feat: assign change utxo index in transaction

packages/mesh-common/src/types/transaction-builder/index.ts

@@ -29,6 +29,7 @@ export type MeshTxBuilderBody = {
   referenceInputs: RefTxIn[];
   mints: MintParam[];
   changeAddress: string;
+  changeAddressOutputIndex?: number;
   metadata: TxMetadata;
   scriptMetadata: ScriptMetadata[];
   validityRange: ValidityRange;

packages/mesh-transaction/src/mesh-tx-builder/index.ts

@@ -313,6 +313,22 @@ export class MeshTxBuilder extends MeshTxBuilderCore {
 
     this.meshTxBuilderBody.fee = selectionSkeleton.fee.toString();
     this.queueAllLastItem();
+
+    const changeLen = selectionSkeleton.change.length;
+    if (this.meshTxBuilderBody.changeAddressOutputIndex !== undefined && changeLen > 0) {
+      let idx = this.meshTxBuilderBody.changeAddressOutputIndex;
+      const changes = this.meshTxBuilderBody.outputs.splice(-changeLen, changeLen);
+
+      const maxIdx = this.meshTxBuilderBody.outputs.length;
+      if (idx > maxIdx) {
+        idx = maxIdx;
+      } else if (idx < 0) {
+        idx = 0;
+      }
+
+      this.meshTxBuilderBody.outputs.splice(idx, 0, ...changes);
+    }
+
     this.removeDuplicateInputs();
     this.sortTxParts();
     this.updateRedeemer(

packages/mesh-transaction/src/mesh-tx-builder/tx-builder-core.ts

@@ -1519,10 +1519,14 @@ export class MeshTxBuilderCore {
   /**
    * Configure the address to accept change UTxO
    * @param addr The address to accept change UTxO
+   * @param outputIndex The index at which the change output should be placed
    * @returns The MeshTxBuilder instance
    */
-  changeAddress = (addr: string) => {
+  changeAddress = (addr: string, outputIndex?: number) => {
     this.meshTxBuilderBody.changeAddress = addr;
+    if (outputIndex !== undefined) {
+      this.meshTxBuilderBody.changeAddressOutputIndex = outputIndex;
+    }
     return this;
   };
 

packages/mesh-transaction/test/mesh-tx-builder/tx.test.ts

@@ -36,6 +36,45 @@ describe("MeshTxBuilder transactions", () => {
     expect(txHex !== "").toBeTruthy();
   });
 
+  it("Basic send tx with change output index", async () => {
+    let mesh = new MeshTxBuilder();
+    let txHex = await mesh
+      .txIn(
+        "2cb57168ee66b68bd04a0d595060b546edf30c04ae1031b883c9ac797967dd85",
+        3,
+        [{ unit: "lovelace", quantity: "9891607895" }],
+        "addr_test1vru4e2un2tq50q4rv6qzk7t8w34gjdtw3y2uzuqxzj0ldrqqactxh",
+        0
+      )
+      .txOut(
+        "addr_test1vru4e2un2tq50q4rv6qzk7t8w34gjdtw3y2uzuqxzj0ldrqqactxh",
+        [{ unit: "lovelace", quantity: "2000000" }],
+      )
+      .txOut(
+        "addr_test1vru4e2un2tq50q4rv6qzk7t8w34gjdtw3y2uzuqxzj0ldrqqactxh",
+        [{ unit: "lovelace", quantity: "3000000" }],
+      )
+      .changeAddress(
+        "addr_test1wql6cyymfrmqe9cjeyfh5d4h945nfszy3yup8d74kkrhsks4dkk0y",
+        1
+      )
+      .complete();
+
+    const cardanoTx = Serialization.Transaction.fromCbor(
+      Serialization.TxCBOR(txHex),
+    );
+    const outputs = cardanoTx.body().outputs();
+    // 3 outputs expected: 2 explicit, 1 change
+    expect(outputs.length).toBe(3);
+
+    // The change output should be at index 1 (between the two explicit outputs)
+    const changeOutput = outputs[1];
+    expect(changeOutput!.address().toBech32()).toBe("addr_test1wql6cyymfrmqe9cjeyfh5d4h945nfszy3yup8d74kkrhsks4dkk0y");
+    // Verify explicit outputs are pushed to indices 0 and 2
+    expect(outputs[0]!.amount().coin().toString()).toBe("2000000");
+    expect(outputs[2]!.amount().coin().toString()).toBe("3000000");
+  });
+
   it("Basic send tx with set fee", () => {
     let mesh = new MeshTxBuilder();
     let txHex = mesh