4.14.4 Tries
Tries (also called digital tree, radix tree or prefix tree maintain a mapping between a variant of a term (see =@=/2) and a value. They have been introduced in SWI-Prolog 7.3.21 as part of the implementation of tabling. The current implementation is rather immature. In particular, the following limitations currently apply:
- Tries are not thread-safe.
- Tries should not be modified while non-deterministic predicates such as trie_gen/3 are running on the trie.
- Terms cannot have attributed variables.
- Terms cannot be cyclic. Possibly this will not change because cyclic terms can only be supported after creating a canonical form of the term.
We give the definition of these predicates for reference and debugging tabled predicates. Future versions are likely to get a more stable and safer implementation. The API to tries should not be considered stable.
- trie_new(-Trie)
- Create a new trie and unify Trie with a handle to the trie. The trie handle is a blob. Tries are subject to atom garbage collection.
- trie_destroy(+Trie)
- Destroy Trie. This removes all nodes from the trie and causes further access to Trie to raise an existence_error exception. The handle itself is reclaimed by atom garbage collection.
- [semidet]is_trie(@Trie)
- True when Trie is a trie object. See also current_trie/1.
- [nondet]current_trie(-Trie)
- True if Trie is a currently existing trie. As this enumerates and then filters all known atoms this predicate is slow and should only be used for debugging purposes. See also is_trie/1.
- trie_insert(+Trie, +Key, +Value)
- Insert the term Key into Trie and associate it
with
Value. Value can be any term. If Key-Value
is already part of Trie, the predicates fails
silently. If Key is in Trie associated with a
different value, a
permission_erroris raised. - trie_update(+Trie, +Key, +Value)
- As trie_insert/3, but if Key is in Trie, its associated value is updated.
- trie_insert(+Trie, +Term, +Value, -Handle)
- As trie_insert/3,
returning a handle to the trie node. This predicate is currently unsafe
as Handle is an integer used to encode a pointer. It was used
to implement a pure Prolog version of the
library(tabling)library. - trie_delete(+Trie, +Key, ?Value)
- Delete Key from Trie if the value associated with Key unifies with Value.
- trie_lookup(+Trie, +Key, -Value)
- True if the term Key is in Trie and associated with Value.
- trie_term(+Handle, -Term)
- True when Term is a copy of the term associated with Handle. The result is undefined (including crashes) if Handle is not a handle returned by trie_insert_new/3 or the node has been removed afterwards.
- [nondet]trie_gen(+Trie, ?Key, -Value)
- True when Key is associated with Value in Trie. Backtracking retrieves all pairs. Currently scans the entire trie, even if Key is partly known. Currently unsafe if Trie is modified while the values are being enumerated.
- [nondet]trie_property(?Trie, ?Property)
- True if Trie exists with Property. Intended for
debugging and statistical purposes. Retrieving some of these properties
visit all nodes of the trie. Defined properties are
- value_count(-Count)
- Number of key-value pairs in the trie.
- node_count(-Count)
- Number of nodes in the trie.
- size(-Bytes)
- Required storage space of the trie.
- hashed(-Count)
- Number of nodes that use a hashed index to its children.