Play Framework/Scala example source code file (OrderPreserving.scala)

This example Play Framework source code file (OrderPreserving.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Play Framework (and Scala) source code examples by using tags.

All credit for the original source code belongs to Play Framework; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Play Framework tags/keywords

collection, k, map, orderpreserving, play, play framework, seq, util, utility, utils, v

The OrderPreserving.scala Play Framework example source code

/*
 * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
 */
package play.utils

import scala.collection.immutable.ListMap
import scala.collection.mutable

object OrderPreserving {

  def groupBy[K, V](seq: Seq[(K, V)])(f: ((K, V)) => K): Map[K, Seq[V]] = {
    // This mutable map will not retain insertion order for the seq, but it is fast for retrieval. The value is
    // a builder for the desired Seq[String] in the final result.
    val m = mutable.Map.empty[K, mutable.Builder[V, Seq[V]]]

    // Run through the seq and create builders for each unique key, effectively doing the grouping
    for ((key, value) <- seq) m.getOrElseUpdate(key, mutable.Seq.newBuilder[V]) += value

    // Create a builder for the resulting ListMap. Note that this one is immutable and will retain insertion order
    val b = ListMap.newBuilder[K, Seq[V]]

    // Note that we are NOT going through m (didn't retain order) but we are iterating over the original seq
    // just to get the keys so we can look up the values in m with them. This is how order is maintained.
    for ((k, v) <- seq.iterator) b += k -> m.getOrElse(k, mutable.Seq.newBuilder[V]).result

    // Get the builder to produce the final result
    b.result
  }
}