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Simplify perfect-square testing loop #2

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Simplify perfect-square testing loop #2

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17 changes: 7 additions & 10 deletions common/smallfact/squfof.c
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,7 @@ uint32 squfof_one_cycle(squfof_data_t *data, uint32 mult_idx,
uint64 scaledn;

for (i = 0; i < num_iter; i++) {
uint32 q, bits, tmp;
uint32 q, tmp;

/* compute (sqrtn+p1)/q1; since this is unity
more than half the time, special case the
Expand Down Expand Up @@ -113,18 +113,15 @@ uint32 squfof_one_cycle(squfof_data_t *data, uint32 mult_idx,
/* if q0 is a perfect square, then the factorization
has probably succeeded. Most of the squareness
tests out there require multiple divisions and
complicated loops. We can approximate these tests
by doing two things: testing that the number of
trailing zeros in q0 is even, and then testing
if q0 shifted right this many places is 1 mod 8. */
complicated loops. We approximate these tests by
ensuring that the last 3 bits are 001 after
dividing (shifting) out all factors of 4. */

bits = 0;
tmp = q0;
while(!(tmp & 1)) {
bits++;
tmp >>= 1;
while(!(tmp & 3)) {
tmp >>= 2;
}
if (!(bits & 1) && ((tmp & 7) == 1)) {
if ((tmp & 7) == 1) {

/* q0 is probably a perfect square. Take the
square root by cheating */
Expand Down