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% vim: ft=mercury ts=4 sw=4 et
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% Copyright (C) 1994-1995, 1997-1999, 2003-2006, 2011 The University of Melbourne.
% Copyright (C) 2014-2016, 2018 The Mercury team.
% This file is distributed under the terms specified in COPYING.LIB.
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%
% File: queue.m.
% Main author: fjh.
% Stability: high.
%
% This file contains a `queue' ADT.
% A queue holds a sequence of values, and provides operations
% to insert values at the end of the queue (put) and remove them from
% the front of the queue (get).
%
% This implementation is in terms of a pair of lists.
% The put and get operations are amortized constant-time.
%
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:- module queue.
:- interface.

:- import_module list.

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:- type queue(T).

    % `init(Queue)' is true iff `Queue' is an empty queue.
    %
:- func init = queue(T).
:- pred init(queue(T)::out) is det.

    % 'queue_equal(Q1, Q2)' is true iff Q1 and Q2 contain the same
    % elements in the same order.
    %
:- pred equal(queue(T)::in, queue(T)::in) is semidet.

    % `is_empty(Queue)' is true iff `Queue' is an empty queue.
    %
:- pred is_empty(queue(T)::in) is semidet.

    % `is_full(Queue)' is intended to be true iff `Queue' is a queue
    % whose capacity is exhausted. This implementation allows arbitrary-sized
    % queues, so is_full always fails.
    %
:- pred is_full(queue(T)::in) is semidet.

    % `put(Elem, Queue0, Queue)' is true iff `Queue' is the queue
    % which results from appending `Elem' onto the end of `Queue0'.
    %
:- func put(queue(T), T) = queue(T).
:- pred put(T::in, queue(T)::in, queue(T)::out) is det.

    % `put_list(Elems, Queue0, Queue)' is true iff `Queue' is the queue
    % which results from inserting the items in the list `Elems' into `Queue0'.
    %
:- func put_list(queue(T), list(T)) = queue(T).
:- pred put_list(list(T)::in, queue(T)::in, queue(T)::out) is det.

    % `first(Queue, Elem)' is true iff `Queue' is a non-empty queue
    % whose first element is `Elem'.
    %
:- pred first(queue(T)::in, T::out) is semidet.

    % `get(Elem, Queue0, Queue)' is true iff `Queue0' is a non-empty
    % queue whose first element is `Elem', and `Queue' the queue which results
    % from removing that element from the front of `Queue0'.
    %
:- pred get(T::out, queue(T)::in, queue(T)::out) is semidet.

    % `length(Queue, Length)' is true iff `Queue' is a queue
    % containing `Length' elements.
    %
:- func length(queue(T)) = int.
:- pred length(queue(T)::in, int::out) is det.

    % `list_to_queue(List, Queue)' is true iff `Queue' is a queue
    % containing the elements of List, with the first element of List at
    % the head of the queue.
    %
:- func list_to_queue(list(T)) = queue(T).
:- pred list_to_queue(list(T)::in, queue(T)::out) is det.

    % A synonym for list_to_queue/1.
    %
:- func from_list(list(T)) = queue(T).

    % `to_list(Queue) = List' is the inverse of from_list/1.
    %
:- func to_list(queue(T)) = list(T).

    % `delete_all(Elem, Queue0, Queue)' is true iff `Queue' is the same
    % queue as `Queue0' with all occurrences of `Elem' removed from it.
    %
:- func delete_all(queue(T), T) = queue(T).
:- pred delete_all(T::in, queue(T)::in, queue(T)::out) is det.

    % `put_on_front(Queue0, Elem) = Queue' pushes `Elem' on to
    % the front of `Queue0', giving `Queue'.
    %
:- func put_on_front(queue(T), T) = queue(T).
:- pred put_on_front(T::in, queue(T)::in, queue(T)::out) is det.

    % `put_list_on_front(Queue0, Elems) = Queue' pushes `Elems'
    % on to the front of `Queue0', giving `Queue' (the N'th member
    % of `Elems' becomes the N'th member from the front of `Queue').
    %
:- func put_list_on_front(queue(T), list(T)) = queue(T).
:- pred put_list_on_front(list(T)::in, queue(T)::in, queue(T)::out)
    is det.

    % `get_from_back(Elem, Queue0, Queue)' removes `Elem' from
    % the back of `Queue0', giving `Queue'.
    %
:- pred get_from_back(T::out, queue(T)::in, queue(T)::out) is semidet.

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