> 2025-07-15 Bruno Haible <br...@clisp.org>
>
> next-prime tests: Update unit test after last change.
> * tests/test-next-prime.c (main): Allow either behaviour.
Oops. It turns out that
- is_prime (3) returns false, leading to next_prime (3) = 5.
- the unit test fails because next_prime (1) = 1.
This fixes it.
2025-07-19 Bruno Haible <br...@clisp.org>
next-prime: Tweaks for GNU gettext.
* lib/next-prime.c (is_prime): Change so that is_prime (3) returns true.
* tests/test-next-prime.c (main): Allow next_prime (1) to be 1.
diff --git a/lib/next-prime.c b/lib/next-prime.c
index 58630db9b0..9aa3264f31 100644
--- a/lib/next-prime.c
+++ b/lib/next-prime.c
@@ -21,22 +21,25 @@
#include <stdint.h> /* for SIZE_MAX */
-/* Return true if CANDIDATE is a prime number. CANDIDATE should be an odd
- number at least equal to 11. */
+/* Return true if CANDIDATE is a prime number or 1.
+ CANDIDATE should be an odd number >= 1. */
static bool _GL_ATTRIBUTE_CONST
is_prime (size_t candidate)
{
size_t divisor = 3;
size_t square = divisor * divisor;
- while (square < candidate && (candidate % divisor))
+ for (;;)
{
+ if (square > candidate)
+ return true;
+ if ((candidate % divisor) == 0)
+ return false;
+ /* Increment divisor by 2. */
divisor++;
square += 4 * divisor;
divisor++;
}
-
- return (candidate % divisor ? true : false);
}
size_t _GL_ATTRIBUTE_CONST
diff --git a/tests/test-next-prime.c b/tests/test-next-prime.c
index b74257ae83..a5a2e9133c 100644
--- a/tests/test-next-prime.c
+++ b/tests/test-next-prime.c
@@ -26,8 +26,9 @@
int
main ()
{
- ASSERT (next_prime (1) >= 3 && next_prime (1) <= 11);
+ ASSERT (next_prime (1) >= 1 && next_prime (1) <= 11);
ASSERT (next_prime (3) >= 3 && next_prime (3) <= 11);
+ ASSERT (next_prime (5) >= 5 && next_prime (5) <= 11);
ASSERT (next_prime (7) >= 7 && next_prime (7) <= 11);
ASSERT (next_prime (10) == 11);
ASSERT (next_prime (11) == 11);
