test-mergesort: use repeatable random numbers

Use MINSTD to generate pseudo-random numbers consistently instead of
using rand(3), whose output can vary from system to system, and reset
its seed before filling in the test values.  This gives repeatable
results across versions and systems, which simplifies sharing and
comparing of results between developers.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

t/helper/test-mergesort.c

@@ -2,6 +2,12 @@
 #include "cache.h"
 #include "mergesort.h"
 
+static uint32_t minstd_rand(uint32_t *state)
+{
+	*state = (uint64_t)*state * 48271 % 2147483647;
+	return *state;
+}
+
 struct line {
 	char *text;
 	struct line *next;
@@ -60,8 +66,9 @@ static void dist_sawtooth(int *arr, int n, int m)
 static void dist_rand(int *arr, int n, int m)
 {
 	int i;
+	uint32_t seed = 1;
 	for (i = 0; i < n; i++)
-		arr[i] = rand() % m;
+		arr[i] = minstd_rand(&seed) % m;
 }
 
 static void dist_stagger(int *arr, int n, int m)
@@ -81,8 +88,9 @@ static void dist_plateau(int *arr, int n, int m)
 static void dist_shuffle(int *arr, int n, int m)
 {
 	int i, j, k;
+	uint32_t seed = 1;
 	for (i = j = 0, k = 1; i < n; i++)
-		arr[i] = (rand() % m) ? (j += 2) : (k += 2);
+		arr[i] = minstd_rand(&seed) % m ? (j += 2) : (k += 2);
 }
 
 #define DIST(name) { #name, dist_##name }