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-rw-r--r--hash-lookup.c131
1 files changed, 131 insertions, 0 deletions
diff --git a/hash-lookup.c b/hash-lookup.c
new file mode 100644
index 0000000..9f0f95e
--- /dev/null
+++ b/hash-lookup.c
@@ -0,0 +1,131 @@
+#include "git-compat-util.h"
+#include "hash.h"
+#include "hash-lookup.h"
+#include "read-cache-ll.h"
+
+static uint32_t take2(const struct object_id *oid, size_t ofs)
+{
+ return ((oid->hash[ofs] << 8) | oid->hash[ofs + 1]);
+}
+
+/*
+ * Conventional binary search loop looks like this:
+ *
+ * do {
+ * int mi = lo + (hi - lo) / 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 oid of element i (between 0 and nr - 1) should be returned
+ * by "fn(i, table)".
+ */
+int oid_pos(const struct object_id *oid, const void *table, size_t nr,
+ oid_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 < the_hash_algo->rawsz - 2; ofs += 2) {
+ lov = take2(fn(0, table), ofs);
+ hiv = take2(fn(nr - 1, table), ofs);
+ miv = take2(oid, ofs);
+ if (miv < lov)
+ return -1;
+ if (hiv < miv)
+ return index_pos_to_insert_pos(nr);
+ if (lov != hiv) {
+ /*
+ * At this point miv could be equal
+ * to hiv (but hash 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;
+ BUG("assertion failed in binary search");
+ }
+ }
+ }
+
+ do {
+ int cmp;
+ cmp = oidcmp(fn(mi, table), oid);
+ if (!cmp)
+ return mi;
+ if (cmp > 0)
+ hi = mi;
+ else
+ lo = mi + 1;
+ mi = lo + (hi - lo) / 2;
+ } while (lo < hi);
+ return index_pos_to_insert_pos(lo);
+}
+
+int bsearch_hash(const unsigned char *hash, const uint32_t *fanout_nbo,
+ const unsigned char *table, size_t stride, uint32_t *result)
+{
+ uint32_t hi, lo;
+
+ hi = ntohl(fanout_nbo[*hash]);
+ lo = ((*hash == 0x0) ? 0 : ntohl(fanout_nbo[*hash - 1]));
+
+ while (lo < hi) {
+ unsigned mi = lo + (hi - lo) / 2;
+ int cmp = hashcmp(table + mi * stride, hash);
+
+ if (!cmp) {
+ if (result)
+ *result = mi;
+ return 1;
+ }
+ if (cmp > 0)
+ hi = mi;
+ else
+ lo = mi + 1;
+ }
+
+ if (result)
+ *result = lo;
+ return 0;
+}