import java.util.concurrent.BlockingQueue
import java.util.concurrent.LinkedBlockingQueue
import java.util.concurrent.TimeUnit

class MaxLeaseRetriesException(message:String = null, cause:Throwable = null) extends Exception(message, cause)

class ConnectionPool[R](maxSize:Int, timeout:Int = 1000, unit: TimeUnit = TimeUnit.MILLISECONDS) {

  val q: BlockingQueue[R] = new LinkedBlockingQueue[R](maxSize) 
  
  /**
   * Given an initialization block, fill the connection pool.
   */
  def construct(block: ()=> R) {
	for (_ <- 0 to maxSize) {
	  q.put(block())
	}
  }
  
  /**
   * Lease a connection from the pool for the duration of a code block
   * Use the default timeout to try and acquire the lease. Retry `maxRetries` times
   * to acquire the lease.
   * 
   * Throws a MaxLeaseRetriesException if the maxRetries is hit with no luck.
   */
  def lease[T](block: R => T, maxRetries:Int=10): T = {
    var con:R = q.poll(timeout, unit)
    var tries:Int = 1
    
    while (con == null && tries <= maxRetries) { 
      con = q.poll(timeout, unit)
      tries+=1
    }
    
    if (con == null) {
      throw new MaxLeaseRetriesException(s"Tried to acquire connection lease ${maxRetries} times with no success!")
    }
    
    val result = block(con)
    q.put(con)
    result
  }
}