Optimized repeat and its tests

lib/coffee-script/helpers.js

@@ -12,8 +12,17 @@
     return literal === string.substr(string.length - len - (back || 0), len);
   };
 
-  exports.repeat = function(string, n) {
-    return (Array(n + 1)).join(string);
+  exports.repeat = function(str, n) {
+    var res;
+    res = '';
+    while (n > 0) {
+      if (n & 1) {
+        res += str;
+      }
+      n >>>= 1;
+      str += str;
+    }
+    return res;
   };
 
   exports.compact = function(array) {

src/helpers.coffee

@@ -12,8 +12,14 @@ exports.ends = (string, literal, back) ->
   literal is string.substr string.length - len - (back or 0), len
 
 # Repeat a string `n` times.
-exports.repeat = (string, n) ->
-  (Array n + 1).join string
+exports.repeat = (str, n) ->
+  # Use clever algorithm to have O(lon(n)) string concatenation operations
+  res = ''
+  while n > 0
+    res += str if n & 1
+    n >>>= 1
+    str += str
+  res
 
 # Trim out all falsy values from an array.
 exports.compact = (array) ->

test/helpers.coffee

@@ -2,7 +2,7 @@
 # -------
 
 # pull the helpers from `CoffeeScript.helpers` into local variables
-{starts, ends, compact, count, merge, extend, flatten, del, last} = CoffeeScript.helpers
+{starts, ends, repeat, compact, count, merge, extend, flatten, del, last} = CoffeeScript.helpers
 
 
 # `starts`
@@ -27,6 +27,15 @@ test "the `ends` helper can take an optional offset", ->
   ok not ends('01234', '234', 6)
 
 
+# `repeat`
+
+test "the `repeat` helper concatenates a given number of times", ->
+  eq 'asdasdasd', repeat('asd', 3)
+
+test "`repeat`ing a string 0 times always returns the empty string", ->
+  eq '', repeat('whatever', 0)
+
+
 # `compact`
 
 test "the `compact` helper removes falsey values from an array, preserves truthy ones", ->