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187 lines
6.7 KiB
Mathematica
187 lines
6.7 KiB
Mathematica
%-----------------------------------------------------------------------------%
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% Copyright (C) 2004-2006 The University of Melbourne.
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% This file may only be copied under the terms of the GNU Library General
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% Public License - see the file COPYING.LIB in the Mercury distribution.
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% vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
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%-----------------------------------------------------------------------------%
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%
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% File: version_store.m.
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% Author: Ralph Becket <rafe@cs.mu.oz.au>
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% Stability: low.
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%
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% (See the header comments in version_types.m for an explanation of version
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% types.)
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%
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% A version_store is similar to, albeit slightly slower than, an ordinary
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% store, but does not depend upon uniqueness.
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%
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% Note that, unlike ordinary stores, liveness of data is via the version store
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% rather than the mutvars. This means that dead data (i.e. whose mutvar is
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% out of scope) in a version_store may not be garbage collected.
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%
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%-----------------------------------------------------------------------------%
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%-----------------------------------------------------------------------------%
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:- module version_store.
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:- interface.
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%-----------------------------------------------------------------------------%
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:- type version_store(S).
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:- type mutvar(T, S).
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% Construct a new version store. This is distinguished from other
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% version stores by its existentially quantified type. This means
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% the compiler can automatically detect any attempt to use a
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% mutvar with the wrong version store.
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%
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:- some [S] func new = version_store(S).
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% new_mutvar(X, Mutvar, VS0, VS) adds a new mutvar with value reference X
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% to the version store.
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%
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:- pred new_mutvar(T::in, mutvar(T, S)::out,
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version_store(S)::in, version_store(S)::out) is det.
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% new_cyclic_mutvar(F, Mutvar, VS0, VS) adds a new mutvar with value
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% reference F(Mutvar) to the version store. This can be used to
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% construct cyclic terms.
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%
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:- pred new_cyclic_mutvar((func(mutvar(T, S)) = T)::in, mutvar(T, S)::out,
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version_store(S)::in, version_store(S)::out) is det.
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% copy_mutvar(Mutvar, NewMutvar, VS0, VS) constructs NewMutvar
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% with the same value reference as Mutvar.
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%
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:- pred copy_mutvar(mutvar(T, S)::in, mutvar(T, S)::out,
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version_store(S)::in, version_store(S)::out) is det.
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% VS ^ elem(Mutvar) returns the element referenced by Mutvar in
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% the version store.
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%
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:- func version_store(S) ^ elem(mutvar(T, S)) = T.
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% lookup(VS, Mutvar) = VS ^ elem(Mutvar).
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%
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% A predicate version is also provided.
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%
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:- func lookup(version_store(S), mutvar(T, S)) = T.
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:- pred get_mutvar(mutvar(T, S)::in, T::out, version_store(S)::in,
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version_store(S)::out) is det.
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% ( VS ^ elem(Mutvar) := X ) updates the version store so that
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% Mutvar now refers to value X.
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%
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:- func ( version_store(S) ^ elem(mutvar(T, S)) := T ) = version_store(S).
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% set(VS, Mutvar, X) = ( VS ^ elem(Mutvar) := X ).
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%
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% A predicate version is also provided.
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%
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:- func set(version_store(S), mutvar(T, S), T) = version_store(S).
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:- pred set_mutvar(mutvar(T, S)::in, T::in, version_store(S)::in,
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version_store(S)::out) is det.
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% unsafe_rewind(VS) produces a version of VS for which all accesses are
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% O(1). Invoking this predicate renders undefined VS and all later
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% versions undefined that were derived by performing individual updates.
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% Only use this when you are absolutely certain there are no live
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% references to VS or later versions of VS.
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%
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% A predicate version is also provided.
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%
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:- func unsafe_rewind(version_store(T)) = version_store(T).
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:- pred unsafe_rewind(version_store(T)::in, version_store(T)::out) is det.
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%-----------------------------------------------------------------------------%
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%-----------------------------------------------------------------------------%
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:- implementation.
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:- import_module counter.
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:- import_module int.
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:- import_module list.
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:- import_module unit.
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:- import_module univ.
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:- import_module version_array.
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%-----------------------------------------------------------------------------%
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% Index 0 of the version_store contains the counter used to assign
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% new version store mutvars. A mutvar is just an index into the
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% version_store.
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%
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:- type version_store(S) ---> version_store(version_array(univ)).
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:- type mutvar(T, S) ---> mutvar(int).
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:- type some_version_store_type ---> some_version_store_type.
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%-----------------------------------------------------------------------------%
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new = version_store(VA) `with_type` version_store(some_version_store_type) :-
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% 256 is just a magic number. The version_store is resized by
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% doubling if necessary when adding a new mutvar. Index 0 of
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% the version_store holds a counter for allocating new mutvars.
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%
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VA = version_array.new(256, univ(counter.init(1) `with_type` counter)).
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%-----------------------------------------------------------------------------%
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new_mutvar(X, Mutvar, VS0, VS) :-
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new_cyclic_mutvar(func(_) = X, Mutvar, VS0, VS).
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%-----------------------------------------------------------------------------%
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new_cyclic_mutvar(F, Mutvar, VS0, VS) :-
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Counter0 = VS0 ^ elem(mutvar(0)),
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counter.allocate(I, Counter0, Counter),
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Mutvar = mutvar(I),
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Size0 = size(VS0),
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VS1 = ( if I >= Size0 then resize(VS0, Size0 + Size0)
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else VS0 ),
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VS = (( VS1 ^ elem(mutvar(0)) := Counter )
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^ elem(Mutvar ) := F(Mutvar) ).
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:- func size(version_store(S)) = int.
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size(version_store(VA)) = size(VA).
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:- func resize(version_store(S), int) = version_store(S).
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resize(version_store(VA), N) = version_store(resize(VA, N, univ(unit))).
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%-----------------------------------------------------------------------------%
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copy_mutvar(Mutvar0, Mutvar, VS0, VS) :-
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X = VS0 ^ elem(Mutvar0),
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new_mutvar(X, Mutvar, VS0, VS).
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%-----------------------------------------------------------------------------%
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version_store(VA) ^ elem(mutvar(I)) = X :-
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UnivX = lookup(VA, I),
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det_univ_to_type(UnivX, X).
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lookup(VS, Mutvar) = VS ^ elem(Mutvar).
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get_mutvar(Mutvar, VS ^ elem(Mutvar), VS, VS).
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%-----------------------------------------------------------------------------%
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( version_store(VA) ^ elem(mutvar(I)) := X ) =
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version_store(VA ^ elem(I) := univ(X)).
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set(VS, Mutvar, X) = ( VS ^ elem(Mutvar) := X ).
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set_mutvar(Mutvar, X, VS, VS ^ elem(Mutvar) := X).
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%-----------------------------------------------------------------------------%
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unsafe_rewind(version_store(VA)) = version_store(unsafe_rewind(VA)).
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unsafe_rewind(VS, unsafe_rewind(VS)).
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%-----------------------------------------------------------------------------%
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%-----------------------------------------------------------------------------%
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