Fix infinite loop in kiss_fft_next_fast_size for n<=0 (issue #121)

kiss_fft_next_fast_size(0) never terminated because the factorization
loop divides m by 2/3/5 forever when m stays 0. Per maintainer decision,
return 1 for n<=0. Add regression test next_fast_size covering n=0,
n<0 and positive inputs.

kiss_fft.c

@@ -411,6 +411,8 @@ void kiss_fft_cleanup(void)
 
 int kiss_fft_next_fast_size(int n)
 {
+    if (n <= 0) /* nothing to factor; avoid infinite loop for n<=0 */
+        return 1;
     while(1) {
         int m=n;
         while ( (m%2) == 0 ) m/=2;

test/CMakeLists.txt

@@ -49,6 +49,8 @@ target_compile_definitions(tkfc PRIVATE KFC_TEST)
 add_kissfft_test_executable(ffr twotonetest.c)
 add_kissfft_test_executable(tr test_real.c)
 
+add_kissfft_test_executable(next_fast_size next_fast_size.c)
+
 add_kissfft_test_executable(testcpp testcpp.cc)
 
 if(KISSFFT_DATATYPE MATCHES "^simd$")

test/next_fast_size.c

@@ -0,0 +1,39 @@
+/*
+ *  Copyright (c) 2003-2010, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ *
+ *  Regression test for github issue #121: kiss_fft_next_fast_size(0)
+ *  used to hang forever in the factorization loop.
+ */
+#include "kiss_fft.h"
+#include <stdio.h>
+
+int main(void)
+{
+    int n;
+    /* n<=0 must return 1 per maintainer decision, not hang */
+    if (kiss_fft_next_fast_size(0) != 1) {
+        fprintf(stderr, "FAIL: kiss_fft_next_fast_size(0) != 1\n");
+        return 1;
+    }
+    if (kiss_fft_next_fast_size(-3) != 1) {
+        fprintf(stderr, "FAIL: kiss_fft_next_fast_size(-3) != 1\n");
+        return 1;
+    }
+    /* positive inputs keep their pre-fix behavior */
+    {
+        int expected[9] = { 0, 1, 2, 3, 4, 5, 6, 8, 8 };
+        for (n = 1; n <= 8; ++n) {
+            if (kiss_fft_next_fast_size(n) != expected[n]) {
+                fprintf(stderr, "FAIL: kiss_fft_next_fast_size(%d) = %d, expected %d\n",
+                        n, kiss_fft_next_fast_size(n), expected[n]);
+                return 1;
+            }
+        }
+    }
+    printf("kiss_fft_next_fast_size: all checks passed\n");
+    return 0;
+}