[nsstring] Trim spaces from end of a NSString

A simple solution to only trim one end instead of both ends in Objective-C:

@implementation NSString (category)

/// trims the characters at the end
- (NSString *)stringByTrimmingSuffixCharactersInSet:(NSCharacterSet *)characterSet {
    NSUInteger i = self.length;
    while (i > 0 && [characterSet characterIsMember:[self characterAtIndex:i - 1]]) {
        i--;
    }
    return [self substringToIndex:i];
}

@end

And a symmetrical utility for trimming the beginning only:

@implementation NSString (category)

/// trims the characters at the beginning
- (NSString *)stringByTrimmingPrefixCharactersInSet:(NSCharacterSet *)characterSet {
    NSUInteger i = 0;
    while (i < self.length && [characterSet characterIsMember:[self characterAtIndex:i]]) {
        i++;
    }
    return [self substringFromIndex:i];
}

@end