namespace LeanCloud.Storage.Internal {
  /// <summary>
  /// A AVFieldOperation represents a modification to a value in a AVObject.
  /// For example, setting, deleting, or incrementing a value are all different kinds of
  /// AVFieldOperations. AVFieldOperations themselves can be considered to be
  /// immutable.
  /// </summary>
  public interface IAVFieldOperation {
    /// <summary>
    /// Converts the AVFieldOperation to a data structure that can be converted to JSON and sent to
    /// LeanCloud 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 AVFieldOperation or this.</returns>
    IAVFieldOperation MergeWithPrevious(IAVFieldOperation previous);

    /// <summary>
    /// Returns a new estimated value based on a previous value and this operation. This
    /// value is not intended to be sent to LeanCloud, 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 AVRelation 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);
  }
}