Class IntroSorter

java.lang.Object
org.apache.lucene.util.Sorter
org.apache.lucene.util.IntroSorter
Direct Known Subclasses:
ArrayIntroSorter, CollectionUtil.ListIntroSorter

public abstract class IntroSorter extends Sorter
Sorter implementation based on a variant of the quicksort algorithm called introsort: when the recursion level exceeds the log of the length of the array to sort, it falls back to heapsort. This prevents quicksort from running into its worst-case quadratic runtime. Selects the pivot using Tukey's ninther median-of-medians, and partitions using Bentley-McIlroy 3-way partitioning. Small ranges are sorted with insertion sort.

This algorithm is NOT stable. It's fast on most data shapes, especially with low cardinality. If the data to sort is known to be strictly ascending or descending, prefer TimSorter.

  • Field Details

    • SINGLE_MEDIAN_THRESHOLD

      static final int SINGLE_MEDIAN_THRESHOLD
      Below this size threshold, the partition selection is simplified to a single median.
      See Also:
  • Constructor Details

    • IntroSorter

      public IntroSorter()
      Create a new IntroSorter.
  • Method Details

    • sort

      public final void sort(int from, int to)
      Description copied from class: Sorter
      Sort the slice which starts at from (inclusive) and ends at to (exclusive).
      Specified by:
      sort in class Sorter
    • sort

      void sort(int from, int to, int maxDepth)
      Sorts between from (inclusive) and to (exclusive) with intro sort.

      Sorts small ranges with insertion sort. Fallbacks to heap sort to avoid quadratic worst case. Selects the pivot with medians and partitions with the Bentley-McIlroy fast 3-ways algorithm (Engineering a Sort Function, Bentley-McIlroy).

    • median

      private int median(int i, int j, int k)
      Returns the index of the median element among three elements at provided indices.
    • setPivot

      protected abstract void setPivot(int i)
      Description copied from class: Sorter
      Save the value at slot i so that it can later be used as a pivot, see Sorter.comparePivot(int).
      Overrides:
      setPivot in class Sorter
    • comparePivot

      protected abstract int comparePivot(int j)
      Description copied from class: Sorter
      Compare the pivot with the slot at j, similarly to compare(i, j).
      Overrides:
      comparePivot in class Sorter
    • compare

      protected int compare(int i, int j)
      Description copied from class: Sorter
      Compare entries found in slots i and j. The contract for the returned value is the same as Comparator.compare(Object, Object).
      Specified by:
      compare in class Sorter