Add/subtract from an IndexPath `item` property overflowing into `section` #objective-c #cocoa #index

indexpath_addition_math.m

- (NSIndexPath *)_indexPathByAdding:(NSInteger)N toIndexPath:(NSIndexPath *)idxPath
{
    const NSInteger SIZE = 8;
    
    NSInteger item = idxPath.item + N;
    NSInteger sectionDelta = trunc(item / SIZE);
    item = abs((int)item) % SIZE;
    NSInteger section = idxPath.section + sectionDelta;
    
    // nil when we go too far back
    if (section < 0) {
        return nil;
    }
    
    return [NSIndexPath indexPathForItem:item inSection:section];
}