path: root/src/library/scala/math/Ordering.scala

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2010, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

// $Id$


package scala.math

import java.util.Comparator

/** A trait for representing total orderings.  It is important to
 * distinguish between a type that has a total order and a representation
 * of total  ordering on some type.  This trait is for representing the
 * latter.
 *
 * A <a href="http://en.wikipedia.org/wiki/Total_order">total ordering</a>
 * is a binary relation on a type <code>T</code> that is also an equivalence relation
 * and partial ordering on values of type <code>T</code>.  This relation is exposed as
 * the <code>compare</code> method of the <code>Ordering</code> trait.
 * This relation must be:
 * <ul>
 * <li>reflexive: <code>compare(x, x) == 0</code>, for any <code>x</code> of
 * type <code>T</code>.</li>
 * <li>symmetry: <code>compare(x, y) == z</code> and <code>compare(y, x) == w</code>
 * then <code>math.signum(z) == -math.signum(w)</code>, for any <code>x</code> and <code>y</code> of
 * type <code>T</code> and <code>z</code> and <code>w</code> of type <code>Int</code>.</li>
 * <li>transitive: if <code>compare(x, y) == z</code> and <code>compare(y, w) == v</code>
 * and <code>math.signum(z) &gt;= 0</code> and <code>math.signum(v) &gt;= 0</code> then
 * <code>compare(x, w) == u</code> and <code>math.signum(z + v) == math.signum(u)</code>,
 * for any <code>x</code>, <code>y</code>,
 * and <code>w</code> of type <code>T</code> and <code>z</code>, <code>v</code>, and <code>u</code>
 * of type <code>Int</code>.</li>
 * </ul>
 *
 * @author Geoffrey Washburn
 * @version 0.9.5, 2008-04-15
 * @since 2.7
 */
@serializable
trait Ordering[T] extends Comparator[T] with PartialOrdering[T] {
  outer =>

  /** An Ordering is defined at all x and y. */
  def tryCompare(x: T, y: T) = Some(compare(x, y))

 /** Returns a negative integer iff <code>x</code> comes before
   * <code>y</code> in the ordering, returns 0 iff <code>x</code>
   * is the same in the ordering as <code>y</code>, and returns a
   * positive number iff <code>x</code> comes after
   * <code>y</code> in the ordering.
   */
  def compare(x: T, y: T): Int

 /** Returns <code>true</code> iff <code>x</code> comes before
   *  <code>y</code> in the ordering.
   */
  override def lteq(x: T, y: T): Boolean = compare(x, y) <= 0

  /** Returns <code>true</code> iff <code>y</code> comes before
   *  <code>x</code> in the ordering.
   */
  override def gteq(x: T, y: T): Boolean = compare(x, y) >= 0

  /** Returns <code>true</code> iff <code>x</code> comes before
   *  <code>y</code> in the ordering and is not the same as <code>y</code>.
   */
  override def lt(x: T, y: T): Boolean = compare(x, y) < 0

  /** Returns <code>true</code> iff <code>y</code> comes before
   *  <code>x</code> in the ordering and is not the same as <code>x</code>.
   */
  override def gt(x: T, y: T): Boolean = compare(x, y) > 0

  /** Returns <code>true</code> iff <code>x</code> is equivalent to
   *  <code>y</code> in the ordering.
   */
  override def equiv(x: T, y: T): Boolean = compare(x, y) == 0

  /** Returns the argument which comes later in the ordering. */
  def max(x: T, y: T): T = if (gteq(x, y)) x else y

  /** Returns the argument which comes earlier in the ordering. */
  def min(x: T, y: T): T = if (lteq(x, y)) x else y

  override def reverse: Ordering[T] = new Ordering[T]{
    override def reverse = outer
    def compare(x: T, y: T) = outer.compare(y, x)
  }

  /** Given a function U => T, creates Ordering[U]. */
  def on[U](f: U => T): Ordering[U] = new Ordering[U] {
    def compare(x: U, y: U) = outer.compare(f(x), f(y))
  }

  class Ops(lhs: T) {
    def <(rhs: T) = lt(lhs, rhs)
    def <=(rhs: T) = lteq(lhs, rhs)
    def >(rhs: T) = gt(lhs, rhs)
    def >=(rhs: T) = gteq(lhs, rhs)
    def equiv(rhs: T) = Ordering.this.equiv(lhs, rhs)
    def max(rhs: T): T = Ordering.this.max(lhs, rhs)
    def min(rhs: T): T = Ordering.this.min(lhs, rhs)
  }
  implicit def mkOrderingOps(lhs: T): Ops = new Ops(lhs)
}

/** This would conflict with all the nice implicit Orderings
 *  available, but thanks to the magic of prioritized implicits
 *  via subclassing we can make Ordered[A] => Ordering[A] only
 *  turn up if nothing else works.
 */
trait LowPriorityOrderingImplicits {
  implicit def ordered[A <: Ordered[A]]: Ordering[A] = new Ordering[A] {
    def compare(x: A, y: A) = x.compare(y)
  }
}

object Ordering extends LowPriorityOrderingImplicits {

  def apply[T](implicit ord : Ordering[T]) = ord

  def fromLessThan[T](cmp: (T, T) => Boolean): Ordering[T] = new Ordering[T] {
    def compare(x: T, y: T) = if (cmp(x, y)) -1 else if (cmp(y, x)) 1 else 0
    // overrides to avoid multiple comparisons
    override def lt(x: T, y: T): Boolean = cmp(x, y)
    override def gt(x: T, y: T): Boolean = cmp(y, x)
    override def gteq(x: T, y: T): Boolean = !cmp(x, y)
    override def lteq(x: T, y: T): Boolean = !cmp(y, x)
  }

  trait UnitOrdering extends Ordering[Unit] {
    def compare(x: Unit, y: Unit) = 0
  }
  implicit object Unit extends UnitOrdering

  trait BooleanOrdering extends Ordering[Boolean] {
    def compare(x: Boolean, y: Boolean) = (x, y) match {
      case (false, true) => -1
      case (true, false) => 1
      case _ => 0
    }
  }
  implicit object Boolean extends BooleanOrdering

  trait ByteOrdering extends Ordering[Byte] {
    def compare(x: Byte, y: Byte) = x.toInt - y.toInt
  }
  implicit object Byte extends ByteOrdering

  trait CharOrdering extends Ordering[Char] {
    def compare(x: Char, y: Char) = x.toInt - y.toInt
  }
  implicit object Char extends CharOrdering

  trait ShortOrdering extends Ordering[Short] {
    def compare(x: Short, y: Short) = x.toInt - y.toInt
  }
  implicit object Short extends ShortOrdering

  trait IntOrdering extends Ordering[Int] {
    def compare(x: Int, y: Int) =
      if (x < y) -1
      else if (x == y) 0
      else 1
  }
  implicit object Int extends IntOrdering

  trait LongOrdering extends Ordering[Long] {
    def compare(x: Long, y: Long) =
      if (x < y) -1
      else if (x == y) 0
      else 1
  }
  implicit object Long extends LongOrdering

  trait FloatOrdering extends Ordering[Float] {
    def compare(x: Float, y: Float) = java.lang.Float.compare(x, y)
  }
  implicit object Float extends FloatOrdering

  trait DoubleOrdering extends Ordering[Double] {
    def compare(x: Double, y: Double) = java.lang.Double.compare(x, y)
  }
  implicit object Double extends DoubleOrdering

  trait BigIntOrdering extends Ordering[BigInt] {
    def compare(x: BigInt, y: BigInt) = x.compare(y)
  }
  implicit object BigInt extends BigIntOrdering

  trait BigDecimalOrdering extends Ordering[BigDecimal] {
    def compare(x: BigDecimal, y: BigDecimal) = x.compare(y)
  }
  implicit object BigDecimal extends BigDecimalOrdering

  trait StringOrdering extends Ordering[String] {
    def compare(x: String, y: String) = x.compareTo(y)
  }
  implicit object String extends StringOrdering

  implicit def Option[T](implicit ord: Ordering[T]) : Ordering[Option[T]] =
    new Ordering[Option[T]] {
      def compare(x : Option[T], y : Option[T]) = (x, y) match {
        case (None, None) => 0
        case (None, _) => -1
        case (_, None) => 1
        case (Some(x), Some(y)) => ord.compare(x, y)
      }
    }

  implicit def Iterable[T](implicit ord: Ordering[T]): Ordering[Iterable[T]] =
    new Ordering[Iterable[T]] {
      def compare(x: Iterable[T], y: Iterable[T]): Int = {
        val xe = x.iterator
        val ye = y.iterator

        while (xe.hasNext && ye.hasNext) {
          val res = ord.compare(xe.next, ye.next)
          if (res != 0) return res
        }

        Boolean.compare(xe.hasNext, ye.hasNext)
      }
    }

  implicit def Tuple2[T1, T2](implicit ord1: Ordering[T1], ord2: Ordering[T2]): Ordering[(T1, T2)] =
    new Ordering[(T1, T2)]{
      def compare(x: (T1, T2), y: (T1, T2)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        0
      }
    }

  implicit def Tuple3[T1, T2, T3](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3]) : Ordering[(T1, T2, T3)] =
    new Ordering[(T1, T2, T3)]{
      def compare(x: (T1, T2, T3), y: (T1, T2, T3)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        0
      }
    }

  implicit def Tuple4[T1, T2, T3, T4](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4]) : Ordering[(T1, T2, T3, T4)] =
    new Ordering[(T1, T2, T3, T4)]{
      def compare(x: (T1, T2, T3, T4), y: (T1, T2, T3, T4)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        0
      }
    }

  implicit def Tuple5[T1, T2, T3, T4, T5](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4], ord5: Ordering[T5]): Ordering[(T1, T2, T3, T4, T5)] =
    new Ordering[(T1, T2, T3, T4, T5)]{
      def compare(x: (T1, T2, T3, T4, T5), y: Tuple5[T1, T2, T3, T4, T5]): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        val compare5 = ord5.compare(x._5, y._5)
        if (compare5 != 0) return compare5
        0
      }
    }

  implicit def Tuple6[T1, T2, T3, T4, T5, T6](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4], ord5: Ordering[T5], ord6: Ordering[T6]): Ordering[(T1, T2, T3, T4, T5, T6)] =
    new Ordering[(T1, T2, T3, T4, T5, T6)]{
      def compare(x: (T1, T2, T3, T4, T5, T6), y: (T1, T2, T3, T4, T5, T6)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        val compare5 = ord5.compare(x._5, y._5)
        if (compare5 != 0) return compare5
        val compare6 = ord6.compare(x._6, y._6)
        if (compare6 != 0) return compare6
        0
      }
    }

  implicit def Tuple7[T1, T2, T3, T4, T5, T6, T7](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4], ord5: Ordering[T5], ord6: Ordering[T6], ord7: Ordering[T7]): Ordering[(T1, T2, T3, T4, T5, T6, T7)] =
    new Ordering[(T1, T2, T3, T4, T5, T6, T7)]{
      def compare(x: (T1, T2, T3, T4, T5, T6, T7), y: (T1, T2, T3, T4, T5, T6, T7)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        val compare5 = ord5.compare(x._5, y._5)
        if (compare5 != 0) return compare5
        val compare6 = ord6.compare(x._6, y._6)
        if (compare6 != 0) return compare6
        val compare7 = ord7.compare(x._7, y._7)
        if (compare7 != 0) return compare7
        0
      }
    }

  implicit def Tuple8[T1, T2, T3, T4, T5, T6, T7, T8](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4], ord5: Ordering[T5], ord6: Ordering[T6], ord7: Ordering[T7], ord8: Ordering[T8]): Ordering[(T1, T2, T3, T4, T5, T6, T7, T8)] =
    new Ordering[(T1, T2, T3, T4, T5, T6, T7, T8)]{
      def compare(x: (T1, T2, T3, T4, T5, T6, T7, T8), y: (T1, T2, T3, T4, T5, T6, T7, T8)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        val compare5 = ord5.compare(x._5, y._5)
        if (compare5 != 0) return compare5
        val compare6 = ord6.compare(x._6, y._6)
        if (compare6 != 0) return compare6
        val compare7 = ord7.compare(x._7, y._7)
        if (compare7 != 0) return compare7
        val compare8 = ord8.compare(x._8, y._8)
        if (compare8 != 0) return compare8
        0
      }
    }

  implicit def Tuple9[T1, T2, T3, T4, T5, T6, T7, T8, T9](implicit ord1: Ordering[T1], ord2: Ordering[T2], ord3: Ordering[T3], ord4: Ordering[T4], ord5: Ordering[T5], ord6: Ordering[T6], ord7: Ordering[T7], ord8 : Ordering[T8], ord9: Ordering[T9]): Ordering[(T1, T2, T3, T4, T5, T6, T7, T8, T9)] =
    new Ordering[(T1, T2, T3, T4, T5, T6, T7, T8, T9)]{
      def compare(x: (T1, T2, T3, T4, T5, T6, T7, T8, T9), y: (T1, T2, T3, T4, T5, T6, T7, T8, T9)): Int = {
        val compare1 = ord1.compare(x._1, y._1)
        if (compare1 != 0) return compare1
        val compare2 = ord2.compare(x._2, y._2)
        if (compare2 != 0) return compare2
        val compare3 = ord3.compare(x._3, y._3)
        if (compare3 != 0) return compare3
        val compare4 = ord4.compare(x._4, y._4)
        if (compare4 != 0) return compare4
        val compare5 = ord5.compare(x._5, y._5)
        if (compare5 != 0) return compare5
        val compare6 = ord6.compare(x._6, y._6)
        if (compare6 != 0) return compare6
        val compare7 = ord7.compare(x._7, y._7)
        if (compare7 != 0) return compare7
        val compare8 = ord8.compare(x._8, y._8)
        if (compare8 != 0) return compare8
        val compare9 = ord9.compare(x._9, y._9)
        if (compare9 != 0) return compare9
        0
      }
    }

}