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package scala.math

/** A trait for totally ordered data.
 *
 * Note that since version 2006-07-24 this trait is no longer covariant in a.
 *
 * It is important that the equals method for an instance of 
 * Ordered[A] be consistent with the compare method.  However, 
 * due to limitations inherent in the type erasure semantics, 
 * there is no reasonable way to provide a default implementation
 * of equality for instances of Ordered[A].  Therefore, if you need
 * to be able to use equality on an instance of Ordered[A] you must 
 * provide it yourself either when inheiriting or instantiating. 
 *
 * It is important that the hashCode method for an instance of 
 * Ordered[A] be consistent with the compare method. However, 
 * it is not possible to provide a sensible default implementation.
 * Therefore, if you need to be able compute the hash of an 
 * instance of Ordered[A] you must provide it yourself either when 
 * inheiriting or instantiating. 
 *
 *  @author  Martin Odersky
 *  @version 1.1, 2006-07-24 
 */
trait Ordered[A] extends java.lang.Comparable[A] {

  /** Result of comparing this with operand that.
   *  returns x where
   *  x < 0    iff    this < that
   *  x == 0   iff    this == that
   *  x > 0    iff    this > that
   */
  def compare(that: A): Int

  def <  (that: A): Boolean = (this compare that) <  0
  def >  (that: A): Boolean = (this compare that) >  0
  def <= (that: A): Boolean = (this compare that) <= 0
  def >= (that: A): Boolean = (this compare that) >= 0
  def compareTo(that: A): Int = compare(that)
}

object Ordered {  
  /** Lens from Ordering[T] to Ordered[T] */
  implicit def orderingToOrdered[T](x: T)(implicit ord: Ordering[T]): Ordered[T] = 
    new Ordered[T] { def compare(that: T): Int = ord.compare(x, that) }
}