com.twitter.algebird.TopKMonoid.scala Maven / Gradle / Ivy

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/*
Copyright 2012 Twitter, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
package com.twitter.algebird

import scala.annotation.tailrec

case class TopK[N](size: Int, items: List[N], max: Option[N])

/**
 * A top-k monoid that is much faster than SortedListTake
 * equivalent to: (left ++ right).sorted.take(k)
 * but doesn't do a total sort
 * You should STRONGLY prefer this to SortedTakeListMonoid which is deprecated and slow
 * If you can handle the mutability, mutable.PriorityQueueMonoid is even faster.
 *
 * NOTE!!!! This assumes the inputs are already sorted! resorting each time kills speed
 */
class TopKMonoid[T](k: Int)(implicit ord: Ordering[T]) extends Monoid[TopK[T]] {

  require(k > 0, "TopK requires at least K>0")

  override lazy val zero = TopK[T](0, List[T](), None)

  def build(t: T): TopK[T] = TopK(1, List(t), Some(t))
  def build(ts: Iterable[T]): TopK[T] = ts.foldLeft(zero) { (acc, t) => plus(acc, build(t)) }

  override def plus(left: TopK[T], right: TopK[T]): TopK[T] = {
    val (bigger, smaller) = if (left.size >= right.size) (left, right) else (right, left)
    if (smaller.size == 0) {
      bigger
    } else if (bigger.size == k) {
      // See if we can just return the bigger:
      val biggerWins = for (biggest <- bigger.max; smallest <- smaller.items.headOption)
        yield (ord.lteq(biggest, smallest))
      if (biggerWins.getOrElse(true)) { // smaller is small, or empty
        bigger
      } else {
        merge(bigger, smaller)
      }
    } else {
      merge(bigger, smaller)
    }
  }
  protected def merge(bigger: TopK[T], smaller: TopK[T]): TopK[T] = {
    import SortedTakeListMonoid.mergeSortR
    // This is the internal loop that does one comparison:
    val newItems = mergeSortR(Nil, bigger.items, smaller.items, k)
    val max = newItems.headOption
    // Now reverse and get the size:
    val (size, reversed) = newItems.foldLeft((0, List[T]())) { (cntItems, v) =>
      val (olds, oldl) = cntItems
      (olds + 1, v :: oldl)
    }
    TopK(size, reversed, max)
  }
}