factor: 30% faster code (estimate max possible factor just once)

function                                             old     new   delta
factorize                                              -     161    +161
isqrt_odd                                              -     102    +102
factor_main                                          281     110    -171
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/1 up/down: 263/-171)           Total: 92 bytes

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

coreutils/factor.c

@@ -20,65 +20,111 @@
 
 #include "libbb.h"
 
+#if 0
+# define dbg(...) bb_error_msg(__VA_ARGS__)
+#else
+# define dbg(...) ((void)0)
+#endif
+
+typedef unsigned long long wide_t;
+#define WIDE_BITS (unsigned)(sizeof(wide_t)*8)
+#define TOPMOST_WIDE_BIT ((wide_t)1 << (WIDE_BITS-1))
+
 #if ULLONG_MAX == (UINT_MAX * UINT_MAX + 2 * UINT_MAX)
 /* "unsigned" is half as wide as ullong */
 typedef unsigned half_t;
+#define HALF_MAX UINT_MAX
 #define HALF_FMT ""
 #elif ULLONG_MAX == (ULONG_MAX * ULONG_MAX + 2 * ULONG_MAX)
 /* long is half as wide as ullong */
 typedef unsigned long half_t;
+#define HALF_MAX ULONG_MAX
 #define HALF_FMT "l"
 #else
 #error Cant find an integer type which is half as wide as ullong
 #endif
 
-static void factorize(const char *numstr)
+/* Returns such x that x+1 > sqrt(N) */
+static inline half_t isqrt(wide_t N)
 {
-	unsigned long long N, factor2;
+	wide_t x;
+	unsigned c;
+
+// Never called with N < 1
+//	if (N == 0)
+//		return 0;
+//
+	/* Count leading zeros */
+	c = 0;
+	while (!(N & TOPMOST_WIDE_BIT)) {
+		c++;
+		N <<= 1;
+	}
+	N >>= c;
+
+	/* Make x > sqrt(n) */
+	x = (wide_t)1 << ((WIDE_BITS + 1 - c) / 2);
+	dbg("x:%llx", x);
+	for (;;) {
+		wide_t y = (x + N/x) / 2;
+		dbg("y:%llx y^2:%llx (y+1)^2:%llx]", y, y*y, (y+1)*(y+1));
+		if (y >= x) {
+			/* Handle degenerate case N = 0xffffffffff...fffffff */
+			if (y == (wide_t)HALF_MAX + 1)
+				y--;
+			dbg("isqrt(%llx)=%llx"HALF_FMT, N, y);
+			return y;
+		}
+		x = y;
+	}
+}
+
+static NOINLINE half_t isqrt_odd(wide_t N)
+{
+	half_t s = isqrt(N);
+	if (s && !(s & 1)) /* even? */
+		s--;
+	return s;
+}
+
+static NOINLINE void factorize(wide_t N)
+{
+	wide_t factor2;
 	half_t factor;
+	half_t max_factor;
 	unsigned count3;
 
-	/* Coreutils compat */
-	numstr = skip_whitespace(numstr);
-	if (*numstr == '+')
-		numstr++;
-
-	N = bb_strtoull(numstr, NULL, 10);
-	if (errno)
-		bb_show_usage();
-
-	printf("%llu:", N);
-
 	if (N < 4)
 		goto end;
+
 	while (!(N & 1)) {
 		printf(" 2");
 		N >>= 1;
 	}
 
+	max_factor = isqrt_odd(N);
 	count3 = 3;
 	factor = 3;
 	factor2 = 3 * 3;
 	for (;;) {
-		unsigned long long nfactor2;
-
 		while ((N % factor) == 0) {
 			N = N / factor;
 			printf(" %u"HALF_FMT"", factor);
+			max_factor = isqrt_odd(N);
 		}
  next_factor:
+		if (factor >= max_factor)
+			break;
 		/* (f + 2)^2 = f^2 + 4*f + 4 = f^2 + 4*(f+1) */
-		nfactor2 = factor2 + 4 * (factor + 1);
+		factor2 = factor2 + 4 * (factor + 1);
-		if (nfactor2 < factor2) /* overflow? */
+		/* overflow is impossible due to max_factor check */
-			break;
+		/* (factor2 > N) is impossible due to max_factor check */
-		factor2 = nfactor2;
-		if (factor2 > N)
-			break;
 		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...
-		 * ^  ^     ^   ^       ^  ^     ^  _     ^  ^     _  ^ (^primes)
+		 * ^ ^   ^  ^     ^  ^     ^  _     ^  ^     _  ^
+		 * (^ = primes, _ = would-be-primes-if-not-divisible-by-5)
 		 */
 		--count3;
 		if (count3 == 0) {
@@ -105,8 +151,20 @@ int factor_main(int argc UNUSED_PARAM, char **argv)
 		bb_show_usage();
 
 	do {
-		factorize(*argv);
+		wide_t N;
+		const char *numstr;
+
+		/* Coreutils compat */
+		numstr = skip_whitespace(*argv);
+		if (*numstr == '+')
+			numstr++;
+
+		N = bb_strtoull(numstr, NULL, 10);
+		if (errno)
+			bb_show_usage();
+		printf("%llu:", N);
+		factorize(N);
 	} while (*++argv);
 
-	fflush_stdout_and_exit(EXIT_SUCCESS);
+	return EXIT_SUCCESS;
 }