%%
%%   Copyright (c) 2012 - 2013, Dmitry Kolesnikov
%%   All Rights Reserved.
%%
%%   Licensed under the Apache License, Version 2.0 (the "License");
%%   you may not use this file except in compliance with the License.
%%   You may obtain a copy of the License at
%%
%%       http://www.apache.org/licenses/LICENSE-2.0
%%
%%   Unless required by applicable law or agreed to in writing, software
%%   distributed under the License is distributed on an "AS IS" BASIS,
%%   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%%   See the License for the specific language governing permissions and
%%   limitations under the License.
%%
%%   @description
%%      binary search tree 
-module(bst).
-behavior(maplike).
-behavior(traversable).
-behavior(foldable).

-include("datum.hrl").

-export([
   new/0,         %% O(1)
   new/1,         %% O(1)

   %%
   %% tree

   %%
   %% map-like
   append/2,      %% O(log n)
   insert/3,      %% O(log n)
   lookup/2,      %% O(log n)
   remove/2,      %% O(log n)
   has/2,         %% O(log n)
   keys/1,        %% O(n)
   apply/3,       %% O(log n)

   %%
   %% traversable
   head/1,
   tail/1,   
   build/1,       %% O(n)
   build/2,       %% O(n)
   list/1,        %% O(n)
   is_empty/1,    %%
   drop/2,        %% O(n)  
   dropwhile/2,   %% O(log n)
   filter/2,      %% O(n)
   foreach/2,     %% O(n)
   map/2,         %% O(n)
   split/2,       %% O(n)
   splitwhile/2,  %% O(log n)
   take/2,        %% O(log n)
   takewhile/2,   %% O(log n)

   %%
   %% foldable
   fold/3,        %% O(n)
   foldl/3,       %% O(n)
   foldr/3,       %% O(n)
   unfold/2,      %% O(n)

   min/1,         %% O(log n)
   max/1          %% O(log n)
]).


%%
%% data types
-type tree()   :: datum:option( {tree(), key(), val(), tree()} ).
-type key()    :: _.
-type val()    :: _.

%%
%% create new binary search tree
-spec new() -> datum:tree(_).

new()  ->
   new(fun datum:compare/2).

%%
%% create new binary search tree
-spec new(datum:compare(_)) -> datum:tree(_).

new(Ord) ->
   #tree{ford = Ord, tree = ?None}.

%%
%% build tree from another traversable structure 
-spec build(_) -> datum:tree(_).

build(List) ->
   maplike:build(?MODULE, List).

%%
%% build tree from another traversable structure
-spec build(datum:compare(_), [_]) -> datum:tree(_).

build(Ord, List) ->
   maplike:build(?MODULE, Ord, List).


%%%----------------------------------------------------------------------------   
%%%
%%% map-like
%%%
%%%----------------------------------------------------------------------------   

%%
%% append a new key/value pair to collection
-spec append({key(), val()}, datum:maplike(_, _)) -> datum:maplike(_, _).

append({Key, Val}, #tree{} = Tree) ->
   insert(Key, Val, Tree);

append(Key, #tree{} = Tree) ->
   insert(Key, ?None, Tree).

%%
%% insert a new a key/value pair to collection
-spec insert(key(), val(), datum:maplike(_, _)) -> datum:maplike(_, _).

insert(Key, Val, #tree{ford = Ord, tree = T} = Tree) ->
   Tree#tree{tree = insert_el(Ord, Key, Val, T)}.

insert_el(_, K, V, ?None) ->
   {?None, K, V, ?None};
insert_el(Ord, K, V, {_, Kx, _, _} = T) ->
   insert_el(Ord(K, Kx), Ord, K, V, T).

insert_el(eq,   _, _, V, {A, Kx,  _, B}) ->
   {A, Kx, V, B};
insert_el(gt, Ord, K, V, {A, Kx, Vx, B}) ->
   {A, Kx, Vx, insert_el(Ord, K, V, B)};
insert_el(lt, Ord, K, V, {A, Kx, Vx, B}) ->
   {insert_el(Ord, K, V, A), Kx, Vx, B}.


%%
%% optionally returns the value associated with key
%%
-spec lookup(key(), datum:maplike(_, _)) -> datum:option( val() ).

lookup(Key, #tree{ford = Ord, tree = T}) ->
   lookup_el(Ord, Key, T).

lookup_el(_, _, ?None) ->
   ?None;
lookup_el(Ord, K, {_, Kx, _, _} = T) ->
   lookup_el(Ord(K, Kx), Ord, K, T).

lookup_el(eq,   _, _, {_, _, Vx, _}) ->
   Vx;
lookup_el(gt, Ord, K, {_, _,  _, B}) -> 
   lookup_el(Ord, K, B);
lookup_el(lt, Ord, K, {A, _,  _, _}) ->
   lookup_el(Ord, K, A).


%%
%% remove key/value pair from collection 
-spec remove(key(), datum:maplike(_, _)) -> datum:maplike(_, _).

remove(K, #tree{ford = Ord, tree = T} = Tree) ->
   Tree#tree{tree = remove_el(Ord, K, T)}.

remove_el(_, _, ?None) ->
   ?None;
remove_el(Ord, K, {_, Kx, _, _} = T) ->
   remove_el(Ord(K, Kx), Ord, K, T).

remove_el(eq, _, _, {A, _, _, ?None}) ->
   A;
remove_el(eq, _, _, {?None, _, _, B}) ->
   B;
remove_el(eq, _, _, {A, _, _, B0}) ->
   {{K, V}, B1} = take_left_node(B0),
   {A, K, V, B1};
remove_el(gt, Ord, K, {A, Kx, Vx, B}) ->
   {A, Kx, Vx, remove_el(Ord, K, B)};
remove_el(lt, Ord, K, {A, Kx, Vx, B}) ->
   {remove_el(Ord, K, A), Kx, Vx, B}.

take_left_node({?None, K, V, B}) ->
   {{K, V}, B};
take_left_node({A0, K, V, B}) ->
   {N, A1} = take_left_node(A0),
   {N, {A1, K, V, B}}.


%%
%% check if the collection has an association
%%
-spec has(key(), datum:maplike(_, _)) -> true | false.

has(Key, Tree) ->
   lookup(Key, Tree) =/= undefined.

%%
%% collects all keys of this collection to list
%%
-spec keys(datum:maplike(_, _)) -> [_].

keys(Tree) ->
   maplike:keys(?MODULE, Tree).

%%
%% apply function on element
-spec apply(key(), fun((datum:option(_)) -> _), datum:maplike(_, _)) -> datum:maplike(_, _).

apply(K, Fun, #tree{ford = Ord, tree = T} = Tree) ->
   Tree#tree{tree = apply_el(Ord, K, Fun, T)}.

apply_el(_, K, Fun, ?None) ->
   {?None, K, Fun(?None), ?None};
apply_el(Ord, K, Fun, {_, Kx, _, _} = T) ->
   apply_el(Ord(K, Kx), Ord, K, Fun, T).

apply_el(eq,   _, _, Fun, {A, Kx, Vx, B}) ->
   {A, Kx, Fun(Vx), B};
apply_el(gt, Ord, K, Fun, {A, Kx, Vx, B}) ->
   {A, Kx, Vx, apply_el(Ord, K, Fun, B)};
apply_el(lt, Ord, K, Fun, {A, Kx, Vx, B}) ->
   {apply_el(Ord, K, Fun, A), Kx, Vx, B}.


%%%----------------------------------------------------------------------------   
%%%
%%% traversable
%%%
%%%----------------------------------------------------------------------------   

%%
%% take collection and return head element of collection
%%
-spec head(datum:traversable(_)) -> datum:option(_).

head(_) ->
   exit(not_implemented).

%%
%% take collection and return its suffix (all elements except the first)
%%
-spec tail(datum:traversable(_)) -> datum:traversable(_).

tail(_) ->
   exit(not_implemented).

%%
%% converts the collection to Erlang list
%%
-spec list(datum:traversable(_)) -> [_].

list(Tree) ->
   foldr(fun(Pair, Acc) -> [Pair | Acc] end, [], Tree).


%%
%% return true if collection is empty 
%%
-spec is_empty(datum:traversable(_)) -> true | false.

is_empty(#tree{tree = ?None}) ->
   true;
is_empty(_) ->
   false.


%%
%% return the suffix of collection that starts at the next element after nth.
%% drop first n elements
%%
-spec drop(integer(), datum:traversable(_)) -> datum:traversable(_).

drop(N, #tree{tree = T} = Tree) ->
   Tree#tree{tree = erlang:element(2, drop_el(N, T))}.

drop_el(N, ?None) ->
   {N, ?None};
drop_el(N, {A, K, V, B}) ->
   case drop_el(N, A) of
      {0, Ax} ->
         {0, {Ax, K, V, B}};
      {M,_Ax} ->
         drop_el(M - 1, B)
   end.

%%
%% drops elements from collection while predicate returns true and 
%% returns remaining stream suffix.
%%
-spec dropwhile(datum:predicate(_), datum:traversable(_)) -> datum:traversable(_).      

dropwhile(Pred, #tree{tree = T} = Tree) ->
   Tree#tree{tree = dropwhile_el(Pred, T)}.

dropwhile_el(_, ?None) ->
   ?None;

dropwhile_el(Pred, {A, K, V, B}) ->
   case Pred({K, V}) of
      false ->
         {dropwhile_el(Pred, A), K, V, B};
      true  ->
         dropwhile_el(Pred, B)
   end.

%%
%% returns a newly-allocated collection that contains only those elements of the 
%% input collection for which predicate is true.
%%
-spec filter(datum:predicate(_), datum:traversable(_)) -> datum:traversable(_).

filter(Pred, #tree{tree = T} = Tree) ->
   Tree#tree{tree = filter_el(Pred, T)}.

filter_el(_, ?None) ->
   ?None;
filter_el(Pred, {A0, K, V, B0}) ->
   A1 = filter_el(Pred, A0),
   case Pred({K, V}) of
      false  ->
         case filter_el(Pred, B0) of
            ?None ->
               A1;
            B1     ->
               {{Kx, Vx}, B2} = take_left_node(B1),
               {A1, Kx, Vx, B2}
         end;
      true ->
         {A1, K, V, filter_el(Pred, B0)}
   end.

%%
%% applies a function to each collection element for its side-effects; 
%% it returns nothing.
%%
-spec foreach(datum:effect(_), datum:traversable(_)) -> ok.

foreach(Pred, #tree{tree = T}) ->
   foreach_el(Pred, T).

foreach_el(_, ?None) ->
   ok;
foreach_el(Pred, {A, K, V, B}) ->
   foreach_el(Pred, A),
   Pred({K, V}),
   foreach_el(Pred, B).


%%
%% create a new collection by apply a function to each element of input collection.
%% 
-spec map(fun((_) -> _), datum:traversable(_)) -> datum:traversable(_).

map(Fun, #tree{tree = T} = Tree) ->
   Tree#tree{tree = map_el(Fun, T)}.

map_el(_, ?None) ->
   ?None;
map_el(Fun, {A, K, V, B}) ->
   {map_el(Fun, A), K, Fun({K, V}), map_el(Fun, B)}.


%%
%% partitions collection into two collection. The split behaves as if it is defined as 
%% consequent take(N, Seq), drop(N, Seq). 
%%
-spec split(integer(), datum:traversable(_)) -> {datum:traversable(_), datum:traversable(_)}.

split(N, #tree{ford = Ord, tree = T}) ->
   {_, A, B} = split_el(N, T),
   {#tree{ford = Ord, tree = A}, #tree{ford = Ord, tree = B}}.

split_el(N, ?None) ->
   {N, ?None, ?None};

split_el(N, {A, K, V, B}) ->
   case split_el(N, A) of
      {0, A1, A2} ->
         {0, A1, {A2, K, V, B}};
      {N1,  _,  _} ->
         {N2, B1, B2} = split_el(N1 - 1, B),
         {N2, {A, K, V, B1}, B2}
   end.


%%
%% partitions stream into two streams according to predicate.
%% The splitwith/2 behaves as if it is defined as consequent 
%% takewhile(Pred, Seq), dropwhile(Pred, Seq)
%%
-spec splitwhile(datum:predicate(_), datum:traversable(_)) -> {datum:traversable(_), datum:traversable(_)}.

splitwhile(Pred, #tree{ford = Ord, tree = T}) ->
   {A, B} = splitwhile_el(Pred, T),
   {#tree{ford = Ord, tree = A}, #tree{ford = Ord, tree = B}}.

splitwhile_el(_, ?None) ->
   {?None, ?None};

splitwhile_el(Fun, {A, K, V, B}) ->
   case Fun({K, V}) of
      false ->
         {Ax, Bx} = splitwhile_el(Fun, A),
         {Ax, {Bx, K, V, B}};
      true  ->
         {Ax, Bx} = splitwhile_el(Fun, B),
         {{A, K, V, Ax}, Bx}
   end.

%%
%% returns a newly-allocated collection containing the first n elements of 
%% the input collection.
%%
-spec take(integer(), datum:traversable(_)) -> datum:traversable(_).


take(N, #tree{tree = T} = Tree) ->
   Tree#tree{tree = erlang:element(2, take_el(N, T))}.

take_el(N, ?None) ->
   {N, ?None};
take_el(N, {A, K, V, B}) ->
   case take_el(N, A) of
      {0, Ax} ->
         {0, Ax};
      {M, Ax} ->
         {R, Bx} = take_el(M - 1, B),
         {R, {Ax, K, V, Bx}}
   end.

%%
%% returns a newly-allocated collection that contains those elements from 
%% input collection while predicate returns true.
%%
-spec takewhile(datum:predicate(_), datum:traversable(_)) -> datum:traversable(_).

takewhile(Pred, #tree{tree = T} = Tree) ->
   Tree#tree{tree = takewhile_el(Pred, T)}.

takewhile_el(_, ?None) ->
   ?None;

takewhile_el(Pred, {A, K, V, B}) ->
   case Pred({K, V}) of
      false ->
         takewhile_el(Pred, A);
      true  ->
         {A, K, V, takewhile_el(Pred, B)}
   end.


%%%----------------------------------------------------------------------------   
%%%
%%% foldable
%%%
%%%----------------------------------------------------------------------------   

%%
%% Combine elements of a structure using a monoid
%% (with an associative binary operation)
%% 
-spec fold(datum:monoid(_), _, datum:foldable(_)) -> _.

fold(Fun, Acc, Tree) ->
   foldl(Fun, Acc, Tree).

%%
%% Left-associative fold of a structure
%%
-spec foldl(datum:monoid(_), _, datum:foldable(_)) -> _.

foldl(Fun, Acc, #tree{tree = T}) ->
   foldl_el(Fun, Acc, T).

foldl_el(_Fun, Acc0, ?None) ->
   Acc0;
foldl_el(Fun, Acc0, {A, K, V, B}) ->
   foldl_el(Fun, Fun({K, V}, foldl_el(Fun, Acc0, A)), B).

%%
%% Right-associative fold of a structure
%%
-spec foldr(datum:monoid(_), _, datum:foldable(_)) -> _.

foldr(Fun, Acc, #tree{tree = T}) ->
   foldr_el(Fun, Acc, T).
   
foldr_el(_Fun, Acc0, ?None) ->
   Acc0;
foldr_el(Fun, Acc0, {A, K, V, B}) ->
   foldr_el(Fun, Fun({K, V}, foldr_el(Fun, Acc0, B)), A).

%% 
%% The fundamental recursive structure constructor, 
%% it applies a function to each previous seed element in turn
%% to determine the next element.
%%
-spec unfold(fun((_) -> _), _) -> datum:foldable(_).

unfold(Fun, Seed) ->
   unfold(Fun, Seed, new()).

unfold(Fun, Seed, Acc) ->
   case Fun(Seed) of
      {Head, Next} ->
         unfold(Fun, Next, append(Head, Acc));
      _ ->
         Acc
   end.


%%
%% return smallest element
-spec min(tree()) -> {key(), val()} | undefined.

min(#tree{tree = T}) ->
   min_el(T).

min_el({?None, K, V, _}) ->
   {K, V};
min_el({A, _, _, _}) ->
   min_el(A);
min_el(?None) ->
   undefined.

%%
%% return largest element
-spec max(tree()) -> {key(), val()}.

max(#tree{tree = T}) ->
   max_el(T).

max_el({_, K, V, ?None}) ->
   {K, V};
max_el({_, _, _, B}) ->
   max_el(B);
max_el(?None) ->
   undefined.

%%
%% map and fold function over tree
% -spec mapfoldl(function(), any(), datum:tree()) -> {datum:tree(), any()}.
%
% mapfoldl(Fun, Acc0, {t, Ord, T}) ->
%    {Tx, Acc} = mapfoldl_el(Fun, Acc0, T),
%    {{t, Ord, Tx}, Acc}.
%
% mapfoldl_el(_Fun, Acc0, ?NULL) ->
%    {?NULL, Acc0};
% mapfoldl_el(Fun, Acc0, {A, K, V, B}) ->
%    {Ax, AccA} = mapfoldl_el(Fun, Acc0, A),
%    {Vx, AccK} = Fun(K, V, AccA),
%    {Bx, AccB} = mapfoldl_el(Fun, AccK, B),
%    {{Ax, K, Vx, Bx}, AccB}.


%%
%% map and fold function over tree
% -spec mapfoldr(function(), any(), datum:tree()) -> {datum:tree(), any()}.
%
% mapfoldr(Fun, Acc0, {t, Ord, T}) ->
%    {Tx, Acc} = mapfoldr_el(Fun, Acc0, T),
%    {{t, Ord, Tx}, Acc}.
%
% mapfoldr_el(_Fun, Acc0, ?NULL) ->
%    {?NULL, Acc0};
% mapfoldr_el(Fun, Acc0, {A, K, V, B}) ->
%    {Bx, AccB} = mapfoldl_el(Fun, Acc0, B),
%    {Vx, AccK} = Fun(K, V, AccB),
%    {Ax, AccA} = mapfoldl_el(Fun, AccK, A),
%    {{Ax, K, Vx, Bx}, AccA}.