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/*
  Rapture, version 2.0.0. Copyright 2010-2016 Jon Pretty, Propensive Ltd.

  The primary distribution site is
  
    http://rapture.io/

  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.
 */

package rapture.core

import scala.collection.mutable

/** Implements a dynamic pool of some resource, e.g. database connections. */
abstract class Pool[Resource] {

  /** Implement to make new resources. */
  protected def make(): Resource

  /** Implement to dispose of surplus resources. */
  protected def dispose(x: Resource): Unit

  /** Implement to check resource is still usable. */
  protected def check(x: Resource): Boolean

  /** Number of resource to always keep in reserve, if we have them. */
  protected def spare = 5

  /** How long to leave surplus resources unused before discarding them. */
  protected def timeout = 10 * 60000L

  private val pool = new mutable.Queue[Resource]
  private var poolCount = 0
  private var lastLow = 0L

  /** Acquire a resource for the duration of the body. */
  def acquireFor[A](body: Resource => A): A = {
    val res = acquireDirect()
    try body(res)
    finally releaseDirect(res)
  }

  /** Acquire a resource without any nesting guarantees. Avoid this method. */
  def acquireDirect(): Resource = pool.synchronized {
    if (poolCount == 0) make()
    else {
      val r = pool.dequeue
      poolCount = poolCount - 1
      if (check(r)) r
      else {
        dispose(r)
        make()
      }
    }
  }

  /** Release a directly-acquired resource. */
  def releaseDirect(r: Resource): Unit = pool.synchronized {
    val now = System.currentTimeMillis()
    if (poolCount < spare) lastLow = now
    if (lastLow > now - timeout) {
      pool.enqueue(r)
      poolCount = poolCount + 1
    } else dispose(r)
  }

  /** Dispose of all resources not currently in use. */
  def disposeAll() = pool.synchronized {
    while (poolCount > 0) {
      dispose(pool.dequeue)
      poolCount = poolCount - 1
    }
  }
}