /sys/man/1/tweak	Sun Feb 14 17:09:05 2010
+++ /sys/man/1/tweak	Sun Feb 14 17:09:04 2010
@@ -5,6 +5,10 @@
 .SH SYNOPSIS
 .B tweak
 [
+.B -b
+.I baseoff
+]
+[
 .I file ...
 ]
 .SH DESCRIPTION
@@ -43,6 +47,10 @@
 .B ascent
 as defined in
 .IR cachechars (2).
+It is assumed that the first character in the first file
+is character 0 unless
+.I baseoff
+is given.
 .PP
 By means described below, magnified views of portions of the images
 may be displayed.
@@ -165,3 +173,8 @@
 For a program written to adjust width tables in fonts,
 .I tweak
 has been pushed unreasonably far.
+.PP
+Fonts with discontinuities will display incorrect character
+values.  It would be necessary to parse
+.IR font (6)
+files instead of subfonts.