Added Luhn algorithm

Author: Nicholas C. Zakas

algorithms/checksums/luhn-algorithm/luhn-algorithm.js

@@ -0,0 +1,67 @@
+/*
+ * Luhn algorithm implementation in JavaScript
+ * Copyright (c) 2009 Nicholas C. Zakas
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * 
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+ 
+
+/**
+ * Uses Luhn algorithm to validate a numeric identifier.
+ * @param {String} identifier The identifier to validate.
+ * @return {Boolean} True if the identifier is valid, false if not.
+ */
+function isValidIdentifier(identifier) {
+     
+    var sum     = 0,
+        alt     = false,
+        i       = identifier.length-1,
+        num;
+
+    while (i >= 0){
+            
+        //get the next digit
+        num = parseInt(identifier.charAt(i), 10);
+
+        //if it's not a valid number, abort
+        if (isNaN(num)){
+            return false;
+        }
+
+        //if it's an alternate number...
+        if (alt) {
+            num *= 2;
+            if (num > 9){
+                num = (num % 10) + 1;
+            }
+        } 
+        
+        //flip the alternate bit
+        alt = !alt;
+        
+        //add to the rest of the sum
+        sum += num;
+
+        //go to next digit
+        i--;
+    }
+
+    //determine if it's valid
+    return (sum % 10 == 0);
+}