NSIndexSet+FastEnumeration.m

#import <Foundation/Foundation.h>

@interface NSIndexSet (FastEnumeration) <NSFastEnumeration>
@end

@implementation NSIndexSet (FastEnumeration)

- (NSUInteger)countByEnumeratingWithState:(nonnull NSFastEnumerationState *)state objects:(id  _Nullable * _Nonnull)buffer count:(NSUInteger)len { 
    /*
     state
     - 0 : remaining count
     - 1 : hash (mutationsPtr)
     */
    
    NSUInteger count = self.count;
    
    if (state->state == 0) {
        // setup initial state
        state->state = 1;
        state->mutationsPtr = &state->extra[1];
        state->extra[0] = count;
    }
    
    state->extra[1] = self.hash;
    state->itemsPtr = buffer;
    
    if (len == 0) {
        return 0;
    }
    
    unsigned long remaining = state->extra[0];
    
    if (remaining == 0) {
        return 0;
    }
    
    NSRange indexRange = NSMakeRange(count - remaining, len);

    NSUInteger *indexes = malloc(sizeof(NSUInteger) * len);
    NSUInteger indexesCount = [self getIndexes:indexes maxCount:len inIndexRange:&indexRange];
    
    for (NSUInteger i = 0; i < len; i++) {
        if (indexesCount <= i) {
            buffer[i] = nil;
        } else {
            buffer[i] = [NSNumber numberWithUnsignedInteger:indexes[i]];
        }
    }
    
    free(indexes);
    
    NSUInteger newRemaining = remaining - indexesCount;
    
    if (newRemaining > 0) {
        state->extra[0] = newRemaining;
        return indexesCount;
    } else {
        // final
        state->extra[0] = 0;
        return remaining;
    }
}

@end



int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSMutableIndexSet *indexSet = [NSMutableIndexSet indexSetWithIndexesInRange:NSMakeRange(0, 10)];
        
        [indexSet addIndexesInRange:NSMakeRange(10, 10)];
        
        // https://mikeash.com/pyblog/friday-qa-2010-04-16-implementing-fast-enumeration.html
        NSAutoreleasePool *pool = [NSAutoreleasePool new];
        
        for (NSNumber *index in indexSet) {
            NSLog(@"%@", index);
            [pool release];
            pool = [NSAutoreleasePool new];
        }
        
        [pool release];
    }
    
    return 0;
}