%---------------------------------------------------------------------------%
% vim: ts=4 sw=4 et ft=mercury
%---------------------------------------------------------------------------%
%
% The "map.rev" Benchmark
% Part of the DPPD Library.
% 
% Specialising the higher-order map/3 (using call and =..) for the
% reverse program. The benchmark program uses built-ins but no
% negations. The benchmark illustrates that partial deduction can be
% used to make declarative higher-order programming in Prolog/LP efficient.

:- module map_rev.

:- interface.

:- pred map_rev is semidet.

:- implementation.

:- import_module list.
:- import_module map_impl.
:- import_module run.

map_rev :-
    map_rev([[a, b], [c, d, e]], [L1, L2]),
    use(L1), use(L2),
    map_rev([[], [a, b], [c, d, e], [f, g, h, i]], Res1),
    use(Res1),
    map_rev([[], [a, b], [c, d, e], [], [f, g, h], [i, j], [k, l], [m], [n]],
        Res2),
    use(Res2).

% The partial deduction query
% 
% :- map(rev, L, R).
% 
% The run-time queries
% 
% :- map(rev, [[a, b], [c, d, e]], [L1, L2]).
% :- map(rev, [[], [a, b], [c, d, e], [f, g, h, i]], Res).
% :- map(rev, [[], [a, b], [c, d, e], [], [f, g, h], [i, j],
%     [k, l], [m], [n]], Res).
% 
% Example solution
% 
% With the ECCE partial deduction system one can obtain the following program
% (which runs almost 10 times faster than the original):
% 
% map__1([], []).
% map__1([X1 | X2], [X3 | X4]) :-
%     rev__2(X1, X3),
%     map__1(X2, X4).
% 
% rev__2([], []).
% rev__2([X1 | X2], X3) :-
%     rev__3(X2, X1, [], X3).
% 
% rev__3([], X1, X2, [X1 | X2]).
% rev__3([X1 | X2], X3, X4, X5) :-
%     rev__3(X2, X1, [X3 | X4], X5).
% 
% Michael Leuschel / K.U. Leuven / michael@cs.kuleuven.ac.be