New utility routine: sort with a context parameter.

I'm about to have a need to sort an array based on auxiliary data held
in a variable that's not globally accessible, so I need a sort routine
that accepts an extra parameter and passes it through to the compare
function.

Sorting algorithm is heapsort, because it's the N log N algorithm I
can implement most reliably.

puzzles.h

@@ -590,6 +590,20 @@ bool findloop_run(struct findloopstate *state, int nvertices,
  */
 bool findloop_is_loop_edge(struct findloopstate *state, int u, int v);
 
+/*
+ * Helper function to sort an array. Differs from standard qsort in
+ * that it takes a context parameter that is passed to the compare
+ * function.
+ *
+ * I wrap it in a macro so that you only need to give the element
+ * count of the array. The element size is determined by sizeof.
+ */
+typedef int (*arraysort_cmpfn_t)(const void *av, const void *bv, void *ctx);
+void arraysort_fn(void *array, size_t nmemb, size_t size,
+                  arraysort_cmpfn_t cmp, void *ctx);
+#define arraysort(array, nmemb, cmp, ctx) \
+    arraysort_fn(array, nmemb, sizeof(*(array)), cmp, ctx)
+
 /*
  * Data structure containing the function calls and data specific
  * to a particular game. This is enclosed in a data structure so