org.apache.lucene.util

## Class IntroSelector

- java.lang.Object
- - org.apache.lucene.util.Selector
  - - org.apache.lucene.util.IntroSelector

-     public abstract class IntroSelector
      extends Selector

  Implementation of the quick select algorithm.

  It uses the median of the first, middle and last values as a pivot and falls back to a median of medians when the number of recursion levels exceeds `2 lg(n)`, as a consequence it runs in linear time on average.

  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `IntroSelector()`

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected int` | `compare(int i, int j)` Compare entries found in slots `i` and `j`.
    `protected abstract int` | `comparePivot(int j)` Compare the pivot with the slot at `j`, similarly to `compare(i, j)`.
    `void` | `select(int from, int to, int k)` Reorder elements so that the element at position `k` is the same as if all elements were sorted and all other elements are partitioned around it: `[from, k)` only contains elements that are less than or equal to `k` and `(k, to)` only contains elements that are greater than or equal to `k`.
    `protected abstract void` | `setPivot(int i)` Save the value at slot `i` so that it can later be used as a pivot, see `comparePivot(int)`.

    - ### Methods inherited from class org.apache.lucene.util.Selector

      `swap`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### IntroSelector

          public IntroSelector()

  - ### Method Detail

    - #### select

          public final void select(int from,
                                   int to,
                                   int k)

      Description copied from class: `Selector`
      Reorder elements so that the element at position `k` is the same as if all elements were sorted and all other elements are partitioned around it: `[from, k)` only contains elements that are less than or equal to `k` and `(k, to)` only contains elements that are greater than or equal to `k`.
      Specified by:
        `select` in class `Selector`

    - #### compare

          protected int compare(int i,
                                int j)

      Compare entries found in slots `i` and `j`. The contract for the returned value is the same as `Comparator.compare(Object, Object)`.

    - #### setPivot

          protected abstract void setPivot(int i)

      Save the value at slot `i` so that it can later be used as a pivot, see `comparePivot(int)`.

    - #### comparePivot

          protected abstract int comparePivot(int j)

      Compare the pivot with the slot at `j`, similarly to `compare(i, j)`.

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