Compute a position string between two other strings for concurrently sorting lists based on a lexicographically sortable position property.

between.m

+ (NSString *)positionBetweenPrevious:(NSString *)previous next:(NSString *)next {
    if (!previous) {
        previous = @"0";
    }
    if (!next) {
        next = @"z";
    }
    NSString *chars = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    NSMutableString *betweenPosition = [NSMutableString string];
    NSUInteger index = 0;
    while (YES) {
        NSRange range = NSMakeRange(index, 1);
        NSUInteger previousIndex = 0;
        NSUInteger nextIndex = chars.length - 1;
        if (index < previous.length) {
            NSString *previousChar = [previous substringWithRange:range];
            previousIndex = [chars rangeOfString:previousChar].location;
        }
        if (index < next.length) {
            NSString *nextChar = [next substringWithRange:range];
            nextIndex = [chars rangeOfString:nextChar].location;
        }
        index += 1;
        NSUInteger betweenIndex;
        if (previousIndex >= nextIndex) {
            betweenIndex = previousIndex;
        } else {
            betweenIndex = (previousIndex + nextIndex) / 2;
        }
        NSString *betweenChar = [chars substringWithRange:NSMakeRange(betweenIndex, 1)];
        [betweenPosition appendString:betweenChar];
        BOOL positionAfterPrevious = [previous compare:betweenPosition] == NSOrderedAscending;
        BOOL positionBeforeNext = [betweenPosition compare:next] == NSOrderedAscending;
        BOOL lastCharNotFirstPossibleChar = betweenIndex != 0;
        if (positionAfterPrevious && positionBeforeNext && lastCharNotFirstPossibleChar) {
            return betweenPosition;
        }
    }
}