Add secp256k1_pubkey_sort

Co-authored-by: Tim Ruffing <crypto@timruffing.de>
Co-authored-by: Russell O'Connor <roconnor@blockstream.io>

src/hsort.h

@@ -0,0 +1,33 @@
+/***********************************************************************
+ * Copyright (c) 2021 Russell O'Connor, Jonas Nick                     *
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_HSORT_H
+#define SECP256K1_HSORT_H
+
+#include <stddef.h>
+#include <string.h>
+
+/* In-place, iterative heapsort with an interface matching glibc's qsort_r. This
+ * is preferred over standard library implementations because they generally
+ * make no guarantee about being fast for malicious inputs.
+ * Remember that heapsort is unstable.
+ *
+ * In/Out: ptr: pointer to the array to sort. The contents of the array are
+ *              sorted in ascending order according to the comparison function.
+ * In:   count: number of elements in the array.
+ *        size: size in bytes of each element.
+ *         cmp: pointer to a comparison function that is called with two
+ *              arguments that point to the objects being compared. The cmp_data
+ *              argument of secp256k1_hsort is passed as third argument. The
+ *              function must return an integer less than, equal to, or greater
+ *              than zero if the first argument is considered to be respectively
+ *              less than, equal to, or greater than the second.
+ *    cmp_data: pointer passed as third argument to cmp.
+ */
+static void secp256k1_hsort(void *ptr, size_t count, size_t size,
+                            int (*cmp)(const void *, const void *, void *),
+                            void *cmp_data);
+#endif