add auto resolver

lib/container.js

@@ -47,6 +47,7 @@ function Container() {
   this._resolvers = [];
 
   this.resolver(require('./resolvers/id')());
+  this.resolver(require('./resolvers/auto')(this));
 }
 
 // Inherit from `EventEmitter`.

lib/resolvers/auto.js

@@ -0,0 +1,36 @@
+/**
+ * Automatic interface resolver.
+ *
+ * Automatically resolves an interface to an object that implements that
+ * interface.  Automatic resolution reduces the amount of configuration that
+ * must be specified.
+ *
+ * For consistency and saftey in the development cycle, such resolution succeeds
+ * if and only if there is one object within the container that implements the
+ * interface.  If multiple objects implement the interface, automatic resolution
+ * would be ambiguous, and is therefore not performed.  In such cases, the exact
+ * object to resolve can be explicitly declared in configuration.
+ *
+ * @return {function}
+ * @protected
+ */
+module.exports = function(container) {
+
+  return function(iface, pid) {
+    var specs = container.components()
+      , candidates = []
+      , spec, i, len;
+    for (i = 0, len = specs.length; i < len; ++i) {
+      spec = specs[i];
+      if (spec.implements.indexOf(iface) !== -1) {
+        candidates.push(spec.id);
+      }
+    }
+
+    if (candidates.length == 1) {
+      return candidates[0];
+    } else if (candidates.length > 1) {
+      throw new Error('Multiple objects implement interface \"' + iface + '\" required by \"' + (pid || 'unknown') + '\". Configure one of: ' + candidates.join(', '));
+    }
+  };
+}