respect cursor position for insert and delte

cursor position was not respected on delete and insert.
new numbers were always inserted on last position
on delte the last digit was erased regardless of cursor position.
cursor postition is respected now, but cursor is rest to last position on delte/insert

Signal/src/view controllers/RegisterViewController.m

@@ -371,7 +371,7 @@
                        forCountry:(NSString *)country {
     _countryCodeLabel.text = code;
     _countryNameLabel.text = country;
-    [self updatePhoneNumberFieldWithString:code];
+    [self updatePhoneNumberFieldWithString:code cursorposition:_enteredPhoneNumber.length];
     [vc dismissViewControllerAnimated:YES completion:nil];
 }
 
@@ -381,23 +381,39 @@
 
 #pragma mark - UITextFieldDelegate
 
+- (NSUInteger) recalculateLocation:(NSUInteger)location {
+    uint offset = 0, phonenumberposition = 0;
+    for (uint i=0;i<location;i++) {
+        if ([_phoneNumberTextField.text characterAtIndex:i] != [_enteredPhoneNumber characterAtIndex:phonenumberposition])
+            offset ++;
+        else
+            phonenumberposition ++;
+    }
+    
+    return offset;
+}
+
 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range
                                                        replacementString:(NSString *)string {
     
+    range.location -= [self recalculateLocation:range.location];
     BOOL handleBackspace = range.length == 1;
     if (handleBackspace) {
-        NSRange backspaceRange = NSMakeRange([_enteredPhoneNumber length] - 1, 1);
+        NSRange backspaceRange = NSMakeRange(range.location, 1);
         [_enteredPhoneNumber replaceCharactersInRange:backspaceRange withString:string];
     } else {
         NSString* sanitizedString = [[string componentsSeparatedByCharactersInSet:[[NSCharacterSet decimalDigitCharacterSet ] invertedSet]] componentsJoinedByString:@""];
-        [_enteredPhoneNumber appendString:sanitizedString];
+        NSMutableString* mutablePhoneNumber = [NSMutableString stringWithString:_enteredPhoneNumber];
+        [mutablePhoneNumber insertString:sanitizedString atIndex:range.location];
+        _enteredPhoneNumber = mutablePhoneNumber;
     }
 
-    [self updatePhoneNumberFieldWithString:_enteredPhoneNumber];
+    [self updatePhoneNumberFieldWithString:_enteredPhoneNumber cursorposition:range.location+1];
     return NO;
 }
 
--(void) updatePhoneNumberFieldWithString:(NSString*) input {
+-(void) updatePhoneNumberFieldWithString:(NSString*)input
+                          cursorposition:(NSUInteger)cursorpos {
     NSString* result = [PhoneNumber bestEffortFormatPartialUserSpecifiedTextToLookLikeAPhoneNumber:_enteredPhoneNumber
                                                                     withSpecifiedCountryCodeString:_countryCodeLabel.text];
     _phoneNumberTextField.text = result;