// Copyright (c) 2015-present, Parse, LLC.  All rights reserved.  This source code is licensed under the BSD-style license found in the LICENSE file in the root directory of this source tree.  An additional grant of patent rights can be found in the PATENTS file in the same directory.

namespace Parse.Internal {
  /// <summary>
  /// A ParseFieldOperation represents a modification to a value in a ParseObject.
  /// For example, setting, deleting, or incrementing a value are all different kinds of
  /// ParseFieldOperations. ParseFieldOperations themselves can be considered to be
  /// immutable.
  /// </summary>
  interface IParseFieldOperation {
    /// <summary>
    /// Converts the ParseFieldOperation to a data structure that can be converted to JSON and sent to
    /// Parse as part of a save operation.
    /// </summary>
    /// <returns>An object to be JSONified.</returns>
    object Encode();

    /// <summary>
    /// Returns a field operation that is composed of a previous operation followed by
    /// this operation. This will not mutate either operation. However, it may return
    /// <code>this</code> if the current operation is not affected by previous changes.
    /// For example:
    ///   {increment by 2}.MergeWithPrevious({set to 5})       -> {set to 7}
    ///         {set to 5}.MergeWithPrevious({increment by 2}) -> {set to 5}
    ///        {add "foo"}.MergeWithPrevious({delete})         -> {set to ["foo"]}
    ///           {delete}.MergeWithPrevious({add "foo"})      -> {delete}        /// </summary>
    /// <param name="previous">The most recent operation on the field, or null if none.</param>
    /// <returns>A new ParseFieldOperation or this.</returns>
    IParseFieldOperation MergeWithPrevious(IParseFieldOperation previous);

    /// <summary>
    /// Returns a new estimated value based on a previous value and this operation. This
    /// value is not intended to be sent to Parse, but it is used locally on the client to
    /// inspect the most likely current value for a field.
    /// 
    /// The key and object are used solely for ParseRelation to be able to construct objects
    /// that refer back to their parents.
    /// </summary>
    /// <param name="oldValue">The previous value for the field.</param>
    /// <param name="key">The key that this value is for.</param>
    /// <returns>The new value for the field.</returns>
    object Apply(object oldValue, string key);
  }
}
