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

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

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