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19716b21a4
A common mistake when writing binary search is to allow possible integer overflow by using the simple average: mid = (min + max) / 2; Instead, use the overflow-safe version: mid = min + (max - min) / 2; This translation is safe since the operation occurs inside a loop conditioned on "min < max". The included changes were found using the following git grep: git grep '/ *2;' '*.c' Making this cleanup will prevent future review friction when a new binary search is contructed based on existing code. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
102 lines
2.6 KiB
C
102 lines
2.6 KiB
C
#include "cache.h"
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#include "sha1-lookup.h"
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static uint32_t take2(const unsigned char *sha1)
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{
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return ((sha1[0] << 8) | sha1[1]);
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}
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/*
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* Conventional binary search loop looks like this:
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*
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* do {
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* int mi = lo + (hi - lo) / 2;
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* int cmp = "entry pointed at by mi" minus "target";
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* if (!cmp)
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* return (mi is the wanted one)
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* if (cmp > 0)
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* hi = mi; "mi is larger than target"
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* else
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* lo = mi+1; "mi is smaller than target"
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* } while (lo < hi);
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*
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* The invariants are:
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*
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* - When entering the loop, lo points at a slot that is never
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* above the target (it could be at the target), hi points at a
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* slot that is guaranteed to be above the target (it can never
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* be at the target).
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*
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* - We find a point 'mi' between lo and hi (mi could be the same
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* as lo, but never can be the same as hi), and check if it hits
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* the target. There are three cases:
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*
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* - if it is a hit, we are happy.
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*
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* - if it is strictly higher than the target, we update hi with
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* it.
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*
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* - if it is strictly lower than the target, we update lo to be
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* one slot after it, because we allow lo to be at the target.
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*
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* When choosing 'mi', we do not have to take the "middle" but
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* anywhere in between lo and hi, as long as lo <= mi < hi is
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* satisfied. When we somehow know that the distance between the
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* target and lo is much shorter than the target and hi, we could
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* pick mi that is much closer to lo than the midway.
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*/
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/*
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* The table should contain "nr" elements.
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* The sha1 of element i (between 0 and nr - 1) should be returned
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* by "fn(i, table)".
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*/
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int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
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sha1_access_fn fn)
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{
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size_t hi = nr;
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size_t lo = 0;
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size_t mi = 0;
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if (!nr)
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return -1;
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if (nr != 1) {
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size_t lov, hiv, miv, ofs;
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for (ofs = 0; ofs < 18; ofs += 2) {
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lov = take2(fn(0, table) + ofs);
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hiv = take2(fn(nr - 1, table) + ofs);
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miv = take2(sha1 + ofs);
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if (miv < lov)
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return -1;
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if (hiv < miv)
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return -1 - nr;
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if (lov != hiv) {
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/*
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* At this point miv could be equal
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* to hiv (but sha1 could still be higher);
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* the invariant of (mi < hi) should be
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* kept.
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*/
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mi = (nr - 1) * (miv - lov) / (hiv - lov);
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if (lo <= mi && mi < hi)
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break;
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die("BUG: assertion failed in binary search");
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}
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}
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}
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do {
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int cmp;
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cmp = hashcmp(fn(mi, table), sha1);
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if (!cmp)
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return mi;
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if (cmp > 0)
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hi = mi;
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else
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lo = mi + 1;
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mi = lo + (hi - lo) / 2;
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} while (lo < hi);
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return -lo-1;
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}
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