#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 - 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 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"); | |

} | |

} | |

} | |

do { | |

int cmp; | |

cmp = hashcmp(fn(mi, table), sha1); | |

if (!cmp) | |

return mi; | |

if (cmp > 0) | |

hi = mi; | |

else | |

lo = mi + 1; | |

mi = lo + (hi - lo) / 2; | |

} while (lo < hi); | |

return -lo-1; | |

} | |

int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo, | |

const unsigned char *table, size_t stride, uint32_t *result) | |

{ | |

uint32_t hi, lo; | |

hi = ntohl(fanout_nbo[*sha1]); | |

lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout_nbo[*sha1 - 1])); | |

while (lo < hi) { | |

unsigned mi = lo + (hi - lo) / 2; | |

int cmp = hashcmp(table + mi * stride, sha1); | |

if (!cmp) { | |

if (result) | |

*result = mi; | |

return 1; | |

} | |

if (cmp > 0) | |

hi = mi; | |

else | |

lo = mi + 1; | |

} | |

if (result) | |

*result = lo; | |

return 0; | |

} |