silentpayments: add sortable recipient struct

Author: josibake

include/secp256k1_silentpayments.h

@@ -24,6 +24,21 @@ extern "C" {
  * implementation without requiring any further elliptic-curve operations from the wallet.
  */
 
+/* This struct serves as an In param for passing the silent payment address data.
+ * The index field is for when more than one address is being sent to in a transaction. Index is
+ * set based on the original ordering of the addresses and used to return the generated outputs
+ * matching the original ordering. When more than one recipient is used the recipient array
+ * will be sorted in place as part of generating the outputs, but the generated outputs will be
+ * outputs will be returned original ordering specified by the index to ensure the caller is able
+ * to match up the generated outputs to the correct silent payment address (e.g. to be able to
+ * assign the correct amounts to the correct generatetd outputs in the final transaction).
+ */
+typedef struct {
+    secp256k1_pubkey scan_pubkey;
+    secp256k1_pubkey spend_pubkey;
+    size_t index;
+} secp256k1_silentpayments_recipient;
+
 #ifdef __cplusplus
 }
 #endif

src/modules/silentpayments/main_impl.h

@@ -9,8 +9,32 @@
 #include "../../../include/secp256k1.h"
 #include "../../../include/secp256k1_silentpayments.h"
 
-/* TODO: implement functions for sender side. */
+/** Sort an array of silent payment recipients. This is used to group recipients by scan pubkey to
+ *  ensure the correct values of k are used when creating multiple outputs for a recipient. */
+static int secp256k1_silentpayments_recipient_sort_cmp(const void* pk1, const void* pk2, void *ctx) {
+    return secp256k1_ec_pubkey_cmp((secp256k1_context *)ctx,
+        &(*(const secp256k1_silentpayments_recipient **)pk1)->scan_pubkey,
+        &(*(const secp256k1_silentpayments_recipient **)pk2)->scan_pubkey
+    );
+}
 
-/* TODO: implement functions for receiver side. */
+int secp256k1_silentpayments_recipient_sort(const secp256k1_context* ctx, const secp256k1_silentpayments_recipient **recipients, size_t n_recipients) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(recipients != NULL);
+
+    /* Suppress wrong warning (fixed in MSVC 19.33) */
+    #if defined(_MSC_VER) && (_MSC_VER < 1933)
+    #pragma warning(push)
+    #pragma warning(disable: 4090)
+    #endif
+
+    secp256k1_hsort(recipients, n_recipients, sizeof(*recipients), secp256k1_silentpayments_recipient_sort_cmp, (void *)ctx);
+
+    #if defined(_MSC_VER) && (_MSC_VER < 1933)
+    #pragma warning(pop)
+    #endif
+
+    return 1;
+}
 
 #endif