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megolm.c: rewrite counter update

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We no longer need to keep track of intermediate values of the counter, which
means we can update it much more easily.
This commit is contained in:
Richard van der Hoff 2016-05-24 17:33:41 +01:00
parent f3c0dd76d7
commit ef8d24f483
1 changed files with 3 additions and 8 deletions
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11
src/megolm.c
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@ -105,26 +105,21 @@ void megolm_advance_to(Megolm *megolm, uint32_t advance_to) {
/* starting with R0, see if we need to update each part of the hash */
for (j = 0; j < (int)MEGOLM_RATCHET_PARTS; j++) {
int shift = (MEGOLM_RATCHET_PARTS-j-1) * 8;
uint32_t increment = 1 << shift;
uint32_t next_counter;
uint32_t mask = (~(uint32_t)0) << shift;
/* how many times to we need to rehash this part? */
int steps = (advance_to >> shift) - (megolm->counter >> shift);
if (steps == 0) {
continue;
}
megolm->counter = megolm->counter & ~(increment - 1);
next_counter = megolm->counter + increment;
/* for all but the last step, we can just bump R(j) without regard
* to R(j+1)...R(3).
*/
while (steps > 1) {
rehash_part(megolm->data, j, j);
megolm->counter = next_counter;
steps --;
next_counter = megolm->counter + increment;
}
/* on the last step (except for j=3), we need to bump at least R(j+1);
@ -152,6 +147,6 @@ void megolm_advance_to(Megolm *megolm, uint32_t advance_to) {
rehash_part(megolm->data, j, k);
k--;
}
megolm->counter = next_counter;
megolm->counter = advance_to & mask;
}
}