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bvector: much faster unranking

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from O(t*n) to O(t*log(n)) for given McE params.
This commit is contained in:
Mirek Kratochvil 2013-01-26 23:48:06 +01:00
parent 805df12176
commit 889970349e
1 changed files with 29 additions and 4 deletions
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33
src/bvector.cpp
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@ -202,9 +202,10 @@ void bvector::colex_unrank (bvector&res, uint n, uint k) const
res.clear();
res.resize (n, 0);
uint p;
for (uint i = k; i > 0; --i) {
p = i;
/* Original code:
uint p = i;
for (;;) {
combination_number (p, i, t);
@ -212,10 +213,34 @@ void bvector::colex_unrank (bvector&res, uint n, uint k) const
++p;
}
combination_number (p - 1, i, t);
* ...that kindof lacks speed. We're actually trying to find
* the smallest value of p for which comb(p,i)>r.
*
* Computing all combination numbers is KIND OF slow and cache
* (as suggested by Barreto and others) doesn't really make big
* difference here (we're usually doing only one or two runs of
* this stuff in one program run). Storing about 50megs of
* precalculated combination numbers is weird as well.
*
* Therefore, with the knowledge that i <= p <= n, we're
* halving the search interval as usual.
*/
uint p, a = i, b = n;
while (a < b) {
p = (a + b) / 2;
combination_number (p, i, t);
if (mpz_cmp (t, r) > 0) b = p;
else a = p + 1;
}
combination_number (b - 1, i, t);
mpz_swap (t2, r);
mpz_sub (r, t2, t);
if (p > n) continue; //overflow protection
//overflow protection (result's wrong anyway now)
if (p > n) continue;
res[p - 1] = 1;
}