% 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.

-module(mango_idx_view).


-export([
    validate/1,
    add/2,
    remove/2,
    from_ddoc/1,
    to_json/1,
    is_usable/2,
    columns/1,
    start_key/1,
    end_key/1,

    indexable_fields/1,
    field_ranges/1,
    field_ranges/2
]).


-include_lib("couch/include/couch_db.hrl").
-include("mango.hrl").
-include("mango_idx.hrl").


validate(#idx{}=Idx) ->
    {ok, Def} = do_validate(Idx#idx.def),
    {ok, Idx#idx{def=Def}}.


add(#doc{body={Props0}}=DDoc, Idx) ->
    Views1 = case proplists:get_value(<<"views">>, Props0) of
        {Views0} -> Views0;
        _ -> []
    end,
    NewView = make_view(Idx),
    Views2 = lists:keystore(element(1, NewView), 1, Views1, NewView),
    Props1 = lists:keystore(<<"views">>, 1, Props0, {<<"views">>, {Views2}}),
    {ok, DDoc#doc{body={Props1}}}.


remove(#doc{body={Props0}}=DDoc, Idx) ->
    Views1 = case proplists:get_value(<<"views">>, Props0) of
        {Views0} ->
            Views0;
        _ ->
            ?MANGO_ERROR({index_not_found, Idx#idx.name})
    end,
    Views2 = lists:keydelete(Idx#idx.name, 1, Views1),
    if Views2 /= Views1 -> ok; true ->
        ?MANGO_ERROR({index_not_found, Idx#idx.name})
    end,
    Props1 = case Views2 of
        [] ->
            lists:keydelete(<<"views">>, 1, Props0);
        _ ->
            lists:keystore(<<"views">>, 1, Props0, {<<"views">>, {Views2}})
    end,
    {ok, DDoc#doc{body={Props1}}}.


from_ddoc({Props}) ->
    case lists:keyfind(<<"views">>, 1, Props) of
        {<<"views">>, {Views}} when is_list(Views) ->
            lists:flatmap(fun({Name, {VProps}}) ->
                Def = proplists:get_value(<<"map">>, VProps),
                {Opts0} = proplists:get_value(<<"options">>, VProps),
                Opts = lists:keydelete(<<"sort">>, 1, Opts0),
                I = #idx{
                    type = <<"json">>,
                    name = Name,
                    def = Def,
                    opts = Opts
                },
                % TODO: Validate the index definition
                [I]
            end, Views);
        _ ->
            []
    end.


to_json(Idx) ->
    {[
        {ddoc, Idx#idx.ddoc},
        {name, Idx#idx.name},
        {type, Idx#idx.type},
        {def, {def_to_json(Idx#idx.def)}}
    ]}.


columns(Idx) ->
    {Props} = Idx#idx.def,
    {<<"fields">>, {Fields}} = lists:keyfind(<<"fields">>, 1, Props),
    [Key || {Key, _} <- Fields].


is_usable(Idx, Selector) ->
    % This index is usable if at least the first column is
    % a member of the indexable fields of the selector.
    Columns = columns(Idx),
    Fields = indexable_fields(Selector),
    lists:member(hd(Columns), Fields).


start_key([]) ->
    [];
start_key([{'$gt', Key, _, _} | Rest]) ->
    case mango_json:special(Key) of
        true ->
            [];
        false ->
            [Key | start_key(Rest)]
    end;
start_key([{'$gte', Key, _, _} | Rest]) ->
    false = mango_json:special(Key),
    [Key | start_key(Rest)];
start_key([{'$eq', Key, '$eq', Key} | Rest]) ->
    false = mango_json:special(Key),
    [Key | start_key(Rest)].


end_key([]) ->
    [{[]}];
end_key([{_, _, '$lt', Key} | Rest]) ->
    case mango_json:special(Key) of
        true ->
            [{[]}];
        false ->
            [Key | end_key(Rest)]
    end;
end_key([{_, _, '$lte', Key} | Rest]) ->
    false = mango_json:special(Key),
    [Key | end_key(Rest)];
end_key([{'$eq', Key, '$eq', Key} | Rest]) ->
    false = mango_json:special(Key),
    [Key | end_key(Rest)].


do_validate({Props}) ->
    {ok, Opts} = mango_opts:validate(Props, opts()),
    {ok, {Opts}};
do_validate(Else) ->
    ?MANGO_ERROR({invalid_index_json, Else}).


def_to_json({Props}) ->
    def_to_json(Props);
def_to_json([]) ->
    [];
def_to_json([{fields, Fields} | Rest]) ->
    [{<<"fields">>, mango_sort:to_json(Fields)} | def_to_json(Rest)];
def_to_json([{<<"fields">>, Fields} | Rest]) ->
    [{<<"fields">>, mango_sort:to_json(Fields)} | def_to_json(Rest)];
def_to_json([{Key, Value} | Rest]) ->
    [{Key, Value} | def_to_json(Rest)].


opts() ->
    [
        {<<"fields">>, [
            {tag, fields},
            {validator, fun mango_opts:validate_sort/1}
        ]}
    ].


make_view(Idx) ->
    View = {[
        {<<"map">>, Idx#idx.def},
        {<<"reduce">>, <<"_count">>},
        {<<"options">>, {Idx#idx.opts}}
    ]},
    {Idx#idx.name, View}.


% This function returns a list of indexes that
% can be used to restrict this query. This works by
% searching the selector looking for field names that
% can be "seen".
%
% Operators that can be seen through are '$and' and any of
% the logical comparisons ('$lt', '$eq', etc). Things like
% '$regex', '$in', '$nin', and '$or' can't be serviced by
% a single index scan so we disallow them. In the future
% we may become more clever and increase our ken such that
% we will be able to see through these with crafty indexes
% or new uses for existing indexes. For instance, I could
% see an '$or' between comparisons on the same field becoming
% the equivalent of a multi-query. But that's for another
% day.

% We can see through '$and' trivially
indexable_fields({[{<<"$and">>, Args}]}) ->
    lists:usort(lists:flatten([indexable_fields(A) || A <- Args]));

% So far we can't see through any other operator
indexable_fields({[{<<"$", _/binary>>, _}]}) ->
    [];

% If we have a field with a terminator that is locatable
% using an index then the field is a possible index
indexable_fields({[{Field, Cond}]}) ->
    case indexable(Cond) of
        true ->
            [Field];
        false ->
            []
    end;

% An empty selector
indexable_fields({[]}) ->
    [].


% Check if a condition is indexable. The logical
% comparisons are mostly straight forward. We
% currently don't understand '$in' which is
% theoretically supportable. '$nin' and '$ne'
% aren't currently supported because they require
% multiple index scans.
indexable({[{<<"$lt">>, _}]}) ->
    true;
indexable({[{<<"$lte">>, _}]}) ->
    true;
indexable({[{<<"$eq">>, _}]}) ->
    true;
indexable({[{<<"$gt">>, _}]}) ->
    true;
indexable({[{<<"$gte">>, _}]}) ->
    true;

% All other operators are currently not indexable.
% This is also a subtle assertion that we don't
% call indexable/1 on a field name.
indexable({[{<<"$", _/binary>>, _}]}) ->
    false.


% For each field, return {Field, Range}
field_ranges(Selector) ->
    Fields = indexable_fields(Selector),
    field_ranges(Selector, Fields).


field_ranges(Selector, Fields) ->
    field_ranges(Selector, Fields, []).


field_ranges(_Selector, [], Acc) ->
    lists:reverse(Acc);
field_ranges(Selector, [Field | Rest], Acc) ->
    case range(Selector, Field) of
        empty ->
            [{Field, empty}];
        Range ->
            field_ranges(Selector, Rest, [{Field, Range} | Acc])
    end.


% Find the complete range for a given index in this
% selector. This works by AND'ing logical comparisons
% together so that we can define the start and end
% keys for a given index.
%
% Selector must have been normalized before calling
% this function.
range(Selector, Index) ->
    range(Selector, Index, '$gt', mango_json:min(), '$lt', mango_json:max()).


% Adjust Low and High based on values found for the
% givend Index in Selector.
range({[{<<"$and">>, Args}]}, Index, LCmp, Low, HCmp, High) ->
    lists:foldl(fun
        (Arg, {LC, L, HC, H}) ->
            range(Arg, Index, LC, L, HC, H);
        (_Arg, empty) ->
            empty
    end, {LCmp, Low, HCmp, High}, Args);

% We can currently only traverse '$and' operators
range({[{<<"$", _/binary>>}]}, _Index, LCmp, Low, HCmp, High) ->
    {LCmp, Low, HCmp, High};

% If the field name matches the index see if we can narrow
% the acceptable range.
range({[{Index, Cond}]}, Index, LCmp, Low, HCmp, High) ->
    range(Cond, LCmp, Low, HCmp, High);

% Else we have a field unrelated to this index so just
% return the current values.
range(_, _, LCmp, Low, HCmp, High) ->
    {LCmp, Low, HCmp, High}.


% The comments below are a bit cryptic at first but they show
% where the Arg cand land in the current range.
%
% For instance, given:
%
%     {$lt: N}
%     Low = 1
%     High = 5
%
% Depending on the value of N we can have one of five locations
% in regards to a given Low/High pair:
%
%     min low mid high max
%
%   That is:
%       min = (N < Low)
%       low = (N == Low)
%       mid = (Low < N < High)
%       high = (N == High)
%       max = (High < N)
%
% If N < 1, (min) then the effective range is empty.
%
% If N == 1, (low) then we have to set the range to empty because
% N < 1 && N >= 1 is an empty set. If the operator had been '$lte'
% and LCmp was '$gte' or '$eq' then we could keep around the equality
% check on Arg by setting LCmp == HCmp = '$eq' and Low == High == Arg.
%
% If 1 < N < 5 (mid), then we set High to Arg and Arg has just
% narrowed our range. HCmp is set the the '$lt' operator that was
% part of the input.
%
% If N == 5 (high), We just set HCmp to '$lt' since its guaranteed
% to be equally or more restrictive than the current possible values
% of '$lt' or '$lte'.
%
% If N > 5 (max), nothing changes as our current range is already
% more narrow than the current condition.
%
% Obviously all of that logic gets tweaked for the other logical
% operators but its all straight forward once you figure out how
% we're basically just narrowing our logical ranges.

range({[{<<"$lt">>, Arg}]}, LCmp, Low, HCmp, High) ->
    case range_pos(Low, Arg, High) of
        min ->
            empty;
        low ->
            empty;
        mid ->
            {LCmp, Low, '$lt', Arg};
        high ->
            {LCmp, Low, '$lt', Arg};
        max ->
            {LCmp, Low, HCmp, High}
    end;

range({[{<<"$lte">>, Arg}]}, LCmp, Low, HCmp, High) ->
    case range_pos(Low, Arg, High) of
        min ->
            empty;
        low when LCmp == '$gte'; LCmp == '$eq' ->
            {'$eq', Arg, '$eq', Arg};
        low ->
            empty;
        mid ->
            {LCmp, Low, '$lte', Arg};
        high ->
            {LCmp, Low, HCmp, High};
        max ->
            {LCmp, Low, HCmp, High}
    end;

range({[{<<"$eq">>, Arg}]}, LCmp, Low, HCmp, High) ->
    case range_pos(Low, Arg, High) of
        min ->
            empty;
        low when LCmp == '$gte'; LCmp == '$eq' ->
            {'$eq', Arg, '$eq', Arg};
        low ->
            empty;
        mid ->
            {'$eq', Arg, '$eq', Arg};
        high when HCmp == '$lte'; HCmp == '$eq' ->
            {'$eq', Arg, '$eq', Arg};
        high ->
            empty;
        max ->
            empty
    end;

range({[{<<"$gte">>, Arg}]}, LCmp, Low, HCmp, High) ->
    case range_pos(Low, Arg, High) of
        min ->
            {LCmp, Low, HCmp, High};
        low ->
            {LCmp, Low, HCmp, High};
        mid ->
            {'$gte', Arg, HCmp, High};
        high when HCmp == '$lte'; HCmp == '$eq' ->
            {'$eq', Arg, '$eq', Arg};
        high ->
            empty;
        max ->
            empty
    end;

range({[{<<"$gt">>, Arg}]}, LCmp, Low, HCmp, High) ->
    case range_pos(Low, Arg, High) of
        min ->
            {LCmp, Low, HCmp, High};
        low ->
            {'$gt', Arg, HCmp, High};
        mid ->
            {'$gt', Arg, HCmp, High};
        high ->
            empty;
        max ->
            empty
    end;

% There's some other un-indexable restriction on the index
% that will be applied as a post-filter. Ignore it and
% carry on our merry way.
range({[{<<"$", _/binary>>, _}]}, LCmp, Low, HCmp, High) ->
    {LCmp, Low, HCmp, High}.


% Returns the value min | low | mid | high | max depending
% on how Arg compares to Low and High.
range_pos(Low, Arg, High) ->
    case mango_json:cmp(Arg, Low) of
        N when N < 0 -> min;
        N when N == 0 -> low;
        _ ->
            case mango_json:cmp(Arg, High) of
                X when X < 0 ->
                    mid;
                X when X == 0 ->
                    high;
                _ ->
                    max
            end
    end.