lazy calc for xpubkey in HDPrivateKey

lib/hdprivatekey.js

@@ -423,14 +423,29 @@ HDPrivateKey.prototype._buildFromBuffers = function(arg) {
     fingerPrint: fingerPrint
   });
 
-  var HDPublicKey = require('./hdpublickey');
-  var hdPublicKey = new HDPublicKey(this);
-
-  JSUtil.defineImmutable(this, {
-    hdPublicKey: hdPublicKey,
-    xpubkey: hdPublicKey.xpubkey
+  this._hdPublicKey = null;
+  Object.defineProperty(this, 'hdPublicKey', {
+    configurable: false,
+    enumerable: true,
+    get: function() {
+      if (!this._hdPublicKey) {
+        var HDPublicKey = require('./hdpublickey');
+        this._hdPublicKey = new HDPublicKey(this);
+      }
+      return this._hdPublicKey;
+    }
+  });
+  Object.defineProperty(this, 'xpubkey', {
+    configurable: false,
+    enumerable: true,
+    get: function() {
+      if (!this._hdPublicKey) {
+        var HDPublicKey = require('./hdpublickey');
+        this._hdPublicKey = new HDPublicKey(this);
+      }
+      return this._hdPublicKey.xpubkey;
+    }
   });
-
   return this;
 };
 

test/hdkeys.js

@@ -76,6 +76,11 @@ describe('BIP32 compliance', function() {
     publicKey.toString().should.equal(publicKey.xpubkey);
   });
 
+  it('cache for xpubkey works for test vector 1', function() {
+    var pk = HDPrivateKey(vector1_m_private);
+    pk.xpubkey.should.equal(pk.xpubkey);
+  });
+
   it('should get the extended public key from the extended private key for test vector 1', function() {
     HDPrivateKey(vector1_m_private).xpubkey.should.equal(vector1_m_public);
   });