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