Switch between sorted arrays with intelligent animations

AffectedRows.m

/**
 Returns the index paths of the rows affected by switching between two
 sorted (but not necessarily unique) arrays. 
 
 @param old The array to be switched from.
 @param new The array to be switched to.
 @param keyPath the key path to compare ("@self" to use raw value)
 
 @return An array of three arrays, the first containing index paths
 for modified rows, the second for deleted rows, and the third for
 inserted rows.
*/
- (NSArray *)affectedRowsForSwitchingFromArray:(NSArray *)old toArray:(NSArray *)new keyPath:(NSString *)keyPath {
    NSInteger i = 0, j = 0;
    NSMutableArray *indexPathsOfModifiedItems = [NSMutableArray array];
    NSMutableArray *indexPathsOfDeletedItems = [NSMutableArray array];
    NSMutableArray *indexPathsOfAddedItems = [NSMutableArray array];
    
    while (i < [new count] || j < [old count]) {
        NSComparisonResult comp;
        if (i < [new count] && j < [old count])
            comp = [[new[i] valueForKeyPath:keyPath] compare:[old[j] valueForKeyPath:keyPath]];
        else if (i >= [new count])
            comp = NSOrderedDescending;
        else if (j >= [old count])
            comp = NSOrderedAscending;
        
        if (comp == NSOrderedSame) {
            [indexPathsOfModifiedItems addObject:[NSIndexPath indexPathForRow:j inSection:0]];
            i++, j++;
        } else if (comp == NSOrderedAscending) {
            [indexPathsOfAddedItems addObject:[NSIndexPath indexPathForRow:i inSection:0]];
            i++;
        } else if (comp == NSOrderedDescending) {
            [indexPathsOfDeletedItems addObject:[NSIndexPath indexPathForRow:j inSection:0]];
            j++;
        }
    }
    
    return @[ indexPathsOfModifiedItems, indexPathsOfDeletedItems, indexPathsOfAddedItems];
}