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Provides fully-fledged multishot delimitied continuations in Kotlin with Coroutines
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import kotlin.coroutines.Continuation
import kotlin.coroutines.CoroutineContext
import kotlin.coroutines.intrinsics.intercepted
import kotlin.jvm.JvmInline

internal expect val Continuation<*>.isCompilerGenerated: Boolean
internal expect val Continuation<*>.completion: Continuation<*>
internal expect fun  Continuation.copy(completion: Continuation<*>): Continuation

@JvmInline
internal value class Frame(val cont: Continuation) {
  fun resumeWith(value: Result, into: Continuation) = cont.copy(into).resumeWith(value)
  fun resumeWithIntercepted(value: Result, into: Continuation) = cont.copy(into).intercepted().resumeWith(value)
}

@JvmInline
internal value class FrameList(val frame: Frame) {
  @Suppress("UNCHECKED_CAST")
  private val completion: Continuation get() = frame.cont.completion as Continuation
  fun head() = frame
  fun tail(): FrameList? =
    if (this.completion is WrapperCont) null else FrameList(Frame(this.completion))
}

internal sealed interface SplitSeq {
  val context: CoroutineContext
}

internal fun  SplitSeq.resumeWith(result: Result, isIntercepted: Boolean) {
  val exception = result.exceptionOrNull()
  if (exception is SeekingStackException) exception.use(this)
  else resumeWithImpl(result, isIntercepted)
}

private tailrec fun  SplitSeq.resumeWithImpl(
  result: Result, isIntercepted: Boolean
) {
  when (this) {
    is EmptyCont -> (if (isIntercepted) underlying.intercepted() else underlying).resumeWith(
      result
    )

    is FramesCont -> if (wrapperCont == null) {
      if (isIntercepted) {
        head.resumeWithIntercepted(result, WrapperCont(tail))
      } else {
        val tail = tail
        val wrapper = WrapperCont(tail, isWaitingForValue = true)
        head.resumeWith(result, wrapper)
        val res = wrapper.result
        if (res != waitingForValue && res != hasBeenIntercepted) {
          val exception = res.exceptionOrNull()
          if (exception is SeekingStackException) exception.use(tail)
          else tail.resumeWithImpl(res, false)
        } else {
          wrapper.result = hasBeenIntercepted
        }
      }
    } else {
      val wrapper = wrapperCont
      val rest = rest!! as SplitSeq
      wrapper.seq = rest
      wrapper.realContext = rest.context
      if (isIntercepted) {
        wrapper.result = hasBeenIntercepted
        frames.head().cont.intercepted().resumeWith(result)
      } else {
        wrapper.result = waitingForValue
        frames.head().cont.resumeWith(result)
        val res = wrapper.result
        if (res != waitingForValue && res != hasBeenIntercepted) {
          val exception = res.exceptionOrNull()
          if (exception is SeekingStackException) exception.use(rest)
          else rest.resumeWithImpl(res, false)
        } else {
          wrapper.result = hasBeenIntercepted
        }
      }
    }

    is PromptCont -> rest!!.resumeWithImpl(result, isIntercepted = isIntercepted)

    is ReaderCont<*, Start, First, End> -> rest!!.resumeWithImpl(result, isIntercepted = isIntercepted)
  }
}

internal tailrec fun  SplitSeq.splitAtAux(
  p: Prompt, seg: Segment
): Pair, SplitSeq> = when (this) {
  is EmptyCont -> error("Prompt not found $p in $seg")
  is FramesCont -> (rest!! as SplitSeq).splitAtAux(p, FramesSegment(frames, seg))
  is PromptCont -> if (p === this.p) {
    // Start and P are now unified, but the compiler doesn't get it
    val pair: Pair, SplitSeq> = seg to rest!!
    @Suppress("UNCHECKED_CAST")
    pair as Pair, SplitSeq>
  } else {
    rest!!.splitAtAux(p, PromptSegment(this.p, seg))
  }

  is ReaderCont<*, Start, First, End> -> rest!!.splitAtAux(p, toSegment(seg))
}

private fun  ReaderCont.toSegment(seg: Segment): ReaderSegment =
  ReaderSegment(p, _state, fork, seg)

internal tailrec fun  SplitSeq.deleteReader(
  p: Reader<*>, previous: ExpectsSequenceStartingWith<*>
): Unit = when (this) {
  is EmptyCont -> error("Reader not found $p")
  is FramesCont -> rest!!.deleteReader(p, this)
  is PromptCont -> rest!!.deleteReader(p, this)
  is ReaderCont<*, Start, *, End> -> if (p === this.p) {
    previous as ExpectsSequenceStartingWith
    previous.sequence = rest!!
  } else rest!!.deleteReader(p, this)
}

internal tailrec fun  SplitSeq.find(p: Prompt
): SplitSeq = when (this) {
  is EmptyCont -> error("Prompt not found $p")

  is FramesCont -> rest!!.find(p)
  is PromptCont -> if (p === this.p) {
    // Start and P are now unified, but the compiler doesn't get it
    @Suppress("UNCHECKED_CAST")
    this.rest!! as SplitSeq
  } else rest!!.find(p)

  is ReaderCont<*, Start, *, End> -> rest!!.find(p)
}

internal tailrec fun  SplitSeq.findGuyBefore(
  p: Prompt
,
  previous: ExpectsSequenceStartingWith?
): ExpectsSequenceStartingWith
? = when (this) {
  is EmptyCont -> error("Prompt not found $p")

  is FramesCont -> (rest!! as SplitSeq).findGuyBefore(p, this)
  is PromptCont -> if (p === this.p) {
    // Start and P are now unified, but the compiler doesn't get it
    @Suppress("UNCHECKED_CAST")
    previous as ExpectsSequenceStartingWith
?
  } else rest!!.findGuyBefore(p, this)

  is ReaderCont<*, Start, *, End> -> rest!!.findGuyBefore(p, this)
}

internal tailrec fun  SplitSeq.find(p: Reader): S = when (this) {
  is EmptyCont -> error("Reader not found $p")
  is FramesCont -> rest!!.find(p)
  is PromptCont -> rest!!.find(p)
  is ReaderCont<*, Start, *, End> -> if (p === this.p) {
    @Suppress("UNCHECKED_CAST")
    this.state as S
  } else rest!!.find(p)
}

internal tailrec fun  SplitSeq.findOrNull(p: Reader): S? = when (this) {
  is EmptyCont -> null
  is FramesCont -> rest!!.findOrNull(p)
  is PromptCont -> rest!!.findOrNull(p)
  is ReaderCont<*, Start, *, End> -> if (p === this.p) {
    @Suppress("UNCHECKED_CAST")
    this.state as S
  } else rest!!.findOrNull(p)
}

internal fun  SplitSeq.splitAt(p: Prompt
): Pair, SplitSeq> =
  splitAtAux(p, EmptySegment)

internal fun  SplitSeq.splitAtOnce(p: Prompt
): Pair, SplitSeq> {
  val box = findGuyBefore(p, null)
  return if (box != null) {
    SingleUseSegment(box, this) to box.sequence!!.also { box.sequence = null } as SplitSeq
  } else {
    EmptySegment to this as SplitSeq
  }
}

internal fun  SplitSeq.pushPrompt(p: Prompt): PromptCont =
  PromptCont(p, this)

internal fun  SplitSeq.pushReader(
  p: Reader, value: S, fork: S.() -> S
): ReaderCont = ReaderCont(p, value, fork, this)

internal data class EmptyCont(val underlying: Continuation) : SplitSeq {
  override val context: CoroutineContext = underlying.context
}

// frame :: frames ::: rest
internal data class FramesCont(
  val frames: FrameList, var rest: SplitSeq?,
  val wrapperCont: WrapperCont?,
) : SplitSeq, ExpectsSequenceStartingWith {
  val head get() = frames.head()

  @Suppress("UNCHECKED_CAST")
  val tail: SplitSeq
    get() = frames.tail()?.let { FramesCont(it, rest, wrapperCont) }
      ?: rest as SplitSeq // First == Last, but the compiler doesn't get it

  override val context: CoroutineContext = rest!!.context
  override var sequence: SplitSeq?
    get() = rest
    set(value) {
      rest = value as SplitSeq?
    }
}

internal fun CoroutineContext.unwrap(): CoroutineContext =
  if (this is WrapperCont<*>) this.realContext else this

internal class WrapperCont(seq: SplitSeq, isWaitingForValue: Boolean = false) : Continuation,
  CoroutineContext {
  var seq: SplitSeq? = seq
  var result: Result = if (isWaitingForValue) waitingForValue else hasBeenIntercepted
  var realContext = seq.context.unwrap()
    set(value) {
      field = value.unwrap()
    }

  override val context: CoroutineContext get() = this

  override fun resumeWith(result: Result) {
    if (this.result == waitingForValue) {
      this.result = result
    } else {
      seq!!.resumeWith(result, isIntercepted = false)
    }
  }

  // TODO improve these implementations
  override fun  fold(initial: R, operation: (R, CoroutineContext.Element) -> R): R =
    realContext.fold(initial, operation)

  override fun plus(context: CoroutineContext): CoroutineContext = realContext.plus(context)

  override fun minusKey(key: CoroutineContext.Key<*>): CoroutineContext =
    realContext.minusKey(key)

  override fun  get(key: CoroutineContext.Key): E? =
    realContext.get(key)
}

private data object WaitingForValue : Throwable()

private val waitingForValue = Result.failure(WaitingForValue)

private data object HasBeenIntercepted : Throwable()

private val hasBeenIntercepted = Result.failure(HasBeenIntercepted)

internal data class PromptCont(
  val p: Prompt, var rest: SplitSeq?
) : SplitSeq, ExpectsSequenceStartingWith {
  override val context = rest!!.context
  override var sequence: SplitSeq?
    get() = rest
    set(value) {
      rest = value as SplitSeq?
    }
}

internal data class ReaderCont(
  val p: Reader, var _state: State, val fork: State.() -> State, var rest: SplitSeq?,
  private var forkOnFirstRead: Boolean = false
) : SplitSeq, ExpectsSequenceStartingWith {
  override val context = rest!!.context
  val state: State
    get() {
      if (forkOnFirstRead) {
        forkOnFirstRead = false
        _state = fork(_state)
      }
      return _state
    }

  override var sequence: SplitSeq?
    get() = rest
    set(value) {
      rest = value as SplitSeq?
    }
}

// sub continuations / stack segments
// mirrors the stack, and so is in reverse order. allows easy access to the state
// stored in the current prompt
internal sealed interface Segment

internal tailrec infix fun  Segment.prependTo(stack: SplitSeq): SplitSeq =
  when (this) {
    is EmptySegment -> {
      // Start == End
      stack as SplitSeq
    }

    is FramesSegment -> init prependTo FramesCont(frames, stack, null)

    is PromptSegment -> init prependTo PromptCont(prompt, stack)
    is ReaderSegment<*, Start, First, End> -> init prependTo ReaderCont(prompt, state, fork, stack, forkOnFirstRead = true)

    is SingleUseSegment -> {
      box.sequence = stack
      cont as SplitSeq
    }
  }

internal data object EmptySegment : Segment

internal data class FramesSegment(
  val frames: FrameList, val init: Segment
) : Segment

internal data class PromptSegment(
  val prompt: Prompt, val init: Segment
) : Segment

internal data class ReaderSegment(
  val prompt: Reader, val state: State, val fork: (State.() -> State), val init: Segment
) : Segment

// Expects that cont eventually refers to box
internal data class SingleUseSegment(
  val box: ExpectsSequenceStartingWith, val cont: SplitSeq
) : Segment

internal sealed interface ExpectsSequenceStartingWith {
  var sequence: SplitSeq?
}

internal fun  collectStack(continuation: Continuation): SplitSeq =
  findNearestWrapperCont(continuation).toFramesCont(continuation)

private fun  WrapperCont.toFramesCont(
  continuation: Continuation
): FramesCont =
  FramesCont(FrameList(Frame(continuation)), seq!!.also { seq = null }, this)

internal fun findNearestSplitSeq(continuation: Continuation<*>): SplitSeq<*, *, *> =
  findNearestWrapperCont(continuation).seq!!

private fun findNearestWrapperCont(continuation: Continuation<*>): WrapperCont<*> =
  continuation.context as? WrapperCont<*> ?: error("No WrapperCont found in stack")