libbb: move isqrt from factor, use it in diff too

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

coreutils/factor.c

@@ -44,24 +44,7 @@ typedef unsigned long half_t;
 #error Cant find an integer type which is half as wide as ullong
 #endif
 
-/* Returns such x that x+1 > sqrt(N) */
+static half_t isqrt_odd(wide_t N)
-static inline half_t isqrt(wide_t N)
-{
-	half_t x;
-	unsigned shift;
-
-	shift = WIDE_BITS - 2;
-	x = 0;
-	do {
-		x = (x << 1) + 1;
-		if ((wide_t)x * x > (N >> shift))
-			x--; /* whoops, that +1 was too much */
-		shift -= 2;
-	} while ((int)shift >= 0);
-	return x;
-}
-
-static NOINLINE half_t isqrt_odd(wide_t N)
 {
 	half_t s = isqrt(N);
 	/* Subtract 1 from even s, odd s won't change: */

editors/diff.c

@@ -295,17 +295,6 @@ static int search(const int *c, int k, int y, const struct cand *list)
 	}
 }
 
-static unsigned isqrt(unsigned n)
-{
-	unsigned x = 1;
-	while (1) {
-		const unsigned y = x;
-		x = ((n / x) + x) >> 1;
-		if (x <= (y + 1) && x >= (y - 1))
-			return x;
-	}
-}
-
 static void stone(const int *a, int n, const int *b, int *J, int pref)
 {
 	const unsigned isq = isqrt(n);

include/libbb.h

@@ -342,6 +342,8 @@ extern int *const bb_errno;
 uint64_t bb_bswap_64(uint64_t x) FAST_FUNC;
 #endif
 
+unsigned long FAST_FUNC isqrt(unsigned long long N);
+
 unsigned long long monotonic_ns(void) FAST_FUNC;
 unsigned long long monotonic_us(void) FAST_FUNC;
 unsigned long long monotonic_ms(void) FAST_FUNC;

libbb/isqrt.c

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2017 Denys Vlasenko <vda.linux@googlemail.com>
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+
+//kbuild:lib-y += isqrt.o
+
+#ifndef ISQRT_TEST
+# include "libbb.h"
+#else
+/* gcc -DISQRT_TEST -Wall -O2 isqrt.c -oisqrt && ./isqrt $((RANDOM*RANDOM)) */
+# include <stdlib.h>
+# include <stdio.h>
+# include <time.h>
+# define FAST_FUNC /* nothing */
+#endif
+
+/* Returns such x that x+1 > sqrt(N) */
+unsigned long FAST_FUNC isqrt(unsigned long long N)
+{
+	unsigned long x;
+	unsigned shift;
+#define LL_WIDTH_BITS (unsigned)(sizeof(N)*8)
+
+	shift = LL_WIDTH_BITS - 2;
+	x = 0;
+	do {
+		x = (x << 1) + 1;
+		if ((unsigned long long)x * x > (N >> shift))
+			x--; /* whoops, that +1 was too much */
+		shift -= 2;
+	} while ((int)shift >= 0);
+	return x;
+}
+
+#ifdef ISQRT_TEST
+int main(int argc, char **argv)
+{
+	unsigned long long n = argv[1] ? strtoull(argv[1], NULL, 0) : time(NULL);
+	for (;;) {
+		unsigned long h;
+		n--;
+		h = isqrt(n);
+		if (!(n & 0xffff))
+			printf("isqrt(%llx)=%lx\n", n, h);
+		if ((unsigned long long)h * h > n) {
+			printf("BAD1: isqrt(%llx)=%lx\n", n, h);
+			return 1;
+		}
+		h++;
+		if ((unsigned long long)h * h != 0 /* this can overflow to 0 - not a bug */
+		 && (unsigned long long)h * h <= n)
+		{
+			printf("BAD2: isqrt(%llx)=%lx\n", n, h);
+			return 1;
+		}
+	}
+}
+#endif