Added commonPrefixWithString method to CPString and tests for it.

Foundation/CPString.j

@@ -537,6 +537,44 @@ var CPStringRegexSpecialCharacters = [
     return hash;
 }
 
+/*!
+    Returns a string containing characters the receiver and a given string have in common, starting from
+    the beginning of each up to the first characters that aren't equivalent.
+    @param aString the string with which to compare the receiver
+*/
+- (CPString)commonPrefixWithString:(CPString)aString
+{
+    return [self commonPrefixWithString: aString options: 0];
+}
+
+/*!
+    Returns a string containing characters the receiver and a given string have in common, starting from
+    the beginning of each up to the first characters that aren't equivalent.
+    @param aString the string with which to compare the receiver
+    @param aMask options for comparision
+*/
+- (CPString)commonPrefixWithString:(CPString)aString options:(int)aMask
+{
+    var len = 0, // length of common prefix
+        min = MIN([aString length], [self length]);
+
+    for ( len = 0; len < min; len++ )
+    {
+        var lhs = [self characterAtIndex: len],
+            rhs = [aString characterAtIndex: len];
+
+        if (aMask & CPCaseInsensitiveSearch)
+        {
+            lhs = [lhs lowercaseString];
+            rhs = [rhs lowercaseString];
+        }
+        
+        if ( lhs != rhs )
+            break;
+    }
+    return [self substringToIndex: len];
+}
+
 /*!
     Returns a copy of the receiver with all the first letters of words capitalized.
 */

Tests/Foundation/CPStringTest.j

@@ -151,6 +151,32 @@
         [self assert:[testStrings[i][0] boolValue] equals:testStrings[i][1]];
 }
 
+- (void)testCommonPrefixWithString
+{
+    var testStringsCase = [
+        ["Hello", "Helicopter", "Hel"],
+        ["Tester", "Taser", "T"],
+        ["Abcd", "Abcd", "Abcd"],
+        ["A long string", "A longer string", "A long"]
+    ];
+    
+    var testStringsCaseless = [
+        ["hElLo", "HeLiCoPtEr", "hEl"],
+        ["tEsTeR", "TaSeR", "t"],
+        ["aBcD", "AbCd", "aBcD"],
+        ["a LoNg StRiNg", "A lOnGeR sTrInG", "a LoNg"]
+    ];
+    
+    for (var i = 0; i < testStringsCase.length; i++)
+        [self assert: [testStringsCase[i][0] commonPrefixWithString:testStringsCase[i][1]]
+              equals: testStringsCase[i][2]];
+
+    for (var i = 0; i < testStringsCaseless.length; i++)
+        [self assert: [testStringsCaseless[i][0] commonPrefixWithString: testStringsCaseless[i][1]
+                                                 options: CPCaseInsensitiveSearch]
+              equals: testStringsCaseless[i][2]];
+}
+
 - (void)testCapitalizedString
 {
     var testStrings = [