keep same target ordering

previously, the silent payment outputs were being added at the end
of the target array, which changes the order from what the caller requested.

instead, keep track of the original index while creating silent payment
outputs and return the targets in the original order.

src/index.ts

@@ -23,7 +23,7 @@ export type Target = {
 
 export type SilentPaymentGroup = {
   Bscan: Buffer;
-  BmValues: Array<[Buffer, number | undefined]>;
+  BmValues: Array<[Buffer, number | undefined, number]>;
 };
 
 function taggedHash(tag: string, data: Buffer): Buffer {
@@ -45,12 +45,13 @@ export class SilentPayment {
    * Numeric values (if present) for targets are passed through.
    */
   createTransaction(utxos: UTXO[], targets: Target[]): Target[] {
-    const ret: Target[] = [];
+    const ret: Target[] = new Array(targets.length);
 
     const silentPaymentGroups: Array<SilentPaymentGroup> = [];
-    for (const target of targets) {
+    for (let i = 0; i < targets.length; i++) {
+      const target = targets[i];
       if (!target.address?.startsWith("sp1")) {
-        ret.push(target); // passthrough
+        ret[i] = target; // passthrough
         continue;
       }
 
@@ -66,11 +67,11 @@ export class SilentPayment {
       // Addresses with the same Bscan key all belong to the same recipient
       const recipient = silentPaymentGroups.find((group) => Buffer.compare(group.Bscan, Bscan) === 0);
       if (recipient) {
-        recipient.BmValues.push([Bm, target.value]);
+        recipient.BmValues.push([Bm, target.value, i]);
       } else {
         silentPaymentGroups.push({
           Bscan: Bscan,
-          BmValues: [[Bm, target.value]],
+          BmValues: [[Bm, target.value, i]],
         });
       }
     }
@@ -87,7 +88,7 @@ export class SilentPayment {
       const ecdh_shared_secret = Buffer.from(ecc.getSharedSecret(ecdh_shared_secret_step1, group.Bscan) as Uint8Array);
 
       let k = 0;
-      for (const [Bm, amount] of group.BmValues) {
+      for (const [Bm, amount, i] of group.BmValues) {
         const tk = taggedHash("BIP0352/SharedSecret", Buffer.concat([ecdh_shared_secret!, SilentPayment._ser32(k)]));
 
         // Let Pmk = tk·G + Bm
@@ -97,7 +98,7 @@ export class SilentPayment {
         const address = SilentPayment.pubkeyToAddress(Pmk.slice(1).toString("hex"));
         const newTarget: Target = { address };
         newTarget.value = amount;
-        ret.push(newTarget);
+        ret[i] = newTarget;
         k += 1;
       }
     }