[9fans] Samterm up down key patch

["Joey Makar" <makarjo@gmail.com>]

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Hello, attached is a patch to samterm to make the up and down arrow
keys move the cursor.

When the down arrow key is pressed it should go to the next newline
plus the number of characters from the previous newline, unless that
is beyond the next next newline, then go to that.

And for the up arrow key: go the previous previous newline plus the
number of characters from the previous newline, unless that is beyond
the previous newline, then go to that.

I'm relatively new to Plan9, any comments welcome.

Joey

[-- Attachment #2: samterm.h.patch --]
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Section	*lastsection(Rasp*, Section*);
long	prevnewline(Rasp*, long);
long	nextnewline(Rasp*, long);
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[-- Attachment #3: main.c.patch --]
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}

Section*
lastsection(Rasp *r, Section *s)
{
	Section *p;
	for(p=r->sect; p; p=p->next)
		if(p->next == s)
			return p;
	return 0;
}

long
prevnewline(Rasp *r, long from)
{
	Section *sect;
	Rune *text;
	long rp, sp;	/* Rasp, Section position */
			/* rp is the integer counterpart of text */

	if(!r->sect)
		return -1;

	/* Go to the from position. */
	sect = r->sect;
	rp = 0;
	while(rp + sect->nrunes < from){
		rp += sect->nrunes;
		sect = sect->next;
	}
	sp = from - rp;
	text = sect->text + sp;
	rp = from;

	/* Search backwards for a newline. */
	while(1){
		char ch[UTFmax];
		int rv;

		if(--sp < 0){
			sect = lastsection(r, sect);
			if(!sect)
				return -1;
			sp = sect->nrunes;
			text = sect->text + sp;
		}
		rp--;
		text--;

		rv = runetochar(ch, text);
		if(rv == 1 && ch[0] == '\n')
			return rp;
	}
}

long
nextnewline(Rasp *r, long from)
{
	Section *sect;
	Rune *text;
	long rp, sp;	/* Rasp, Section position */
			/* rp is the integer counterpart of text */

	if(!r->sect)
		return 0;

	/* Go to the from position. */
	sect = r->sect;
	rp = 0;
	while(rp + sect->nrunes < from){
		rp += sect->nrunes;
		sect = sect->next;
	}
	sp = from - rp;
	text = sect->text + sp;
	rp = from;

	/* Search for a newline. */
	while(1){
		char ch[UTFmax];
		int rv;

		rv = runetochar(ch, text);
		if(rv == 1 && ch[0] == '\n')
			return rp;

		if(++sp > sect->nrunes - 1){
			sect = sect->next;
			if(!sect)
				return r->nrunes;
			sp = 0;
			text = sect->text - 1;
		}
		rp++;
		text++;
	}

	return 0; /* Won't execute; silences a compiler warning. */
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	}else if(c == SCROLLKEY){
		long pn, nn, nnn;	/* Previous, next, next next newline */
		flushtyping(0);
		pn = prevnewline(&t->rasp, l->p0);
		nn = nextnewline(&t->rasp, l->p0);
		if(nn != t->rasp.nrunes)
			nnn = nextnewline(&t->rasp, nn + 1);
		else
			nnn = t->rasp.nrunes;
		/* pn may be -1. */
		a0 = (l->p0 - pn) + nn;
		a0 = a0 > nnn ? nnn : a0;
		flsetselect(l, a0, a0);
		center(l, a0);
	}else if(c == BACKSCROLLKEY){
		long ppn, pn;	/* Previous previous, previous newline */
		flushtyping(0);
		pn = prevnewline(&t->rasp, l->p0);
		if(pn != -1)
			ppn = prevnewline(&t->rasp, pn);
		else
			ppn = -1;
		if(pn == -1)
			a0 = 0;
		else{
			/* ppn may be -1. */
			a0 = (l->p0 - pn) + ppn;
			a0 = a0 > pn ? pn : a0;
		}
		flsetselect(l, a0, a0);
		center(l, a0);
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	}else if(c==PAGEUP){
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	if(c==PAGEDOWN){
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