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package com.avsystem.commons
package misc

import scala.language.implicitConversions

object OptRef {
  def apply[A >: Null](value: A): OptRef[A] = new OptRef[A](value)
  def unapply[A >: Null](opt: OptRef[A]) = opt //name-based extractor

  def some[A >: Null](value: A): OptRef[A] =
    if (value != null) new OptRef[A](value)
    else throw new NullPointerException

  object Boxed {
    def unapply[A, B >: Null](optRef: OptRef[B])(implicit unboxing: Unboxing[A, B]): Opt[A] =
      if (optRef.isEmpty) Opt.Empty else Opt(unboxing.fun(optRef.get))
  }

  implicit def opt2Iterable[A >: Null](xo: OptRef[A]): Iterable[A] = xo.toList

  final val Empty: OptRef[Null] = new OptRef[Null](null)

  def empty[A >: Null]: OptRef[A] = Empty

  private val nullFunc: Any => Null = _ => null

  final class WithFilter[+A >: Null] private[OptRef](self: OptRef[A], p: A => Boolean) {
    def map[B >: Null](f: A => B): OptRef[B] = self filter p map f
    def flatMap[B >: Null](f: A => OptRef[B]): OptRef[B] = self filter p flatMap f
    def foreach[U](f: A => U): Unit = self filter p foreach f
    def withFilter(q: A => Boolean): WithFilter[A] = new WithFilter[A](self, x => p(x) && q(x))
  }
}

/**
  * Like [[Opt]] but has better Java interop thanks to the fact that wrapped value has type `A` instead of `Any`.
  * For example, Scala method defined like this:
  * {{{
  *   def takeMaybeString(str: OptRef[String]): Unit
  * }}}
  * will be seen by Java as:
  * {{{
  *   public void takeMaybeString(String str);
  * }}}
  * and `null` will be used to represent absence of value.
  * 
  * This comes at the cost of `A` having to be a nullable type. Also, empty value is represented internally using `null`
  * which unfortunately makes [[OptRef]] suffer from SI-7396 (`hashCode` fails on `OptRef.Empty` which means that you
  * can't add [[OptRef]] values into hash sets or use them as hash map keys).
  */
final class OptRef[+A >: Null] private(private val value: A) extends AnyVal with Serializable {
  @inline def isEmpty: Boolean = value == null
  @inline def isDefined: Boolean = !isEmpty
  @inline def nonEmpty: Boolean = isDefined

  @inline def get: A =
    if (isEmpty) throw new NoSuchElementException("empty OptRef") else value

  @inline def toOpt: Opt[A] =
    Opt(value)

  @inline def toOption: Option[A] =
    Option(value)

  @inline def getOrElse[B >: A](default: => B): B =
    if (isEmpty) default else value

  @inline def orNull[B >: A](implicit ev: Null <:< B): B =
    value.asInstanceOf[B]

  @inline def map[B >: Null](f: A => B): OptRef[B] =
    if (isEmpty) OptRef.Empty else OptRef(f(value))

  @inline def fold[B >: Null](ifEmpty: => B)(f: A => B): B =
    if (isEmpty) ifEmpty else f(value)

  @inline def flatMap[B >: Null](f: A => OptRef[B]): OptRef[B] =
    if (isEmpty) OptRef.Empty else f(value)

  @inline def filter(p: A => Boolean): OptRef[A] =
    if (isEmpty || p(value)) this else OptRef.Empty

  @inline def withFilter(p: A => Boolean): OptRef.WithFilter[A] =
    new OptRef.WithFilter[A](this, p)

  @inline def filterNot(p: A => Boolean): OptRef[A] =
    if (isEmpty || !p(value)) this else OptRef.Empty

  @inline def contains[A1 >: A](elem: A1): Boolean =
    !isEmpty && value == elem

  @inline def exists(p: A => Boolean): Boolean =
    !isEmpty && p(value)

  @inline def forall(p: A => Boolean): Boolean =
    isEmpty || p(value)

  @inline def foreach[U](f: A => U): Unit = {
    if (!isEmpty) f(value)
  }

  @inline def collect[B >: Null](pf: PartialFunction[A, B]): OptRef[B] =
    if (!isEmpty) new OptRef(pf.applyOrElse(value, OptRef.nullFunc)) else OptRef.Empty

  @inline def orElse[B >: A](alternative: => OptRef[B]): OptRef[B] =
    if (isEmpty) alternative else this

  @inline def iterator: Iterator[A] =
    if (isEmpty) Iterator.empty else Iterator.single(value)

  @inline def toList: List[A] =
    if (isEmpty) List() else new ::(value, Nil)

  @inline def toRight[X](left: => X) =
    if (isEmpty) Left(left) else Right(value)

  @inline def toLeft[X](right: => X) =
    if (isEmpty) Right(right) else Left(value)

  override def toString =
    if (isEmpty) "OptRef.Empty" else s"OptRef($value)"
}