Is there such a thing as bidirectional maps in Scala?

Question

I'd like to link 2 columns of unique identifiers and be able to get a first column value by a second column value as well as a second column value by a first column value. Something like

Map(1 <-> "one", 2 <-> "two", 3 <-> "three")

Is there such a facility in Scala?

Actually I need even more: 3 columns to select any in a triplet by another in a triplet (individual values will never be met more than once in the entire map). But a 2-column bidirectional map can help too.

Answer 1

Guava has a bimap that you can use along with

import scala.collection.JavaConversions._

Answer 2

My BiMap approach:

object BiMap {
  private[BiMap] trait MethodDistinctor
  implicit object MethodDistinctor extends MethodDistinctor
}

case class BiMap[X, Y](map: Map[X, Y]) {
  def this(tuples: (X,Y)*) = this(tuples.toMap)
  private val reverseMap = map map (_.swap)
  require(map.size == reverseMap.size, "no 1 to 1 relation")
  def apply(x: X): Y = map(x)
  def apply(y: Y)(implicit d: BiMap.MethodDistinctor): X = reverseMap(y)
  val domain = map.keys
  val codomain = reverseMap.keys
}

val biMap = new BiMap(1 -> "A", 2 -> "B")
println(biMap(1)) // A
println(biMap("B")) // 2

Of course one can add syntax for <-> instead of ->.

Answer 3

Here's a quick Scala wrapper for Guava's BiMap.

import com.google.common.{collect => guava}
import scala.collection.JavaConversions._
import scala.collection.mutable
import scala.languageFeature.implicitConversions

class MutableBiMap[A, B] private (
    private val g: guava.BiMap[A, B] = new guava.HashBiMap[A, B]()) {

  def inverse: MutableBiMap[B, A] = new MutableBiMap[B, A](g.inverse)
}

object MutableBiMap {

  def empty[A, B]: MutableBiMap[A, B] = new MutableBiMap()

  implicit def toMap[A, B] (x: MutableBiMap[A, B]): mutable.Map[A,B] = x.g
}

Answer 4

I have a really simple BiMap in Scala:

  case class BiMap[A, B](elems: (A, B)*) {

    def groupBy[X, Y](pairs: Seq[(X, Y)]) = pairs groupBy {_._1} mapValues {_ map {_._2} toSet}

    val (left, right) = (groupBy(elems), groupBy(elems map {_.swap}))

    def apply(key: A) = left(key)
    def apply[C: ClassTag](key: B) = right(key)
  }

Usage:

  val biMap = BiMap(1 -> "x", 2 -> "y", 3 -> "x", 1 -> "y")
  assert(biMap(1) == Set("x", "y"))
  assert(biMap("x") == Set(1, 3))