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129 lines
3.1 KiB
129 lines
3.1 KiB
#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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/* |
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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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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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int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo, |
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const unsigned char *table, size_t stride, uint32_t *result) |
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{ |
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uint32_t hi, lo; |
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hi = ntohl(fanout_nbo[*sha1]); |
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lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout_nbo[*sha1 - 1])); |
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while (lo < hi) { |
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unsigned mi = lo + (hi - lo) / 2; |
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int cmp = hashcmp(table + mi * stride, sha1); |
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if (!cmp) { |
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if (result) |
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*result = mi; |
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return 1; |
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} |
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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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} |
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if (result) |
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*result = lo; |
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return 0; |
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}
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