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## org.apache.lucene.search.function
Class ReverseOrdFieldSource

    java.lang.Object
      org.apache.lucene.search.function.ValueSource
          org.apache.lucene.search.function.ReverseOrdFieldSource

All Implemented Interfaces:
  Serializable

    public class ReverseOrdFieldSource

extends ValueSource

Expert: obtains the ordinal of the field value from the default Lucene `FieldCache` using getStringIndex() and reverses the order.

The native lucene index order is used to assign an ordinal value for each field value.

Field values (terms) are lexicographically ordered by unicode value, and numbered starting at 1.
Example of reverse ordinal (rord):
If there were only three field values: "apple","banana","pear"
then rord("apple")=3, rord("banana")=2, ord("pear")=1

WARNING: rord() depends on the position in an index and can thus change when other documents are inserted or deleted, or if a MultiSearcher is used.

See Also:
  Serialized Form
WARNING: This API is experimental and might change in incompatible ways in the next release.

  NOTE: with the switch in 2.9 to segment-based searching, if `getValues(org.apache.lucene.index.IndexReader)` is invoked with a composite (multi-segment) reader, this can easily cause double RAM usage for the values in the FieldCache. It's best to switch your application to pass only atomic (single segment) readers to this API.

Field Summary
`String` | `field`

Constructor Summary
`ReverseOrdFieldSource(String field)` Contructor for a certain field.

Method Summary
`String` | `description()` description of field, used in explain()
`boolean` | `equals(Object o)` Needed for possible caching of query results - used by `ValueSourceQuery.equals(Object)`.
`DocValues` | `getValues(IndexReader reader)` Return the DocValues used by the function query.
`int` | `hashCode()` Needed for possible caching of query results - used by `ValueSourceQuery.hashCode()`.

Methods inherited from class org.apache.lucene.search.function.ValueSource
`toString`

Methods inherited from class java.lang.Object
`clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

Field Detail

### field

    public String field

Constructor Detail

### ReverseOrdFieldSource

    public ReverseOrdFieldSource(String field)

  Contructor for a certain field.

  Parameters:
    `field` - field whose values reverse order is used.

Method Detail

### description

    public String description()

  Description copied from class: `ValueSource`
  description of field, used in explain()

  Specified by:
    `description` in class `ValueSource`

### getValues

    public DocValues getValues(IndexReader reader)
                        throws IOException

  Description copied from class: `ValueSource`
  Return the DocValues used by the function query.

  Specified by:
    `getValues` in class `ValueSource`
  Parameters:
    `reader` - the IndexReader used to read these values. If any caching is involved, that caching would also be IndexReader based.
  Throws:
    `IOException` - for any error.

### equals

    public boolean equals(Object o)

  Description copied from class: `ValueSource`
  Needed for possible caching of query results - used by `ValueSourceQuery.equals(Object)`.

  Specified by:
    `equals` in class `ValueSource`
  See Also:
    `Object.equals(Object)`

### hashCode

    public int hashCode()

  Description copied from class: `ValueSource`
  Needed for possible caching of query results - used by `ValueSourceQuery.hashCode()`.

  Specified by:
    `hashCode` in class `ValueSource`
  See Also:
    `Object.hashCode()`

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