lawrencelomax / llbinaryoperators

Binary Enumeration Operators on NSArray. Fully unit tested

LLBinaryOperators

Binary Enumeration Operators on NSArray. Fully unit tested.

Rationale

The NSArray methods for binary enumeration only work when dealing with object equality:

- (NSUInteger)indexOfObject:(id)obj inSortedRange:(NSRange)r options:(NSBinarySearchingOptions)opts usingComparator:(NSComparator)cmp

Sometimes, you may wish to get the performance gain of a binary enumeration within a sorted array, without requiring object equality. It isn't always possible or desirable to override isEqual: to allow for object equality. For example in a hashed collection, you cannot express intersection via a hashCode

Usage

You may have a object that contains some layout information for a large number of items ordered by position. To obtain the layout information for a given query rectangle, you could use linear enumeration, however it would be much more efficient to use a Binary Search. As you cannot use the standard NSArray methods due to requiring object equality, you can use LLBinaryOperators to obtain the layout information that intersects with the query rectangle.

Use the category method. method:

- (void) ll_binaryEnumerate:(LLBinaryEnumerationBlock)block;

You can do this to obtain the rectangles that intersect with the lowest value in the x dimension:

NSArray * arrayOfCGRects = ....;
CGRect intersectionRect = ....;
__block NSUInteger intersectionIndex = NSNotFound;
[arrayOfCGRects ll_binaryEnumerate:NSComparisonResult^(NSUInteger index, LayoutInformation * object, NSRange range, BOOL * stop) {
  CGRect * rect = [object bounds];
  if(CGRectIntersectsRect(rect, intersectionRect) {
    intersectionIndex = index;
    // We have an index that intersects, but it could be anywhere in a range
    // we should go downwards to find the lowest index that intersects
    // When we have this index we can enumerate linearly up to find the highest index
    return NSOrderedDescending;
  } else if (CGRectGetMinX(rect) > CGRectGetMaxX(intersectionRect)) {
    // Move right (up) the array
    return NSOrderedAscending;
  } else if (CGRectGetMaxX(rect) < CGRectGetMinX(intersectionRect)) {
    // Move left (down) along the array
    return NSOrderedDescending;
  }
  
  *stop = YES;
  return NSOrderedSame;
}];

We have the leftmost index, now linearly enumerate upwards until we hit the rightmost index

NSMutableArray * intersectionObjects = [NSMutableArray array];
if(intersectionIndex != NSNotFound) {
    [arrayOfCGRects enumerateObjectsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(intersectionIndex, [array count] - indersectionIndex)] options:0 usingBlock:^(SomeObject * obj, NSUInteger idx, BOOL *stop) {
        if(CGRectIntersectsRect([obj bounds], intersectionRect)) {
            [intersectionObjects addObject:obj];
            return;
        }
        *stop = YES;
    }];
    
    NSLog(@"Objects %@ intersect rect %@", intersectionObjects, NSStringFromCGRect(intersectionRect));
}