CGAL::Multiset<Type,Compare,Allocator>

Definition

An instance s of the parametrized data type Multiset<Type,Compare,Allocator> is a multi-set of elements of type Type, represented as a red-black tree (see [CLRS01, Chapter 13] for an excellent introduction to red-black trees). The main difference between Multiset<Type,Compare,Allocator> and STL's multiset is that the latter uses a less-than functor with a Boolean return type, while our Multiset<Type,Compare,Allocator> class is parameterized by a comparison functor Compare that returns the three-valued Comparison_result (namely it returns either SMALLER, EQUAL, or LARGER). It is thus possible to maintain the underlying red-black tree with less invocations of the comparison functor. This leads to a speedup of about 5% even if we maintain a set of integers. When each comparison of two elements of type Type is an expensive operation (for example, when they are geometric entities represented using exact arithmetic), the usage of a three-valued comparison functor can lead to considerable decrease in the running times.

Moreover, Multiset<Type,Compare,Allocator> allows the insertion of an element into the set given its exact position, and not just using an insertion hint, as done by std::multiset. This can further reduce the running times, as additional comparison operations can be avoided.

In addition, the Multiset<Type,Compare,Allocator> guarantees that the order of elements sent to the comparison functor is fixed. For example, if we insert a new element x into the set (or erase an element from the set), then we always invoke Compare() (x, y) (and never Compare() (y, x)), where y is an element already stored in the set. This behavior, not supported by std::multiset, is sometimes crucial for designing more efficient comparison predicates.

Multiset<Type,Compare,Allocator> also allows for look-up of keys whose type may differ from Type, as long as users supply a comparison functor CompareKey, where CompareKey() (key, y) returns the three-valued Comparison_result (key is the look-up key and y is an element of type Type). Indeed, it is very convenient to look-up equivalent objects in the set given just by their key. We note however that it is also possible to use a key of type Type and to employ the default Compare functor for the look-up, as done when using the std::multiset class.

advanced

Finally, Multiset<Type,Compare,Allocator> introduces the catenate() and split() functions. The first function operates on s and accepts a second set s', such that the maximum element in s is not greater than the minimal element in s', and concatenates s' to s. The second function splits s into two sets,one containing all the elements that are less than a given key, and the other contains all elements greater than (or equal to) this key.

advanced

Parameters

The Multiset class-template has three parameters:

• Type - the type of the stored elements.
• Compare - the comparison-functor type. This type should provide the following operator for comparing two Type elements, namely:
  Comparison_result operator() (const Type& t1, const Type& t2) const;
  The CGAL::Compare<Type> functor is used by default. In this case, Type must support an equality operator (operator==) and a less-than operator (operator<).
• Allocator - the allocator type.
  CGAL_ALLOCATOR is used by default.

#include <CGAL/Multiset.h>

Assertions

The assertion and precondition flags for the Multiset class use MULTISET in their names (i.e., CGAL_MULTISET_NO_ASSERTIONS and CGAL_MULTISET_NO_PRECONDITIONS).

Types

In compliance with STL, the types value_type and key_type (both equivalent to Type), reference and const_reference (reference to a value-type), key_compare and value_compare (both equivalent to Compare), size_type and difference_type are defined as well.

Creation

Access Member Functions

Comparison Operations

Insertion Methods

Removal Methods

Look-up Methods

All methods listed in this section can also accept a Type element as a look-up key. In this case, it is not necessary to supply a CompareKey functor, as the Compare functor will be used by default.

advanced

Special Operations

void	s.replace ( iterator position, Type x)
		replaces the element stored at the given position with x. Precondition:  The operation does not violate the set order - that is, x is not less that position's predecessor and not greater than its successor.
void	s.swap ( iterator pos1, iterator pos2)
		swaps places between the two elements given by pos1 and pos2. Precondition:  The operation does not violate the set order - that is, pos1 and pos2 store equivalent elements.
void	s.catenate ( Self& s_prime)	concatenates all elements in s_prime into s and clears s_prime. All iterators to s and to s_prime remain valid. Precondition:  The maximal element in s is not greater than the minimal element in s_prime.
template <class Key, class CompareKey>
void	s.split ( Key key, CompareKey comp_key, Self& s_prime)
		splits s such that it contains all elements that are less than the given key and such that s_prime contains all other elements. Precondition:  s_prime is initially empty.
void	s.split ( iterator position, Self& s_prime)
		splits s such that it contains all set elements in the range [begin, position) and such that s_prime contains all elements in the range [position, end()). Precondition:  s_prime is initially empty.

advanced

Implementation

Multiset uses a proprietary implementation of a red-black tree data-structure. The red-black tree invariants guarantee that the height of a tree containing $$n elements is $$O(log$$n) (more precisely, it is bounded by $$2 log$$₂n). As a consequence, all methods that accept an element and need to locate it in the tree (namely insert(x), erase(x), find(x), count(x), lower_bound(x) , upper_bound(x), find_lower(x) and equal_range(x)) take $$O(log$$n) time and perform $$O(log$$n) comparison operations.

On the other hand, the set operations that accept a position iterator (namely insert_before(pos, x), insert_after(pos, x) and erase(pos)) are much more efficient as they can be performed at a constant amortized cost (see [GS78] and [Tar83] for more details). More important, these set operations require no comparison operations. Therefore, it is highly recommended to maintain the set via iterators to the stored elements, whenever possible. The function insert(pos, x) is safer to use, but it takes amortized $$O(min$${d,log$$n}) time, where $$d is the distance between the given position and the true position of x. In addition, it always performs at least two comparison operations.

advanced

The catenate() and split() functions are also very efficient, and can be performed in $$O(log$$n) time, where $$n is the total number of elements in the sets, and without performing any comparison operations (see [Tar83] for the details). Note however that the size of two sets resulting from a split operation is initially unknown, as it is impossible to compute it in less than linear time. Thus, the first invocation of size() on such a set takes linear time, and not constant time.

advanced

The design is derived from the STL multiset class-template (see, e.g, [MS96]), where the main differences between the two classes are highlighted in the class definition above.