facility to inject a non-random random generator

This should be handy for writing tests that make use of randomization.

src/tests.c

@@ -23,6 +23,7 @@
 #include <util/language.h>
 #include <util/message.h>
 #include <util/log.h>
+#include <util/rand.h>
 
 #include <CuTest.h>
 
@@ -103,6 +104,8 @@ void test_cleanup(void)
         errno = 0;
         log_error("errno: %d", error);
     }
+
+    random_source_reset();
 }
 
 terrain_type *

src/util/rand.c

@@ -21,6 +21,7 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #include "rng.h"
 
 #include <assert.h>
+#include <stdlib.h>
 #include <string.h>
 #include <math.h>
 #include <float.h>
@@ -65,3 +66,66 @@ bool chance(double x)
         return true;
     return rng_double() < x;
 }
+
+extern double genrand_real2(void);
+
+
+typedef struct random_source {
+    double (*double_source) (void);
+} random_source;
+
+random_source *r_source = 0;
+
+double rng_injectable_double(void) {
+    if (r_source)
+        return r_source->double_source();
+    return genrand_real2();
+}
+
+static double constant_value;
+
+static double constant_source (void) {
+    return constant_value;
+}
+
+struct random_source constant_provider = {
+    constant_source
+};
+
+void random_source_inject_constant(double value) {
+    constant_value = value;
+    r_source = &constant_provider;
+}
+
+static int i = 0;
+static double *values;
+static int value_size = 0;
+
+static double array_source (void) {
+    assert(i<value_size);
+    return values[i++];
+}
+
+struct random_source array_provider = {
+    array_source
+};
+
+void random_source_inject_array(double inject[], int size) {
+    assert(size > 0);
+    value_size = size;
+    if (values)
+        free(values);
+    values = malloc(sizeof(double) * size);
+    for (i=0; i < size; ++i) {
+        values[i] = inject[i];
+    }
+    i = 0;
+    r_source = &array_provider;
+}
+
+void random_source_reset(void) {
+    if (values)
+        free(values);
+    values = NULL;
+    r_source = NULL;
+}

src/util/rand.h

@@ -31,6 +31,15 @@ extern "C" {
     extern int ntimespprob(int n, double p, double mod);
     extern bool chance(double x);
 
+    /* a random source that generates numbers in [0, 1).
+       By calling the random_source_inject... functions you can set a special random source,
+       which is handy for testing */
+    double rng_injectable_double(void);
+
+    void random_source_inject_constant(double value);
+    void random_source_inject_array(double inject[], int size);
+    void random_source_reset(void);
+
 #ifdef __cplusplus
 }
 #endif

src/util/rng.h

@@ -23,7 +23,7 @@ extern "C" {
   extern unsigned long genrand_int32(void);
 
   /* generates a random number on [0,1)-real-interval */
-  extern double genrand_real2(void);
+  extern double rng_injectable_double(void);
 
   /* generates a random number on [0,0x7fffffff]-interval */
   long genrand_int31(void);
@@ -31,7 +31,7 @@ extern "C" {
 # define rng_init(seed) init_genrand(seed)
 # define rng_int (int)genrand_int31
 # define rng_uint (unsigned int)genrand_int32
-# define rng_double genrand_real2
+# define rng_double rng_injectable_double
 # define RNG_RAND_MAX 0x7fffffff
 #else
 # include <stdlib.h>