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/*
 * Copyright 2012 Taro L. Saito
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

//--------------------------------------
//
// ValueHolder.scala
// Since: 2012/11/06 5:40 PM
//
//--------------------------------------

package xerial.lens

import xerial.core.log.Logger

/**
 * Holder of structured data consisting of named values. ValueHolder is immutable, so
 * the set operations in this class return another ValueHolder and never modify the original ValueHolder.
 *
 *  * A(a, B(b, c))
 *
 * { a: apple, B:{b:book, c:car} }
 *
 * val n1 = Empty.set("a", apple)  =>  Node(a -> Leaf(apple))
 * val n2 = n1.set("B.b", "book")
 *       => Node(a -> Leaf(apple), B -> Empty.set("b", "book"))
 *       => Node(a -> apple, B->Node(b -> Leaf(book)))
 * val n3 = n2.set("B.c", "car") => Node(a ->apple, B->Node(b -> Leaf(book), c->Leaf(car)))
 *
 * 
 *
 * @author Taro L. Saito
 *
 */
trait ValueHolder[+A] {

  def +[B >: A](e:(Path, B)) : ValueHolder[B] = set(e._1, e._2)
  def ++[B >: A](it:Iterable[(Path, B)]) : ValueHolder[B] = it.foldLeft[ValueHolder[B]](this){ (h, e) => h.set(e._1, e._2) }

  /**
   * Set a value at the specified path
   * @param path string representation of [[xerial.lens.Path]]
   * @param value
   * @return updated value holder
   */
  def set[B >: A](path:String, value:B) : ValueHolder[B] = set(Path(path), value)

  /**
   * Set a value at the specified path
   * @param path path
   * @param value String value to set
   * @return updated value holder
   */
  def set[B >: A](path:Path, value:B) : ValueHolder[B]

  /**
   * Extract a part of the value holder under the path
   * @param path
   * @return value holder under the path
   */
  def get(path:String) : ValueHolder[A] = get(Path(path))

  /**
   * Extract a part of the value holder under the path
   * @param path
   * @return value holder under the path
   */
  def get(path:Path) : ValueHolder[A]

  /**
   * Depth first search iterator
   * @return
   */
  def dfs : Iterator[(Path, A)]  = dfs(Path.current)

  /**
   * Depth first search iterator under the specified path
   * @param path
   * @return
   */
  def dfs(path:Path) : Iterator[(Path, A)]
  /**
   * Depth first search iterator under the specified path
   * @param path
   * @return
   */
  def dfs(path:String) : Iterator[(Path, A)] = dfs(Path(path))

  def isEmpty = false

}



object ValueHolder extends Logger {
  import collection.immutable.{Map => IMap}


  def apply[A](elems:Iterable[(Path, A)]) : ValueHolder[A] = Empty.++[A](elems)

  val empty : ValueHolder[Nothing] = Empty

  private case object Empty extends ValueHolder[Nothing] {
    def set[B >: Nothing](path: Path, value: B) = {
      if(path.isEmpty)
        Leaf(value)
      else
        Node(IMap.empty[String, ValueHolder[B]]).set(path, value)
    }
    def get(path:Path) = Empty
    def extract(path:Path) = Empty

    def dfs(path:Path) = Iterator.empty
    override def isEmpty = true
  }

  private case class Node[A](child:IMap[String, ValueHolder[A]]) extends ValueHolder[A] {
    override def toString = "{%s}".format(child.map{e => "%s:%s".format(e._1, e._2) }.mkString(", "))

    def set[B >: A](path: Path, value: B) : ValueHolder[B] = {
      if(path.isEmpty)
        throw new IllegalStateException("path cannot be empty")
      else {
        val p = child.getOrElse(path.head, Empty).set(path.tailPath, value)
        Node(child + (path.head -> p))
      }
    }

    def get(path:Path) = {
      if(path.isEmpty)
        this
      else
        child.get(path.head) map { _.get(path.tailPath) } getOrElse Empty
    }
    
    def dfs(path:Path) = (for((name, h) <- child) yield h.dfs(path / name)).reduce(_ ++ _)
  }

  private case class Leaf[A](value:A) extends ValueHolder[A] {
    override def toString = value.toString
    def set[B >: A](path: Path, value: B) = {
      SeqLeaf(Seq(this, Empty.set[B](path, value)))
    }

    def get(path: Path) = {
      if(path.isEmpty)
        this
      else
        Empty
    }

    def dfs(path: Path) = Iterator.single(path -> value)
  }

  private case class SeqLeaf[A](elems:Seq[ValueHolder[A]]) extends ValueHolder[A] {
    override def toString = "[%s]".format(elems.mkString(", "))

    def set[B >: A](path: Path, value: B) = {
      SeqLeaf(elems :+ Empty.set(path, value))
    }

    def get(path: Path) =
      if(path.isEmpty)
        this
      else
        Empty

    def dfs(path: Path) = elems.map(e => e.dfs(path)).reduce(_ ++ _)


  }

}