add #buildLookup method to turn collection in to a fast lookup hash

implement #without in terms of it
add tests and internal docs

test/collections.js

@@ -164,5 +164,12 @@ $(document).ready(function() {
   test('collections: size', function() {
     equals(_.size({one : 1, two : 2, three : 3}), 3, 'can compute the size of an object');
   });
+  
+  test('collections: buildLookup', function() {
+    same(_.buildLookup([1,'hi']), {1:true, 'hi':true}, 'defaults values to true');
+    same(_.buildLookup([1,'hi'], 1), {1:1, 'hi':1}, 'can override value');
+    same(_.buildLookup({5:'five'}), {five: true}, 'making a map from an object uses its values');
+  });
+  
 
 });

test/objects.js

@@ -11,7 +11,7 @@ $(document).ready(function() {
   });
 
   test("objects: functions", function() {
-    var expected = ["alias", "all", "any", "bind", "bindAll", "breakLoop", "clone", "compact",
+    var expected = ["alias", "all", "any", "bind", "bindAll", "breakLoop", "buildLookup", "clone", "compact",
     "compose","defer", "delay", "detect", "each", "every", "extend", "filter", "first",
     "flatten", "foldl", "foldr", "forEach", "functions", "head", "identity", "include",
     "indexOf", "inject", "intersect", "invoke", "isArguments", "isArray", "isDate", "isElement", "isEmpty", "isEqual",

underscore.js

@@ -230,6 +230,24 @@
   _.size = function(obj) {
     return _.toArray(obj).length;
   };
+  
+  // Build a lookup map from a collection.
+  // Pay a little memory upfront to make searching a collection for a 
+  // value faster later
+  // e.g:
+  //
+  //  var lookup = _.buildLookup([1,2,8])  // {1: true, 2: true, 8: true}
+  //  lookup[key]   // instead of: _.include(array, key)
+  //
+  // See example usage in #without
+  // By default sets the value to true, can pass in a value to use instead
+  _.buildLookup = function (obj, useValue) {
+    useValue = (useValue === undefined)?  true :  useValue;
+    return _.reduce(obj, {}, function (memo, value) {  
+      memo[value] = useValue;  
+      return memo;
+    });
+  };   
 
   // -------------------------- Array Functions: ------------------------------
 
@@ -269,8 +287,8 @@
 
   // Return a version of the array that does not contain the specified value(s).
   _.without = function(array) {
-    var values = _.rest(arguments);
+    var lookup = _.buildLookup(_.rest(arguments));
-    return _.select(array, function(value){ return !_.include(values, value); });
+    return _.filter(array, function(value){ return !lookup[value]; });
   };
 
   // Produce a duplicate-free version of the array. If the array has already