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diff --git a/sha1-lookup.c b/sha1-lookup.c
index da357479cf19aad4bebc64f874c76fdf8566712b..c4dc55d1f5cd07adcf46865354f841cc587c51f6 100644 (file)
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
#include "cache.h"
#include "sha1-lookup.h"
+static uint32_t take2(const unsigned char *sha1)
+{
+ return ((sha1[0] << 8) | sha1[1]);
+}
+
+/*
+ * Conventional binary search loop looks like this:
+ *
+ * do {
+ * int mi = (lo + hi) / 2;
+ * int cmp = "entry pointed at by mi" minus "target";
+ * if (!cmp)
+ * return (mi is the wanted one)
+ * if (cmp > 0)
+ * hi = mi; "mi is larger than target"
+ * else
+ * lo = mi+1; "mi is smaller than target"
+ * } while (lo < hi);
+ *
+ * The invariants are:
+ *
+ * - When entering the loop, lo points at a slot that is never
+ * above the target (it could be at the target), hi points at a
+ * slot that is guaranteed to be above the target (it can never
+ * be at the target).
+ *
+ * - We find a point 'mi' between lo and hi (mi could be the same
+ * as lo, but never can be the same as hi), and check if it hits
+ * the target. There are three cases:
+ *
+ * - if it is a hit, we are happy.
+ *
+ * - if it is strictly higher than the target, we update hi with
+ * it.
+ *
+ * - if it is strictly lower than the target, we update lo to be
+ * one slot after it, because we allow lo to be at the target.
+ *
+ * When choosing 'mi', we do not have to take the "middle" but
+ * anywhere in between lo and hi, as long as lo <= mi < hi is
+ * satisfied. When we somehow know that the distance between the
+ * target and lo is much shorter than the target and hi, we could
+ * pick mi that is much closer to lo than the midway.
+ */
+/*
+ * The table should contain "nr" elements.
+ * The sha1 of element i (between 0 and nr - 1) should be returned
+ * by "fn(i, table)".
+ */
+int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
+ sha1_access_fn fn)
+{
+ size_t hi = nr;
+ size_t lo = 0;
+ size_t mi = 0;
+
+ if (!nr)
+ return -1;
+
+ if (nr != 1) {
+ size_t lov, hiv, miv, ofs;
+
+ for (ofs = 0; ofs < 18; ofs += 2) {
+ lov = take2(fn(0, table) + ofs);
+ hiv = take2(fn(nr - 1, table) + ofs);
+ miv = take2(sha1 + ofs);
+ if (miv < lov)
+ return -1;
+ if (hiv < miv)
+ return -1 - nr;
+ if (lov != hiv) {
+ /*
+ * At this point miv could be equal
+ * to hiv (but sha1 could still be higher);
+ * the invariant of (mi < hi) should be
+ * kept.
+ */
+ mi = (nr - 1) * (miv - lov) / (hiv - lov);
+ if (lo <= mi && mi < hi)
+ break;
+ die("BUG: assertion failed in binary search");
+ }
+ }
+ if (18 <= ofs)
+ die("cannot happen -- lo and hi are identical");
+ }
+
+ do {
+ int cmp;
+ cmp = hashcmp(fn(mi, table), sha1);
+ if (!cmp)
+ return mi;
+ if (cmp > 0)
+ hi = mi;
+ else
+ lo = mi + 1;
+ mi = (hi + lo) / 2;
+ } while (lo < hi);
+ return -lo-1;
+}
+
/*
* Conventional binary search loop looks like this:
*