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diff --git a/patch-ids.c b/patch-ids.c
--- /dev/null
+++ b/patch-ids.c
@@ -0,0 +1,192 @@
+#include "cache.h"
+#include "diff.h"
+#include "commit.h"
+#include "patch-ids.h"
+
+static int commit_patch_id(struct commit *commit, struct diff_options *options,
+ unsigned char *sha1)
+{
+ if (commit->parents)
+ diff_tree_sha1(commit->parents->item->object.sha1,
+ commit->object.sha1, "", options);
+ else
+ diff_root_tree_sha1(commit->object.sha1, "", options);
+ diffcore_std(options);
+ return diff_flush_patch_id(options, sha1);
+}
+
+static uint32_t take2(const unsigned char *id)
+{
+ return ((id[0] << 8) | id[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.
+ */
+static int patch_pos(struct patch_id **table, int nr, const unsigned char *id)
+{
+ int hi = nr;
+ int lo = 0;
+ int mi = 0;
+
+ if (!nr)
+ return -1;
+
+ if (nr != 1) {
+ unsigned lov, hiv, miv, ofs;
+
+ for (ofs = 0; ofs < 18; ofs += 2) {
+ lov = take2(table[0]->patch_id + ofs);
+ hiv = take2(table[nr-1]->patch_id + ofs);
+ miv = take2(id + 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 id 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("oops");
+ }
+ }
+ if (18 <= ofs)
+ die("cannot happen -- lo and hi are identical");
+ }
+
+ do {
+ int cmp;
+ cmp = hashcmp(table[mi]->patch_id, id);
+ if (!cmp)
+ return mi;
+ if (cmp > 0)
+ hi = mi;
+ else
+ lo = mi + 1;
+ mi = (hi + lo) / 2;
+ } while (lo < hi);
+ return -lo-1;
+}
+
+#define BUCKET_SIZE 190 /* 190 * 21 = 3990, with slop close enough to 4K */
+struct patch_id_bucket {
+ struct patch_id_bucket *next;
+ int nr;
+ struct patch_id bucket[BUCKET_SIZE];
+};
+
+int init_patch_ids(struct patch_ids *ids)
+{
+ memset(ids, 0, sizeof(*ids));
+ diff_setup(&ids->diffopts);
+ ids->diffopts.recursive = 1;
+ if (diff_setup_done(&ids->diffopts) < 0)
+ return error("diff_setup_done failed");
+ return 0;
+}
+
+int free_patch_ids(struct patch_ids *ids)
+{
+ struct patch_id_bucket *next, *patches;
+
+ free(ids->table);
+ for (patches = ids->patches; patches; patches = next) {
+ next = patches->next;
+ free(patches);
+ }
+ return 0;
+}
+
+static struct patch_id *add_commit(struct commit *commit,
+ struct patch_ids *ids,
+ int no_add)
+{
+ struct patch_id_bucket *bucket;
+ struct patch_id *ent;
+ unsigned char sha1[20];
+ int pos;
+
+ if (commit_patch_id(commit, &ids->diffopts, sha1))
+ return NULL;
+ pos = patch_pos(ids->table, ids->nr, sha1);
+ if (0 <= pos)
+ return ids->table[pos];
+ if (no_add)
+ return NULL;
+
+ pos = -1 - pos;
+
+ bucket = ids->patches;
+ if (!bucket || (BUCKET_SIZE <= bucket->nr)) {
+ bucket = xcalloc(1, sizeof(*bucket));
+ bucket->next = ids->patches;
+ ids->patches = bucket;
+ }
+ ent = &bucket->bucket[bucket->nr++];
+ hashcpy(ent->patch_id, sha1);
+
+ if (ids->alloc <= ids->nr) {
+ ids->alloc = alloc_nr(ids->nr);
+ ids->table = xrealloc(ids->table, sizeof(ent) * ids->alloc);
+ }
+ if (pos < ids->nr)
+ memmove(ids->table + pos + 1, ids->table + pos,
+ sizeof(ent) * (ids->nr - pos));
+ ids->nr++;
+ ids->table[pos] = ent;
+ return ids->table[pos];
+}
+
+struct patch_id *has_commit_patch_id(struct commit *commit,
+ struct patch_ids *ids)
+{
+ return add_commit(commit, ids, 1);
+}
+
+struct patch_id *add_commit_patch_id(struct commit *commit,
+ struct patch_ids *ids)
+{
+ return add_commit(commit, ids, 0);
+}