camelCaseToSnakeCaseString

- (nonnull NSString *)camelCaseToSnakeCaseString
{
    if ([self length] == 0) {
        return @"";
    }

    NSMutableString *output = [NSMutableString string];

    NSCharacterSet *digitSet = [NSCharacterSet decimalDigitCharacterSet];
    NSCharacterSet *uppercaseSet = [NSCharacterSet uppercaseLetterCharacterSet];
    NSCharacterSet *lowercaseSet = [NSCharacterSet lowercaseLetterCharacterSet];

    for (NSInteger idx = 0; idx < [self length]; idx += 1) {
        unichar c = [self characterAtIndex:idx];

        // if it's the last one then just append lowercase of character
        if (idx == [self length] - 1) {
            if ([uppercaseSet characterIsMember:c]) {
                [output appendFormat:@"%@", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
            }
            else {
                [output appendFormat:@"%C", c];
            }
            continue;
        }

        unichar nextC = [self characterAtIndex:(idx+1)];
        // this logic finds the boundaries between lowercase/uppercase/digits and lets the string be split accordingly.
        if ([lowercaseSet characterIsMember:c] && [uppercaseSet characterIsMember:nextC]) {
            [output appendFormat:@"%@_", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
        }
        else if ([lowercaseSet characterIsMember:c] && [digitSet characterIsMember:nextC]) {
            [output appendFormat:@"%@_", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
        }
        else if ([digitSet characterIsMember:c] && [uppercaseSet characterIsMember:nextC]) {
            [output appendFormat:@"%@_", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
        }
        else {
            // Append lowercase of character
            if ([uppercaseSet characterIsMember:c]) {
                [output appendFormat:@"%@", [[NSString stringWithCharacters:&c length:1] lowercaseString]];
            }
            else {
                [output appendFormat:@"%C", c];
            }
        }
    }
    return output;
}