don't make quickselect "destructive"; recompile

squiggle_more.c

@@ -6,6 +6,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h> // memcpy
 
 /* Parallel sampler */
 void sampler_parallel(double (*sampler)(uint64_t* seed), double* results, int n_threads, int n_samples)
@@ -105,15 +106,24 @@ static int partition(int low, int high, double xs[], int length)
 static double quickselect(int k, double xs[], int n)
 {
     // https://en.wikipedia.org/wiki/Quickselect
+
+    double *ys = malloc((size_t)n * sizeof(double));
+    memcpy(ys, xs, (size_t)n * sizeof(double));
+    // ^: don't make this operation "destructive"
+
     int low = 0;
     int high = n - 1;
     for (;;) {
         if (low == high) {
-            return xs[low];
+            double result = ys[low];
+            free(ys);
+            return result;
         }
-        int pivot = partition(low, high, xs, n);
+        int pivot = partition(low, high, ys, n);
         if (pivot == k) {
-            return xs[pivot];
+            double result = ys[pivot];
+            free(ys);
+            return result;
         } else if (k < pivot) {
             high = pivot - 1;
         } else {