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page: make lookahead fetch work in backwards direction

--- a/sys/src/cmd/page.c

+++ b/sys/src/cmd/page.c

@@ -34,6 +34,7 @@

 int newwin;

 int rotate;

 int viewgen;

+int forward;	/* read ahead direction: >= 0 forwards, < 0 backwards */

 Point resize, pos;

 Page *root, *current;

 Page lru;

@@ -45,7 +46,7 @@

 	MiB	= 1024*1024,

 };

-ulong imemlimit = 8*MiB;

+ulong imemlimit = 16*MiB;

 ulong imemsize;

 Image *frame, *paper, *ground;

@@ -882,17 +883,23 @@

 	p->lprev = nil;

 }

-void

-loadpage(Page *p)

+static void

+llinkhead(Page *p)

 {

-	int fd;

-

-	qlock(&lru);

 	lunlink(p);

 	p->lnext = lru.lnext;

 	p->lprev = &lru;

 	p->lnext->lprev = p;

 	p->lprev->lnext = p;

+}

+

+void

+loadpage(Page *p)

+{

+	int fd;

+

+	qlock(&lru);

+	llinkhead(p);

 	qunlock(&lru);

 	if(p->open && p->image == nil){

@@ -943,7 +950,7 @@

 void

 loadpages(Page *p, int oviewgen)

 {

-	while(p && viewgen == oviewgen){

+	while(p != nil && viewgen == oviewgen){

 		qlock(p);

 		loadpage(p);

 		if(viewgen != oviewgen){

@@ -965,9 +972,9 @@

 			unlockdisplay(display);

 		}

 		qunlock(p);

-		if(p != current || imemsize >= imemlimit)

-			break;

-		p = nextpage(p);

+		if(p != current && imemsize >= imemlimit)

+			break;		/* only one page ahead once we reach the limit */

+		p = forward < 0 ? prevpage(p) : nextpage(p);

 	}

 }

@@ -1327,8 +1334,7 @@

 	if(p == nil)

 		return;

 	drawlock(0);

-	if(p->image == nil)

-		unloadpages(imemlimit/2);

+	unloadpages(imemlimit);

 	showpage1(p);

 	drawlock(1);

 }

@@ -1338,6 +1344,7 @@

 {

 	Page *p;

+	forward = 1;

 	for(p = nextpage(current); p; p = nextpage(p))

 		if(p->image || p->open)

 			break;

@@ -1349,6 +1356,7 @@

 {

 	Page *p;

+	forward = -1;

 	for(p = prevpage(current); p; p = prevpage(p))

 		if(p->image || p->open)

 			break;

@@ -1730,6 +1738,7 @@

 					pagemenu.lasthit = pageindex(current);

 					x = pageat(emenuhit(3, &m, &pagemenu));

 					qunlock(&pagelock);

+					forward = 0;

 					showpage(x);

 				}

 			} else if(m.buttons & 8){

@@ -1767,8 +1776,10 @@

 				   (e.kbdc & 0xFF00) == Spec)

 					break;

 				snprint(buf, sizeof(buf), "%C", (Rune)e.kbdc);

-				if(eenter("Go to", buf, sizeof(buf), &m) > 0)

+				if(eenter("Go to", buf, sizeof(buf), &m) > 0){

+					forward = 0;

 					showpage(findpage(buf));

+				}

 			}

 			break;

 		case Eplumb:

@@ -1806,6 +1817,7 @@

 					j = addpage(root, s, popenfile, s, fd);

 					drawlock(1);

 				}

+				forward = 0;

 				showpage(j);

 			}

 		Plumbfree: