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-rw-r--r--sha1-lookup.c33
1 files changed, 26 insertions, 7 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c
index 4faa638caa..da357479cf 100644
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
@@ -50,6 +50,12 @@
* the midway of the table. It can reasonably be expected to be near
* 87% (222/256) from the top of the table.
*
+ * However, we do not want to pick "mi" too precisely. If the entry at
+ * the 87% in the above example turns out to be higher than the target
+ * we are looking for, we would end up narrowing the search space down
+ * only by 13%, instead of 50% we would get if we did a simple binary
+ * search. So we would want to hedge our bets by being less aggressive.
+ *
* The table at "table" holds at least "nr" entries of "elem_size"
* bytes each. Each entry has the SHA-1 key at "key_offset". The
* table is sorted by the SHA-1 key of the entries. The caller wants
@@ -119,11 +125,25 @@ int sha1_entry_pos(const void *table,
if (hiv < kyv)
return -1 - hi;
- if (kyv == lov && lov < hiv - 1)
- kyv++;
- else if (kyv == hiv - 1 && lov < kyv)
- kyv--;
-
+ /*
+ * Even if we know the target is much closer to 'hi'
+ * than 'lo', if we pick too precisely and overshoot
+ * (e.g. when we know 'mi' is closer to 'hi' than to
+ * 'lo', pick 'mi' that is higher than the target), we
+ * end up narrowing the search space by a smaller
+ * amount (i.e. the distance between 'mi' and 'hi')
+ * than what we would have (i.e. about half of 'lo'
+ * and 'hi'). Hedge our bets to pick 'mi' less
+ * aggressively, i.e. make 'mi' a bit closer to the
+ * middle than we would otherwise pick.
+ */
+ kyv = (kyv * 6 + lov + hiv) / 8;
+ if (lov < hiv - 1) {
+ if (kyv == lov)
+ kyv++;
+ else if (kyv == hiv)
+ kyv--;
+ }
mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
if (debug_lookup) {
@@ -142,8 +162,7 @@ int sha1_entry_pos(const void *table,
if (cmp > 0) {
hi = mi;
hi_key = mi_key;
- }
- else {
+ } else {
lo = mi + 1;
lo_key = mi_key + elem_size;
}