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% vim: ft=mercury ts=4 sts=4 sw=4 et
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% Copyright (C) 2021 The Mercury team.
% This file is distributed under the terms specified in COPYING.LIB.
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%
% File: random.system_rng.m.
% Main author: Julien Fischer
%
% This module provides access to the system random number generator.
% This is a platform specific cryptographically secure random number generator
% that is seeded from the OS entropy pool.
%
% The intended use of this generator is to provide small amounts of
% high-quality random material suitable, for example, for seeding other
% random number generators. It is not intended for generating large amounts
% of random material.
%
% On the C backends, the system RNG depends on the operating system.
% For macOS, Cygwin, OpenBSD, NetBSD and versions of FreeBSD from 12 onwards,
% we use the arc4random() family of functions.
% For Windows, we use the rand_s() function.
% For Linux, AIX, Solaris and versions of FreeBSD before 12, we read randomness
% from /dev/urandom; on these system each open system RNG handle will require
% an open file descriptor.
% On other operating systems the system RNG is not available; on these systems
% attempting to open a system RNG handle will throw an exception.
%
% On the C# backend, the system RNG is an instance of the
% System.Security.Cryptography.RandomNumberGenerator class that uses the
% default cryptographic random number generator implementation.
%
% On the Java backend, the system RNG is an instance of the
% java.security.SecureRandom class using the default random number algorithm.
%
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:- module random.system_rng.
:- interface.

:- import_module maybe.

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    % A handle through which the system RNG can be accessed.
    %
    % In general, it is *not* safe to share system RNG handles across
    % multiple threads. In circumstances where multiple threads require access
    % to the system RNG, each thread should open its own handle.
    %
:- type system_rng.

:- instance urandom(system_rng, io).

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    % True if this platform provides a system RNG.
    %
:- pred have_system_rng is semidet.

    % open_system_rng(MaybeHandle, !IO):
    %
    % Returns a handle through which the system RNG can be accessed.
    %
:- pred open_system_rng(maybe_error(system_rng)::out, io::di, io::uo)
    is det.

    % close_system_rng(Handle, !IO):
    %
    % Release the system RNG handle along with any OS resources associated
    % with it. Throws an exception if an error occurs.
    %
:- pred close_system_rng(system_rng::in, io::di, io::uo) is det.

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    % Generate a uniformly distributed unsigned integer of 8, 16, 32 or
    % 64 bits respectively using the system RNG.
    %
:- pred generate_uint8(system_rng::in, uint8::out, io::di, io::uo) is det.
:- pred generate_uint16(system_rng::in, uint16::out, io::di, io::uo) is det.
:- pred generate_uint32(system_rng::in, uint32::out, io::di, io::uo) is det.
:- pred generate_uint64(system_rng::in, uint64::out, io::di, io::uo) is det.

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