[libmath-prime-util-perl] 30/54: Small totient speedup

[Partha P. Mukherjee]

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ppm-guest pushed a commit to annotated tag v0.38
in repository libmath-prime-util-perl.

commit 141a8de526660972d4884099b98a6fa842585f5d
Author: Dana Jacobsen <dana at acm.org>
Date:   Fri Feb 14 11:51:44 2014 -0800

    Small totient speedup
---
 util.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/util.c b/util.c
index 2485e87..a85ee4b 100644
--- a/util.c
+++ b/util.c
@@ -1112,8 +1112,12 @@ int kronecker_ss(IV a, IV b) {
 UV totient(UV n) {
   UV i, nfacs, totient, lastf, facs[MPU_MAX_FACTORS+1];
   if (n <= 1) return n;
-  nfacs = factor(n, facs);
   totient = 1;
+  /* phi(2m) = 2phi(m) if m even, phi(m) if m odd */
+  while ((n & 0x3) == 0) { n >>= 1; totient <<= 1; }
+  if ((n & 0x1) == 0) { n >>= 1; }
+  /* factor and calculate totient */
+  nfacs = factor(n, facs);
   lastf = 0;
   for (i = 0; i < nfacs; i++) {
     UV f = facs[i];

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