added merge ability

index.js

@@ -58,3 +58,13 @@ exports.stringify = function(obj) {
 
   return parts.join('/');
 };
+
+exports.merge = function(str, objToMerge) {
+  var obj = exports.parse(str);
+  for (var prop in objToMerge) {
+    if (['type', 'id', 'subtype', 'version', 'index'].indexOf(prop) > -1) {
+      obj[prop] = objToMerge[prop];
+    }
+  }
+  return exports.stringify(obj);
+};

test/merge_test.js

@@ -0,0 +1,42 @@
+'use strict';
+
+var test = require('tap').test;
+var merge = require('..').merge;
+
+test('changing type component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {type:'new_type'}), 'new_type/id/subtype/version/index', 'should return docuri string with type changed');
+  t.end();
+});
+
+test('changing id component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {id:'new_id'}), 'type/new_id/subtype/version/index', 'should return docuri string with id changed');
+  t.end();
+});
+
+test('changing subtype component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {subtype:'new_subtype'}), 'type/id/new_subtype/version/index', 'should return docuri string with subtype changed');
+  t.end();
+});
+
+test('changing version component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {version:'new_version'}), 'type/id/subtype/new_version/index', 'should return docuri string with version changed');
+  t.end();
+});
+
+test('changing index component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {index:'new_index'}), 'type/id/subtype/version/new_index', 'should return docuri string with index changed');
+  t.end();
+});
+
+test('changing unknown component', function(t) {
+  t.equal(merge('type/id/subtype/version/index', {unknown:'i dont know'}), 'type/id/subtype/version/index', 'should return unchanged docuri');
+  t.end();
+});
+
+
+test('missing second argument', function(t) {
+  t.equal(merge('type/id/subtype/version/index'), 'type/id/subtype/version/index', 'should return unchanged docuri string');
+  t.end();
+});
+
+