Shifting an NSMutableIndexSet<\/tt> by a negative number will drop an index in some cases.<\/p>\n
Example Code:<\/p>\n
\r\nNSMutableIndexSet *set = [NSMutableIndexSet indexSetWithIndex:0];\r\n[set addIndex:2];\r\n[set shiftIndexesStartingAtIndex:1 by:-1];\r\nNSLog(@\"%@\", set);\r\n<\/pre>\nThe set should contain 0-1 but instead contains only 1.<\/p>\n
The Reason<\/h3>\n
NSIndexSet<\/tt> is a series of NSRange<\/tt>-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<\/p>\n
\r\n[set shiftIndexesStartingAtIndex:3 by:-2];\r\n<\/pre>\nthen we should get a set with a single range 1-4.<\/p>\n
However, the implementation of shiftIndexesStartingAtIndex:by:<\/tt> 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.<\/p>\n
The Workaround<\/h3>\n
Luckily, the methods addIndex:<\/tt> and addIndexesInRange:<\/tt> do correctly unify ranges. And so the workaround is to first call one of these methods, and only then shift:<\/p>\n
\r\n[set addIndexesInRange:NSMakeRange(3, 2)];\r\n[set shiftIndexesStartingAtIndex:3 by:-2];\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"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. … Continue reading