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| author | Denys Vlasenko <vda.linux@googlemail.com> | 2017-04-09 23:12:38 +0200 |
|---|---|---|
| committer | Denys Vlasenko <vda.linux@googlemail.com> | 2017-04-09 23:12:38 +0200 |
| commit | 7e5f2f3b5163a3c7c6b20a311f1177f9c4f04b81 (patch) | |
| tree | 7f3b277b887c0a841120eac95db964f6ac63c5c6 /coreutils/factor.c | |
| parent | 1d232fd44026deae94c88200766152aec9d6c5e5 (diff) | |
| download | busybox-7e5f2f3b5163a3c7c6b20a311f1177f9c4f04b81.tar.gz | |
factor: expand comments
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Diffstat (limited to 'coreutils/factor.c')
| -rw-r--r-- | coreutils/factor.c | 11 |
1 files changed, 7 insertions, 4 deletions
diff --git a/coreutils/factor.c b/coreutils/factor.c index e01190150..1c01e3f27 100644 --- a/coreutils/factor.c +++ b/coreutils/factor.c @@ -107,9 +107,12 @@ static NOINLINE void factorize(wide_t N) factor = 3; factor2 = 3 * 3; for (;;) { - while ((N % factor) == 0) { + /* The division is the most costly part of the loop. + * On 64bit CPUs, takes at best 12 cycles, often ~20. + */ + while ((N % factor) == 0) { /* not likely */ N = N / factor; - printf(" %u"HALF_FMT"", factor); + printf(" %u"HALF_FMT, factor); max_factor = isqrt_odd(N); } next_factor: @@ -122,8 +125,8 @@ static NOINLINE void factorize(wide_t N) factor += 2; /* Rudimentary wheel sieving: skip multiples of 3: * Every third odd number is divisible by three and thus isn't a prime: - * 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37... - * ^ ^ ^ ^ ^ ^ ^ _ ^ ^ _ ^ + * 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47... + * ^ ^ ^ ^ ^ ^ ^ _ ^ ^ _ ^ ^ ^ ^ * (^ = primes, _ = would-be-primes-if-not-divisible-by-5) */ --count3; |