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packfiles: use bsearch

This is a surprisingly small speedup, but it still matters
when doing lookups on larger repositories. Drops repack time
on libvpx by 10%.

--- a/pack.c

+++ b/pack.c

@@ -609,8 +609,9 @@

 vlong

 searchindex(char *idx, int nidx, Hash h)

 {

-	int lo, hi, hidx, i, nent;

+	int lo, hi, hidx, i, r, nent;

 	vlong o, oo;

+	void *s;

 	o = 8;

 	if(nidx < 8 + 256*4)

@@ -638,21 +639,23 @@

 	/*

 	 * Now that we know the range of hashes that the

-	 * entry may exist in, read them in so we can do

-	 * a bsearch.

+	 * entry may exist in, search them

 	 */

+	r = -1;

 	hidx = -1;

-	o = Hashsz*lo + 8 + 256*4;

-	for(i = 0; i < hi - lo; i++){

-		if(o < 0 || o + sizeof(h.h) > nidx)

-			goto err;

-		if(memcmp(h.h, idx+o, sizeof(h.h)) == 0){

-			hidx = lo + i;

+	o = 8 + 256*4;

+	while(lo < hi){

+		hidx = (hi + lo)/2;

+		s = idx + o + hidx*sizeof(h.h);

+		r = memcmp(h.h, s, sizeof(h.h));

+		if(r < 0)

+			hi = hidx;

+		else if(r > 0)

+			lo = hidx + 1;

+		else

 			break;

-		}

-		o += sizeof(h.h);

 	}

-	if(hidx == -1)

+	if(r != 0)

 		goto notfound;

 	/*