2 * Copyright (C) 2017 Denys Vlasenko <vda.linux@googlemail.com>
4 * Licensed under GPLv2, see file LICENSE in this source tree.
7 //kbuild:lib-y += isqrt.o
12 /* gcc -DISQRT_TEST -Wall -O2 isqrt.c -oisqrt && ./isqrt $((RANDOM*RANDOM)) */
16 # define FAST_FUNC /* nothing */
19 /* Returns such x that x+1 > sqrt(N) */
20 unsigned long FAST_FUNC isqrt(unsigned long long N)
24 #define LL_WIDTH_BITS (unsigned)(sizeof(N)*8)
26 shift = LL_WIDTH_BITS - 2;
30 if ((unsigned long long)x * x > (N >> shift))
31 x--; /* whoops, that +1 was too much */
33 } while ((int)shift >= 0);
38 int main(int argc, char **argv)
40 unsigned long long n = argv[1] ? strtoull(argv[1], NULL, 0) : time(NULL);
46 printf("isqrt(%llx)=%lx\n", n, h);
47 if ((unsigned long long)h * h > n) {
48 printf("BAD1: isqrt(%llx)=%lx\n", n, h);
52 if ((unsigned long long)h * h != 0 /* this can overflow to 0 - not a bug */
53 && (unsigned long long)h * h <= n)
55 printf("BAD2: isqrt(%llx)=%lx\n", n, h);