Version 5.0 The LEDA User Manual

7.7 Partially Persistent Dictionaries ( pp dictionary
)
1. Definition
A partially persistent dictionary stores the history of a set of items (type
pp dic item). Here k is a key from a linearly ordered type K, and i is an information from
the data type I. An instance D of the parameterized data type pp dictionary
references a version of the set in the history.
An update operation may only be performed on an instance D containing the newest
version v. The operation changes the set of items and creates a new version v ′ which
represents the new state of the set. After the operation, D references version v ′ , and all
other instances which have referenced version v remain unchanged. An access operation
may be applied to any instance D. Let v denote the version referenced by D. Then the
operation is carried out on the set which existed when v was created.
The main advantage of a partially persistent dictionary compared to a dictionary is that
copy and assignment operations use only constant time and space, because only the versions
and not the whole sets are assigned.
The linear order for the data type K is given by a compare object of type CMP. It must
provide a function call operator which has the same syntax and semantics as a LEDA
compare function (see Section 2.3). The data type pp dictionary may be instantitiated
without specifying the third template argument, i.e. as type pp dictionary. This
data type uses the linear order which is defined by the compare function for the type K.
#include < LEDA/core/pp dictionary.h >
2. Creation
pp dictionary D(const CMP& cmp = CMP( ));
creates an instance D of type pp dictionary and initializes
D to an empty persistent dictionary.
When the argument cmp is provided, the object is used as compare
function object which defines the linear order for the data type K.
3. Operations
3.1 Update operations
pp dic item D.insert(const K& k, const I& i)
associates the information i with the key k.
If there is an item in the set then it
is deleted. A new item is added and
returned.