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-rw-r--r--libbb/isqrt.c60
1 files changed, 60 insertions, 0 deletions
diff --git a/libbb/isqrt.c b/libbb/isqrt.c
new file mode 100644
index 000000000..817b7d036
--- /dev/null
+++ b/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