feat(utxo-lib): parse basic info from the psbt

TICKET: BTC-637

modules/utxo-lib/src/bitgo/index.ts

@@ -10,6 +10,7 @@ export * from './dash';
 export * from './parseInput';
 export * from './signature';
 export * from './transaction';
+export * from './transactionAmounts';
 export * from './types';
 export * from './Unspent';
 export * from './UtxoPsbt';

modules/utxo-lib/src/bitgo/transactionAmounts.ts

@@ -0,0 +1,37 @@
+import { UtxoPsbt } from './UtxoPsbt';
+import { createTransactionFromBuffer } from './transaction';
+
+export function getTransactionAmountsFromPsbt(psbt: UtxoPsbt): {
+  inputCount: number;
+  outputCount: number;
+  inputAmount: bigint;
+  outputAmount: bigint;
+  fee: bigint;
+} {
+  const inputCount = psbt.data.inputs.length;
+  const outputCount = psbt.data.outputs.length;
+  const txInputs = psbt.txInputs;
+  const txOutputs = psbt.txOutputs;
+  const inputAmount = psbt.data.inputs.reduce((acc, input, inputIndex) => {
+    if (input.witnessUtxo) {
+      return acc + BigInt(input.witnessUtxo.value);
+    } else if (input.nonWitnessUtxo) {
+      const tx = createTransactionFromBuffer(input.nonWitnessUtxo, psbt.network, { amountType: 'bigint' });
+      return acc + tx.outs[txInputs[inputIndex].index].value;
+    } else {
+      throw new Error('missing witnessUtxo and nonWitnessUtxo');
+    }
+  }, BigInt(0));
+  const outputAmount = psbt.data.outputs.reduce(
+    (acc, output, outputIndex) => acc + txOutputs[outputIndex].value,
+    BigInt(0)
+  );
+  const fee = inputAmount - outputAmount;
+  return {
+    inputCount,
+    outputCount,
+    inputAmount,
+    outputAmount,
+    fee,
+  };
+}

modules/utxo-lib/test/bitgo/psbt/Psbt.ts

@@ -33,6 +33,8 @@ import {
   deleteWitnessUtxoForNonSegwitInputs,
   getPsbtInputScriptType,
   withUnsafeNonSegwit,
+  getTransactionAmountsFromPsbt,
+  WalletUnspent,
 } from '../../../src/bitgo';
 import {
   createOutputScript2of3,
@@ -876,8 +878,9 @@ function testUtxoPsbt(coinNetwork: Network) {
   describe(`Testing UtxoPsbt (de)serialization for ${getNetworkName(coinNetwork)} network`, function () {
     let psbt: UtxoPsbt;
     let psbtHex: string;
+    let unspents: (WalletUnspent<bigint> | Unspent<bigint>)[];
     before(async function () {
-      const unspents = mockUnspents(rootWalletKeys, ['p2sh'], BigInt('10000000000000'), coinNetwork);
+      unspents = mockUnspents(rootWalletKeys, ['p2sh'], BigInt('10000000000000'), coinNetwork);
       const txBuilderParams = {
         signer: 'user',
         cosigner: 'bitgo',
@@ -905,6 +908,15 @@ function testUtxoPsbt(coinNetwork: Network) {
       assert.deepStrictEqual(createPsbtFromHex(psbtHex, coinNetwork, false).toBuffer(), psbt.toBuffer());
     });
 
+    it('should be able to get transaction info from psbt', function () {
+      const txInfo = getTransactionAmountsFromPsbt(psbt);
+      assert.strictEqual(txInfo.fee, FEE);
+      assert.strictEqual(txInfo.inputCount, unspents.length);
+      assert.strictEqual(txInfo.inputAmount, BigInt('10000000000000') * BigInt(unspents.length));
+      assert.strictEqual(txInfo.outputAmount, BigInt('10000000000000') * BigInt(unspents.length) - FEE);
+      assert.strictEqual(txInfo.outputCount, psbt.data.outputs.length);
+    });
+
     function deserializeBip32PathsCorrectly(bip32PathsAbsolute: boolean): void {
       function checkDerivationPrefix(bip32Derivation: { path: string }): void {
         const path = bip32Derivation.path.split('/');