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

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

4 files changed, 63 insertions, 29 deletions

diff --git a/coreutils/factor.c b/coreutils/factor.c
index f910fdb44..11cc04f92 100644
--- a/coreutils/factor.c
+++ b/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 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)
+static half_t isqrt_odd(wide_t N)
 {
         half_t s = isqrt(N);
         /* Subtract 1 from even s, odd s won't change: */
diff --git a/editors/diff.c b/editors/diff.c
index 0eb825cfb..3304edb26 100644
--- a/editors/diff.c
+++ b/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);
diff --git a/include/libbb.h b/include/libbb.h
index a2c699b54..04071639a 100644
--- a/include/libbb.h
+++ b/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;
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