text: /**
   *    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.
   */
  
  #pragma once
  
  #include "mongo/base/status.h"
  #include "mongo/base/string_data.h"
  #include "mongo/bson/mutable/element.h"
  #include "mongo/db/field_ref.h"
  #include "mongo/db/jsobj.h"
  
  namespace mongo {
  
      class LogBuilder;
  
      /**
       * Abstract base class for update "modifiers" (a.k.a "$ operators"). To create a new
       * operator, implement a new derived class.
       *
       * A typical call sequence for the class is:
       *
       *   + init() with the mod arguments
       *
       *   + For each document that is being touched on that update, the following methods are
       *     going to be called once for that document and in the order the calls appear here.
       *
       *     + prepare() to check if mod is viable over the document
       *
       *     + apply(), effectively computing the update
       *
       *     + log() registering the change in the log for replication purposes
       *
       * Again, a modifier implementation may rely on these last three calls being made and in
       * that particular order and therefore can keep and reuse state between these calls, when
       * appropriate.
       *
       * TODO:
       * For a reference implementation, see modifier_identity.{h,cpp} used in tests.
       */
      class ModifierInterface {
      public:
          virtual ~ModifierInterface() { }
  
          struct Options;
          /**
           * Returns OK and extracts the parameters for this given mod from 'modExpr'. For
           * instance, for a $inc, extracts the increment value. The init() method would be
           * called only once per operand, that is, if a { $inc: { a: 1, b: 1 } } is issued,
           * there would be one instance of the operator working on 'a' and one on 'b'. In each
           * case, init() would be called once with the respective bson element.
           *
           * If 'modExpr' is invalid, returns an error status with a reason description.
           *
           * The optional bool out parameter 'positional', if provided, will be set to 'true' if
           * the mod requires matched field details to be provided when calling 'prepare'. The
           * field is optional since this is a hint to the caller about what work is needed to
           * correctly invoke 'prepare'. It is always legal to provide any match details
           * unconditionally. The value set in 'positional' if any, is only meaningful if 'init'
           * returns an OK status.
           *
           * Note:
           *
           *   + An operator may assume the modExpr passed here will be unchanged throughout all
           *     the mod object lifetime and also that the modExrp's lifetime exceeds the life
           *     time of this mod. Therefore, taking references to elements inside modExpr is
           *     valid.
           */
          virtual Status init(const BSONElement& modExpr, const Options& opts,
                              bool* positional = NULL) = 0;
  
          /**
           * Returns OK if it would be correct to apply this mod over the document 'root' (e.g, if
           * we're $inc-ing a field, is that field numeric in the current doc?).
           *
           * If the field this mod is targeted to contains a $-positional parameter, that value
           * can be bound with 'matchedField', passed by the caller.
           *
           * In addition, the call also identifies which fields(s) of 'root' the mod is interested
           * in changing (note that the modifier may want to add a field that's not present in
           * the document). The call also determines whether it could modify the document in
           * place and whether it is a no-op for the given document. All this information is in
           * the passed 'execInfo', which is filled inside the call.
           *
           * If the mod cannot be applied over 'root', returns an error status with a reason
           * description.
           *
           * Note that you must provide a meaningful 'matchedField' here, unless 'init' set
           * 'positional' to 'false', in which case you may pass an empty StringData object.
           */
          struct ExecInfo;
          virtual Status prepare(mutablebson::Element root,
                                 StringData matchedField,
                                 /* IN-OUT */ ExecInfo* execInfo) = 0;
  
          /**
           * Returns OK and modifies (or adds) an element (or elements) from the 'root' passed on
           * the prepareMod call. This may act on multiple fields but should only be called once
           * per operator.
           *
           * For this call to be issued, the call to 'prepareElem' must have necessarily turned
           * off 'ExecInfo.noOp', ie this mod over this document is not a no-op.
           *
           * If the mod could not be applied, returns an error status with a reason description.
           */
          virtual Status apply() const = 0 ;
  
          /**
           * Returns OK and records the result of this mod in the provided LogBuilder. The mod
           * must have kept enough state to be able to produce the log record (see idempotency
           * note below). This call may be issued even if apply() was not.
           *
           * If the mod could not be logged, returns an error status with a reason description.
           *
           * Idempotency Note:
           *
           *   + The modifier must log a mod that is idempotent, ie, applying it more than once
           *     to a base collection would produce the same result as applying it only once. For
           *     example, a $inc can be switched to a $set for the resulting incremented value,
           *     for logging purposes.  An array based operator may check the contents of the
           *     array before operating on it.
           */
          virtual Status log(LogBuilder* logBuilder) const = 0;
      };
  
      /**
       * Options used to control Modifier behavior
       */
      struct ModifierInterface::Options {
          Options() : fromReplication(false), enforceOkForStorage(true) {}
          Options(bool repl, bool ofs) : fromReplication(repl), enforceOkForStorage(ofs) {}
  
          static Options normal() { return Options(false, true); }
          static Options fromRepl() { return Options(true, false); }
          static Options unchecked() { return Options(false, false); }
  
          bool fromReplication;
          bool enforceOkForStorage;
      };
  
      struct ModifierInterface::ExecInfo {
          static const int MAX_NUM_FIELDS = 2;
  
          /**
           * An update mod may specify that it wishes to the applied only if the context
           * of the update turns out a certain way.
           */
          enum UpdateContext {
              // This mod wants to be applied only if the update turns out to be an insert.
              INSERT_CONTEXT,
  
              // This mod wants to be applied only if the update is not an insert.
              UPDATE_CONTEXT,
  
              // This mod doesn't care if the update will be an update or an upsert.
              ANY_CONTEXT
          };
  
          ExecInfo() : noOp(false), context(ANY_CONTEXT) {
              for (int i = 0; i < MAX_NUM_FIELDS; i++) {
                  fieldRef[i] = NULL;
              }
          }
  
          // The fields of concern to the driver: no other op may modify the fields listed here.
          FieldRef* fieldRef[MAX_NUM_FIELDS]; // not owned here
          bool noOp;
          UpdateContext context;
      };
  
  } // namespace mongo
  