package scala
package reflect
package macros

/**
 * <span class="badge badge-red" style="float: right;">EXPERIMENTAL</span>
 *
 *  Attachments provide a way to associate custom metadata with symbols and trees.
 *
 *  Along with `symbol` and `tpe`, which represent core metadata of trees, each tree
 *  carries the `attachments` field that can store other metadata: compiler-defined (e.g. positions) or user-defined.
 *  Same story is true for symbols, which also have extensible metadata by the virtue
 *  of the same `attachments` field.
 *
 *  Typically attachments just store a [[scala.reflect.api.Position]], but they can be extended to
 *  encompass arbitrary payloads. Payloads are stored in type-indexed slots, which can be read with `get[T]` and written
 *  with `update[T]` and `remove[T]`.
 *
 *  This API doesn't have much use in the runtime reflection API (the [[scala.reflect.api]] package), but it might be of help
 *  for macro writers, providing a way to coordinate multiple macros operating on the same code. Therefore the `attachments`
 *  field is only declared in trees and symbols belonging to [[scala.reflect.macros.Universe]].
 */
abstract class Attachments { self =>

  /** The position type of this attachment */
  type Pos >: Null

  /** The underlying position */
  def pos: Pos

  /** Creates a copy of this attachment with the position replaced by `newPos` */
  def withPos(newPos: Pos): Attachments { type Pos = self.Pos }

  /** The underlying payload with the guarantee that no two elements have the same type. */
  def all: Set[Any] = Set.empty

  private def matchesTag[T: ClassTag](datum: Any) =
    classTag[T].runtimeClass.isInstance(datum)

  /** An underlying payload of the given class type `T`. */
  def get[T: ClassTag]: Option[T] =
    (all filter matchesTag[T]).headOption.asInstanceOf[Option[T]]

  /** Check underlying payload contains an instance of type `T`. */
  def contains[T: ClassTag]: Boolean =
    !isEmpty && (all exists matchesTag[T])

  /** Creates a copy of this attachment with the payload slot of T added/updated with the provided value.
   *  Replaces an existing payload of the same type, if exists.
   */
  def update[T: ClassTag](attachment: T): Attachments { type Pos = self.Pos } =
    new NonemptyAttachments[Pos](this.pos, remove[T].all + attachment)

  /** Creates a copy of this attachment with the payload of the given class type `T` removed. */
  def remove[T: ClassTag]: Attachments { type Pos = self.Pos } = {
    val newAll = all filterNot matchesTag[T]
    if (newAll.isEmpty) pos.asInstanceOf[Attachments { type Pos = self.Pos }]
    else new NonemptyAttachments[Pos](this.pos, newAll)
  }

  def isEmpty: Boolean = true
}

// SI-7018: This used to be an inner class of `Attachments`, but that led to a memory leak in the
// IDE via $outer pointers.
private final class NonemptyAttachments[P >: Null](override val pos: P, override val all: Set[Any]) extends Attachments {
  type Pos = P
  def withPos(newPos: Pos) = new NonemptyAttachments(newPos, all)
  override def isEmpty: Boolean = false
}
