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-rw-r--r--sha1-lookup.c272
1 files changed, 272 insertions, 0 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c
new file mode 100644
index 0000000..c4dc55d
--- /dev/null
+++ b/sha1-lookup.c
@@ -0,0 +1,272 @@
+#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:
+ *
+ * unsigned lo, hi;
+ * do {
+ * unsigned 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 as 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 set it to hi,
+ * and repeat the search.
+ *
+ * - if it is strictly lower than the target, we update lo to
+ * one slot after it, because we allow lo to be at the target.
+ *
+ * If the loop exits, there is no matching entry.
+ *
+ * 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.
+ *
+ * Now, we can take advantage of the fact that SHA-1 is a good hash
+ * function, and as long as there are enough entries in the table, we
+ * can expect uniform distribution. An entry that begins with for
+ * example "deadbeef..." is much likely to appear much later than in
+ * 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
+ * to find the entry with "key", and knows that the entry at "lo" is
+ * not higher than the entry it is looking for, and that the entry at
+ * "hi" is higher than the entry it is looking for.
+ */
+int sha1_entry_pos(const void *table,
+ size_t elem_size,
+ size_t key_offset,
+ unsigned lo, unsigned hi, unsigned nr,
+ const unsigned char *key)
+{
+ const unsigned char *base = table;
+ const unsigned char *hi_key, *lo_key;
+ unsigned ofs_0;
+ static int debug_lookup = -1;
+
+ if (debug_lookup < 0)
+ debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
+
+ if (!nr || lo >= hi)
+ return -1;
+
+ if (nr == hi)
+ hi_key = NULL;
+ else
+ hi_key = base + elem_size * hi + key_offset;
+ lo_key = base + elem_size * lo + key_offset;
+
+ ofs_0 = 0;
+ do {
+ int cmp;
+ unsigned ofs, mi, range;
+ unsigned lov, hiv, kyv;
+ const unsigned char *mi_key;
+
+ range = hi - lo;
+ if (hi_key) {
+ for (ofs = ofs_0; ofs < 20; ofs++)
+ if (lo_key[ofs] != hi_key[ofs])
+ break;
+ ofs_0 = ofs;
+ /*
+ * byte 0 thru (ofs-1) are the same between
+ * lo and hi; ofs is the first byte that is
+ * different.
+ */
+ hiv = hi_key[ofs_0];
+ if (ofs_0 < 19)
+ hiv = (hiv << 8) | hi_key[ofs_0+1];
+ } else {
+ hiv = 256;
+ if (ofs_0 < 19)
+ hiv <<= 8;
+ }
+ lov = lo_key[ofs_0];
+ kyv = key[ofs_0];
+ if (ofs_0 < 19) {
+ lov = (lov << 8) | lo_key[ofs_0+1];
+ kyv = (kyv << 8) | key[ofs_0+1];
+ }
+ assert(lov < hiv);
+
+ if (kyv < lov)
+ return -1 - lo;
+ if (hiv < kyv)
+ return -1 - hi;
+
+ /*
+ * 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) {
+ printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
+ printf("ofs %u lov %x, hiv %x, kyv %x\n",
+ ofs_0, lov, hiv, kyv);
+ }
+ if (!(lo <= mi && mi < hi))
+ die("assertion failure lo %u mi %u hi %u %s",
+ lo, mi, hi, sha1_to_hex(key));
+
+ mi_key = base + elem_size * mi + key_offset;
+ cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
+ if (!cmp)
+ return mi;
+ if (cmp > 0) {
+ hi = mi;
+ hi_key = mi_key;
+ } else {
+ lo = mi + 1;
+ lo_key = mi_key + elem_size;
+ }
+ } while (lo < hi);
+ return -lo-1;
+}
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