Update comment

Progress on #117.

ryu/d2s.c

@@ -307,11 +307,10 @@ static inline floating_decimal_64 d2d(const uint64_t ieeeMantissa, const uint32_
         --vp;
       }
     } else if (q < 63) { // TODO(ulfjack): Use a tighter bound here.
-      // We need to compute min(ntz(mv), pow5Factor(mv) - e2) >= q - 1
-      // <=> ntz(mv) >= q - 1 && pow5Factor(mv) - e2 >= q - 1
-      // <=> ntz(mv) >= q - 1 (e2 is negative and -e2 >= q)
-      // <=> (mv & ((1 << (q - 1)) - 1)) == 0
-      // We also need to make sure that the left shift does not overflow.
+      // We want to know if the full product has at least q trailing zeros.
+      // We need to compute min(p2(mv), p5(mv) - e2) >= q
+      // <=> p2(mv) >= q && p5(mv) - e2 >= q
+      // <=> p2(mv) >= q (because -e2 >= q)
       vrIsTrailingZeros = multipleOfPowerOf2(mv, q);
 #ifdef RYU_DEBUG
       printf("vr is trailing zeros=%s\n", vrIsTrailingZeros ? "true" : "false");