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circe core
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package io.circe

import cats.{ Eq, Show }
import java.io.Serializable

sealed abstract class CursorOp extends Product with Serializable {

  /**
   * Does this operation require the current focus (not context) to be an
   * object?
   */
  def requiresObject: Boolean

  /**
   * Does this operation require the current focus (not context) to be an array?
   */
  def requiresArray: Boolean
}

object CursorOp {
  abstract sealed class ObjectOp extends CursorOp {
    final def requiresObject: Boolean = true
    final def requiresArray: Boolean = false
  }

  abstract sealed class ArrayOp extends CursorOp {
    final def requiresObject: Boolean = false
    final def requiresArray: Boolean = true
  }

  abstract sealed class UnconstrainedOp extends CursorOp {
    final def requiresObject: Boolean = false
    final def requiresArray: Boolean = false
  }

  final case object MoveLeft extends UnconstrainedOp
  final case object MoveRight extends UnconstrainedOp
  @deprecated("Use Up and DownArray", "0.12.0")
  final case object MoveFirst extends UnconstrainedOp
  final case object MoveUp extends UnconstrainedOp
  @deprecated("Use MoveRight", "0.12.0")
  final case class LeftN(n: Int) extends UnconstrainedOp
  @deprecated("Use MoveLeft", "0.12.0")
  final case class RightN(n: Int) extends UnconstrainedOp
  final case class Field(k: String) extends UnconstrainedOp
  final case class DownField(k: String) extends ObjectOp
  final case object DownArray extends ArrayOp
  final case class DownN(n: Int) extends ArrayOp
  final case object DeleteGoParent extends UnconstrainedOp

  implicit final val showCursorOp: Show[CursorOp] = Show.show {
    case MoveLeft       => "<-"
    case MoveRight      => "->"
    case MoveFirst      => "|<-"
    case MoveUp         => "_/"
    case LeftN(n)       => "-<-:(" + n + ")"
    case RightN(n)      => ":->-(" + n + ")"
    case Field(f)       => "--(" + f + ")"
    case DownField(f)   => "--\\(" + f + ")"
    case DownArray      => "\\\\"
    case DownN(n)       => "=\\(" + n + ")"
    case DeleteGoParent => "!_/"
  }

  implicit final val eqCursorOp: Eq[CursorOp] = Eq.fromUniversalEquals

  val eqCursorOpList: Eq[List[CursorOp]] = cats.instances.list.catsKernelStdEqForList[CursorOp]

  /**
   * Represents JavaScript-style selections into a JSON structure.
   */
  private[this] sealed trait Selection
  private[this] case class SelectField(field: String) extends Selection
  private[this] case class SelectIndex(index: Int) extends Selection
  private[this] case class Op(op: CursorOp) extends Selection

  /** Shows history as JS style selections, i.e. ".foo.bar[3]" */
  def opsToPath(history: List[CursorOp]): String = {

    // Fold into sequence of selections (reducing array ops etc. into single selections)
    val selections = history.foldRight(List.empty[Selection]) {
      case (DownField(k), acc)                 => SelectField(k) :: acc
      case (DownArray, acc)                    => SelectIndex(0) :: acc
      case (MoveUp, _ :: tail)                 => tail
      case (MoveRight, SelectIndex(i) :: tail) => SelectIndex(i + 1) :: tail
      case (MoveLeft, SelectIndex(i) :: tail)  => SelectIndex(i - 1) :: tail
      case (RightN(n), SelectIndex(i) :: tail) => SelectIndex(i + n) :: tail
      case (LeftN(n), SelectIndex(i) :: tail)  => SelectIndex(i - n) :: tail
      case (op, acc)                           => Op(op) :: acc
    }

    selections.foldLeft("") {
      case (str, SelectField(f)) => s".$f$str"
      case (str, SelectIndex(i)) => s"[$i]$str"
      case (str, Op(op))         => s"{${Show[CursorOp].show(op)}}$str"
    }
  }
}