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18 counter

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% vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
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% Copyright (C) 2000, 2005-2006, 2011 The University of Melbourne.
% This file may only be copied under the terms of the GNU Library General
% Public License - see the file COPYING.LIB in the Mercury distribution.
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%
% File: counter.m.
% Author: zs.
% Stability: high.
%
% Predicates for dealing with counters, which are mechanisms for allocating
% consecutively numbered integers. The abstraction barrier eliminates the
% possibility of confusion along the lines of "does this counter record
% the next number to be handed out, or the last number that was handed out?".
%
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:- module counter.
:- interface.

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:- type counter.

    % init(N, Counter) returns a counter whose first allocation will be the
    % integer N.
    %
:- pred init(int::in, counter::out) is det.

    % A function version of init/2.
    %
:- func init(int) = counter.

    % allocate(N, Counter0, Counter) takes a counter, and returns (a) the next
    % integer to be allocated from that counter, and (b) the updated state of
    % the counter.
    %
:- pred allocate(int::out, counter::in, counter::out) is det.

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