Add functionality to randomize outputs

lib/transaction/transaction.js

@@ -805,6 +805,40 @@ Transaction.prototype.removeOutput = function(index) {
   this._updateChangeOutput();
 };
 
+/**
+ * Randomize this transaction's outputs ordering. The shuffling algorithm is a
+ * version of the Fisher-Yates shuffle, provided by lodash's _.shuffle().
+ *
+ * @return {Transaction} this
+ */
+Transaction.prototype.shuffleOutputs = function() {
+  return this.sortOutputs(_.shuffle);
+};
+
+/**
+ * Sort this transaction's outputs, according to a given sorting function that
+ * takes an array as argument and returns a new array, with the same elements
+ * but with a different order. The argument function MUST NOT modify the order
+ * of the original array
+ *
+ * @param {Function} sortingFunction
+ * @return {Transaction} this
+ */
+Transaction.prototype.sortOutputs = function(sortingFunction) {
+  var outs = sortingFunction(this.outputs);
+  return this._newOutputOrder(outs);
+};
+
+Transaction.prototype._newOutputOrder = function(newOutputs) {
+  var changeIndex = 0;
+  while (this.outputs[this._changeIndex] !== newOutputs[changeIndex]) {
+    changeIndex++;
+  }
+  this.outputs = newOutputs;
+  this._changeIndex = changeIndex;
+  return this;
+};
+
 Transaction.prototype.removeInput = function(txId, outputIndex) {
   var index;
   if (!outputIndex && _.isNumber(txId)) {

package.json

@@ -91,7 +91,8 @@
     "bitcore-build": "bitpay/bitcore-build",
     "brfs": "^1.2.0",
     "chai": "^1.10.0",
-    "gulp": "^3.8.10"
+    "gulp": "^3.8.10",
+    "sinon": "^1.13.0"
   },
   "license": "MIT"
 }

test/transaction/transaction.js

@@ -5,6 +5,7 @@
 var should = require('chai').should();
 var expect = require('chai').expect;
 var _ = require('lodash');
+var sinon = require('sinon');
 
 var bitcore = require('../..');
 var Transaction = bitcore.Transaction;
@@ -619,6 +620,52 @@ describe('Transaction', function() {
     });
   });
 
+  describe('output ordering', function() {
+
+    var tenth = 1e7;
+    var fourth = 25e6;
+    var half = 5e7;
+    var transaction, out1, out2, out3, out4;
+
+    beforeEach(function() {
+      transaction = new Transaction()
+        .from(simpleUtxoWith1BTC)
+        .to(toAddress, tenth)
+        .to(toAddress, fourth)
+        .to(toAddress, half)
+        .change(changeAddress);
+      out1 = transaction.outputs[0];
+      out2 = transaction.outputs[1];
+      out3 = transaction.outputs[2];
+      out4 = transaction.outputs[3];
+    });
+
+    it('allows the user to sort outputs according to a criteria', function() {
+      var sorting = function(array) {
+        return [array[3], array[2], array[1], array[0]];
+      };
+      transaction.sortOutputs(sorting);
+      transaction.outputs[0].should.equal(out4);
+      transaction.outputs[1].should.equal(out3);
+      transaction.outputs[2].should.equal(out2);
+      transaction.outputs[3].should.equal(out1);
+    });
+
+    it('allows the user to randomize the output order', function() {
+      var shuffle = sinon.stub(_, 'shuffle');
+      shuffle.onFirstCall().returns([out2, out1, out4, out3]);
+
+      transaction._changeIndex.should.equal(3);
+      transaction.shuffleOutputs();
+      transaction.outputs[0].should.equal(out2);
+      transaction.outputs[1].should.equal(out1);
+      transaction.outputs[2].should.equal(out4);
+      transaction.outputs[3].should.equal(out3);
+      transaction._changeIndex.should.equal(2);
+
+      _.shuffle.restore();
+    });
+  });
 });
 
 var tx_empty_hex = '01000000000000000000';