1:- module(priorityq,
    2   [  pq_insert/4
    3   ,  pq_remove/4
    4   ,  pq_size/2
    5   ,  pq_peek/3
    6   ,  pq_empty/1
    7   ]).

   12:- use_module(library(heaps)).

 pq_empty(?Q:pq(A)) is det

Unifies Q with an empty queue.

   16pq_empty(H) :- empty_heap(H).

 pq_peek(-P:prio, -X:A, +Q:pq(A)) is semidet

Unify X with the item at the head of the queue if non-empty.

   20pq_peek(P, V, H) :- min_of_heap(H,P1,V), P is -P1.

 pq_insert(+P:prio, +X:A, +Q1:pq(A), -Q2:pq(A)) is det

Insert an item into a queue.

   24pq_insert(P, V, H1, H2) :- P1 is -P, add_to_heap(H1, P1, V, H2).

 pq_remove(-P:prio, -X:A, +Q1:pq(A), -Q2:pq(A)) is semidet

Removes topmost value in the queue. Fails if queue empty.

   28pq_remove(P, V, H1, H2) :- get_from_heap(H1, P1, V, H2), P is -P1.

 pq_size(+Q:pq(A), -N:natural) is det

Unifies N with the number of items in the queue.

   32pq_size(H, N) :- heap_size(H, N)