factor: improve comments for sieving logic

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/coreutils/factor.c b/coreutils/factor.c
index 7adc2385a..5e23c6ca7 100644
--- a/coreutils/factor.c
+++ b/coreutils/factor.c
@@ -73,7 +73,7 @@ static NOINLINE void factorize(wide_t N)
                 MULTIPLE_OF_3 = 1 << 2,
                 MULTIPLE_OF_5 = 1 << 6,
                 MULTIPLE_OF_7 = 1 << 11,
-                MULTIPLE_3_5_7 = MULTIPLE_OF_3 | MULTIPLE_OF_5 | MULTIPLE_OF_7,
+                MULTIPLE_DETECTED = MULTIPLE_OF_3 | MULTIPLE_OF_5 | MULTIPLE_OF_7,
         };
         unsigned sieve_word;
 
@@ -102,9 +102,13 @@ static NOINLINE void factorize(wide_t N)
         // count5 = 6;
         // count7 = 9;
         sieve_word = 0
+                /* initial count for SHIFT_n is (n-1)/2*3: */
                 + (MULTIPLE_OF_3 - 3 * SHIFT_3)
                 + (MULTIPLE_OF_5 - 6 * SHIFT_5)
                 + (MULTIPLE_OF_7 - 9 * SHIFT_7)
+                //+ (MULTIPLE_OF_11 - 15 * SHIFT_11)
+                //+ (MULTIPLE_OF_11 - 18 * SHIFT_13)
+                //+ (MULTIPLE_OF_11 - 24 * SHIFT_17)
         ;
         factor = 3;
         for (;;) {
@@ -133,7 +137,7 @@ static NOINLINE void factorize(wide_t N)
                 // if (count3 && count5 && count7)
                 //      continue;
                 sieve_word += INCREMENT_EACH;
-                if (!(sieve_word & MULTIPLE_3_5_7))
+                if (!(sieve_word & MULTIPLE_DETECTED))
                         continue;
                 /*
                  * "factor" is multiple of 3 33% of the time (count3 reached 0),