The Bug

Shifting an NSMutableIndexSet by a negative number will drop an index in some cases.

Example Code:

NSMutableIndexSet *set = [NSMutableIndexSet indexSetWithIndex:0];
[set addIndex:2];
[set shiftIndexesStartingAtIndex:1 by:-1];
NSLog(@"%@", set);

The set should contain 0-1 but instead contains only 1.

The Reason

NSIndexSet is a series of NSRange-s. If the shift method removes empty space between ranges, than they should become a single unified range. For example, if a set contains the range 1-2 and the range 5-6, and we do

[set shiftIndexesStartingAtIndex:3 by:-2];

then we should get a set with a single range 1-4.

However, the implementation of shiftIndexesStartingAtIndex:by: fails to unify ranges, and also assumes that separate ranges have at least one empty space between them. And so we get a set containing the ranges 1-1 and 3-4.

The Workaround

Luckily, the methods addIndex: and addIndexesInRange: do correctly unify ranges. And so the workaround is to first call one of these methods, and only then shift:

[set addIndexesInRange:NSMakeRange(3, 2)];
[set shiftIndexesStartingAtIndex:3 by:-2];