/**
 *    Copyright (C) 2013 10gen Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the GNU Affero General Public License in all respects
 *    for all of the code used other than as permitted herein. If you modify
 *    file(s) with this exception, you may extend this exception to your
 *    version of the file(s), but you are not obligated to do so. If you do not
 *    wish to do so, delete this exception statement from your version. If you
 *    delete this exception statement from all source files in the program,
 *    then also delete it in the license file.
 */

#include "mongo/db/matcher/expression.h"

#pragma once

namespace mongo {

    /**
     * Logic for how indices can be used with an expression.
     */
    class Indexability {
    public:
        /**
         * Is an index over me->path() useful?
         * This is the same thing as being sargable, if you have a RDBMS background.
         */
        static bool nodeCanUseIndexOnOwnField(const MatchExpression* me) {
            if (me->path().empty()) {
                return false;
            }

            if (arrayUsesIndexOnOwnField(me)) {
                return true;
            }

            return isIndexOnOwnFieldTypeNode(me);
        }

        /**
         * This array operator doesn't have any children with fields and can use an index.
         *
         * Example: a: {$elemMatch: {$gte: 1, $lte: 1}}.
         */
        static bool arrayUsesIndexOnOwnField(const MatchExpression* me) {
            if (!me->isArray()) {
                return false;
            }

            if (MatchExpression::ELEM_MATCH_VALUE != me->matchType()) {
                return false;
            }

            // We have an ELEM_MATCH_VALUE expression. In order to be
            // considered "indexable" all children of the ELEM_MATCH_VALUE
            // must be "indexable" type expressions as well.
            for (size_t i = 0; i < me->numChildren(); i++) {
                if (!isIndexOnOwnFieldTypeNode(me->getChild(i))) {
                    return false;
                }
            }

            return true;
        }

        /**
         * Certain array operators require that the field for that operator is prepended
         * to all fields in that operator's children.
         *
         * Example: a: {$elemMatch: {b:1, c:1}}.
         */
        static bool arrayUsesIndexOnChildren(const MatchExpression* me) {
            return me->isArray() && MatchExpression::ELEM_MATCH_OBJECT == me->matchType();
        }

        /**
         * Returns true if 'me' is a NOT, and the child of the NOT can use
         * an index on its own field.
         */
        static bool isBoundsGeneratingNot(const MatchExpression* me) {
            return MatchExpression::NOT == me->matchType() &&
                   nodeCanUseIndexOnOwnField(me->getChild(0));
        }

        /**
         * Returns true if either 'me' is a bounds generating NOT,
         * or 'me' can use an index on its own field.
         */
        static bool isBoundsGenerating(const MatchExpression* me) {
            return isBoundsGeneratingNot(me) || nodeCanUseIndexOnOwnField(me);
        }

    private:
        /**
         * Returns true if 'me' is "sargable" but is not a negation and
         * is not an array node such as ELEM_MATCH_VALUE.
         *
         * Used as a helper for nodeCanUseIndexOnOwnField().
         */
        static bool isIndexOnOwnFieldTypeNode(const MatchExpression* me) {
            return    me->matchType() == MatchExpression::LTE
                   || me->matchType() == MatchExpression::LT
                   || me->matchType() == MatchExpression::EQ
                   || me->matchType() == MatchExpression::GT
                   || me->matchType() == MatchExpression::GTE
                   || me->matchType() == MatchExpression::REGEX
                   || me->matchType() == MatchExpression::MOD
                   || me->matchType() == MatchExpression::MATCH_IN
                   || me->matchType() == MatchExpression::TYPE_OPERATOR
                   || me->matchType() == MatchExpression::GEO
                   || me->matchType() == MatchExpression::GEO_NEAR
                   || me->matchType() == MatchExpression::EXISTS
                   || me->matchType() == MatchExpression::TEXT;
        }
    };

}  // namespace mongo
