merkleblock: add indexOf.

lib/bcoin/merkleblock.js

@@ -109,12 +109,29 @@ MerkleBlock.prototype.addTX = function addTX(tx) {
  */
 
 MerkleBlock.prototype.hasTX = function hasTX(hash) {
+  return this.indexOf(hash) !== -1;
+};
+
+/**
+ * Test the block's _matched_ transaction vector against a hash.
+ * @param {Hash|TX} hash
+ * @returns {Number} Index.
+ */
+
+MerkleBlock.prototype.indexOf = function indexOf(hash) {
+  var index;
+
   if (hash instanceof bcoin.tx)
     hash = hash.hash('hex');
 
   this.verifyPartial();
 
-  return this.map[hash] != null;
+  index = this.map[hash];
+
+  if (index == null)
+    return -1;
+
+  return index;
 };
 
 /**