/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2005-2024, VU University Amsterdam
                              CWI, Amsterdam
                              SWI-Prolog Solutions b.v.
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    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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*/

:- module(nb_set,
          [ empty_nb_set/1,		 % -EmptySet
            add_nb_set/2,		 % +Key, !Set
            add_nb_set/3,		 % +Key, !Set, ?New
            size_nb_set/2,		 % +Set, -Size
            nb_set_to_list/2,		 % +Set, -List
            gen_nb_set/2                 % +Set, -Key
          ]).
:- autoload(library(apply), [maplist/2]).
:- autoload(library(terms), [term_factorized/3]).
:- use_module(library(debug), [assertion/1]).

/** <module> Non-backtrackable sets

This library provides  a  non-backtrackabe  _set_   of  terms  that  are
variants of each other. It is primarily intended to implement distinct/1
from library(solution_sequences). The set is implemented as a hash table
that is built using non-backtrackable primitives, notably nb_setarg/3.

The original version of this library   used  binary trees which provides
immediate ordering. As the trees were   not  balanced, performance could
get   really   poor.   The   complexity   of   balancing   trees   using
non-backtrackable primitives is too high. The  next iteration used _open
hash tables_, while the current incarnation   uses _closed hash tables_,
providing better perfomance and less space usage.

@author Jan Wielemaker
*/

initial_capacity(4).                       % initial hash-table size

%!  empty_nb_set(-Set)
%
%   Create an empty non-backtrackable set.

empty_nb_set(NbSet) :-
    initial_capacity(Capacity),
    Empty = empty(1),
    '$filled_array'(Buckets, buckets, Capacity, Empty),
    NbSet = nb_set(Empty, Capacity, 0, Buckets).

%!  add_nb_set(+Key, !Set) is det.
%!  add_nb_set(+Key, !Set, ?New) is semidet.
%
%   Insert Key into the set. If  a   variant  (see  =@=/2) of Key is
%   already in the set, the set is unchanged and New is unified with
%   `false`. Otherwise, New is unified with   `true` and a _copy of_
%   Key is added to the set.
%
%   @tbd    Computing the hash for cyclic terms is performed with
%           the help of term_factorized/3, which performs rather
%           poorly.

add_nb_set(Key, Set) :-
    add_nb_set(Key, Set, _).
add_nb_set(Key, Set, New) :-
    Set = nb_set(Empty, Capacity, Size, Buckets),
    key_hash(Key, Hash),
    index(Hash, Capacity, KIndex),
        arg(KIndex, Buckets, StoredKey),
        (   same_term(StoredKey, Empty)
        ->  !,
            New = true,
            nb_setarg(KIndex, Buckets, Key),
            NSize is Size+1,
            nb_setarg(3, Set, NSize),
            (   NSize > Capacity*0.5
            ->  rehash(Set)
            ;   true
            )
        ;   Key =@= StoredKey
        ->  !,
            New = false
        ).

%!  index(+Hash, +Capacity, -Index) is nondet.
%
%   Generate  candidate  values  for  Index,  starting  from  `Hash  mod
%   Capacity`, round tripping to 1 when Capacity is reached.

index(Hash, Capacity, KIndex) :-
    KIndex0 is (Hash mod Capacity) + 1,
    next(KIndex0, Capacity, KIndex).

next(KIndex, _, KIndex).
next(KIndex0, Capacity, KIndex) :-
    KIndex1 is KIndex0+1,
    (   KIndex1 < Capacity
    ->  next(KIndex1, Capacity, KIndex)
    ;   next(1, Capacity, KIndex)
    ).

rehash(Set) :-
    Set = nb_set(Empty, Capacity, Size, Buckets),
    NCapacity is Capacity*2,
    '$filled_array'(NBuckets, buckets, NCapacity, Empty),
    nb_setarg(2, Set, NCapacity),
    nb_setarg(3, Set, 0),
    nb_linkarg(4, Set, NBuckets),
    forall(between(1, Capacity, I),
           reinsert(I, Empty, Buckets, Set)),
    arg(3, Set, NewSize),
    assertion(NewSize == Size).

:- det(reinsert/4).
reinsert(KIndex, Empty, Buckets, Set) :-
    arg(KIndex, Buckets, Key),
    (   same_term(Key, Empty)
    ->  true
    ;   add_nb_set(Key, Set, true)
    ).

%!  hash_key(+Key, -Hash:integer) is det.
%
%   Compute a hash for Term. Note that variant_hash/2 currently does
%   not handle cyclic terms, so use  term_factorized/3 to get rid of
%   the cycles. This means that  this   library  is rather slow when
%   cyclic terms are involved.

:- if(catch((A = f(A), variant_hash(A,_)), _, fail)).
key_hash(Key, Hash) :-
    variant_hash(Key, Hash).
:- else.
key_hash(Key, Hash) :-
    acyclic_term(Key),
    !,
    variant_hash(Key, Hash).
key_hash(Key, Hash) :-
    term_factorized(Key, Skeleton, Substiution),
    variant_hash(Skeleton+Substiution, Hash).
:- endif.

%!  nb_set_to_list(+NBSet, -OrdSet) is det.
%!  nb_set_to_list(-NBSet, +List) is det.
%
%   Get the elements of a an nb_set.   OrdSet  is sorted to the standard
%   order of terms, providing a set representation that is compatible to
%   library(ordsets).

nb_set_to_list(NBSet, Set),
    NBSet = nb_set(Empty, Capacity, _Size, Buckets) =>
    buckets_to_list(1, Empty, Capacity, Buckets, List0),
    sort(List0, Set).

buckets_to_list(KIndex, Empty, Capacity, Buckets, List) :-
    (   arg(KIndex, Buckets, Key)
    ->  (   same_term(Empty, Key)
        ->  KIndex1 is KIndex+1,
            buckets_to_list(KIndex1, Empty, Capacity, Buckets, List)
        ;   List = [Key|List1],
            KIndex1 is KIndex+2,
            buckets_to_list(KIndex1, Empty, Capacity, Buckets, List1)
        )
    ;   List = []
    ).

%!  gen_nb_set(+Set, -Key) is nondet.
%
%   Enumerate the members of a set in the standard order of terms.

gen_nb_set(nb_set(Empty, Capacity, _Size, Buckets), Key) =>
    between(1, Capacity, KIndex),
    arg(KIndex, Buckets, Key),
    \+ same_term(Empty, Key).

%!  size_nb_set(+Set, -Size) is det.
%
%   Unify Size with the number of elements in the set

size_nb_set(nb_set(_Empty, _Capacity, Sz, _Buckets), Size) =>
    Size = Sz.
