org.apache.lucene.util

## Class FixedBitSet

- java.lang.Object
- - org.apache.lucene.util.BitSet
  - - org.apache.lucene.util.FixedBitSet

- All Implemented Interfaces:
    Accountable, Bits

      public final class FixedBitSet
      extends BitSet
      implements Bits, Accountable

  BitSet of fixed length (numBits), backed by accessible (`getBits()`) long[], accessed with an int index, implementing `Bits` and `DocIdSet`. If you need to manage more than 2.1B bits, use `LongBitSet`.
  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from interface org.apache.lucene.util.Bits

      `Bits.MatchAllBits, Bits.MatchNoBits`

  - ### Field Summary

    - ### Fields inherited from interface org.apache.lucene.util.Bits

      `EMPTY_ARRAY`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `FixedBitSet(int numBits)` Creates a new LongBitSet.
    `FixedBitSet(long[] storedBits, int numBits)` Creates a new LongBitSet using the provided long[] array as backing store.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `and(FixedBitSet other)` this = this AND other
    `void` | `andNot(FixedBitSet other)` this = this AND NOT other
    `static long` | `andNotCount(FixedBitSet a, FixedBitSet b)` Returns the popcount or cardinality of "a and not b" or "intersection(a, not(b))".
    `Bits` | `asReadOnlyBits()` Convert this instance to read-only `Bits`.
    `static int` | `bits2words(int numBits)` returns the number of 64 bit words it would take to hold numBits
    `int` | `cardinality()` Returns number of set bits.
    `void` | `clear(int index)` Clear the bit at `i`.
    `void` | `clear(int startIndex, int endIndex)` Clears a range of bits.
    `FixedBitSet` | `clone()`
    `static FixedBitSet` | `copyOf(Bits bits)` Make a copy of the given bits.
    `static FixedBitSet` | `ensureCapacity(FixedBitSet bits, int numBits)` If the given `FixedBitSet` is large enough to hold `numBits+1`, returns the given bits, otherwise returns a new `FixedBitSet` which can hold the requested number of bits.
    `boolean` | `equals(Object o)`
    `void` | `flip(int index)` Flip the bit at the provided index.
    `void` | `flip(int startIndex, int endIndex)` Flips a range of bits
    `boolean` | `get(int index)` Returns the value of the bit with the specified `index`.
    `boolean` | `getAndClear(int index)`
    `boolean` | `getAndSet(int index)`
    `long[]` | `getBits()` Expert.
    `int` | `hashCode()`
    `static long` | `intersectionCount(FixedBitSet a, FixedBitSet b)` Returns the popcount or cardinality of the intersection of the two sets.
    `boolean` | `intersects(FixedBitSet other)` returns true if the sets have any elements in common
    `int` | `length()` Returns the number of bits in this set
    `int` | `nextSetBit(int index)` Returns the index of the first set bit starting at the index specified.
    `void` | `or(DocIdSetIterator iter)` Does in-place OR of the bits provided by the iterator.
    `void` | `or(FixedBitSet other)` this = this OR other
    `int` | `prevSetBit(int index)` Returns the index of the last set bit before or on the index specified.
    `long` | `ramBytesUsed()` Return the memory usage of this object in bytes.
    `boolean` | `scanIsEmpty()` Scans the backing store to check if all bits are clear.
    `void` | `set(int index)` Set the bit at `i`.
    `void` | `set(int startIndex, int endIndex)` Sets a range of bits
    `static long` | `unionCount(FixedBitSet a, FixedBitSet b)` Returns the popcount or cardinality of the union of the two sets.
    `void` | `xor(DocIdSetIterator iter)` Does in-place XOR of the bits provided by the iterator.
    `void` | `xor(FixedBitSet other)` this = this XOR other

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

      `approximateCardinality, checkUnpositioned, of`

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

      `finalize, getClass, notify, notifyAll, toString, wait, wait, wait`

    - ### Methods inherited from interface org.apache.lucene.util.Accountable

      `getChildResources`

- - ### Constructor Detail

    - #### FixedBitSet

          public FixedBitSet(int numBits)

      Creates a new LongBitSet. The internally allocated long array will be exactly the size needed to accommodate the numBits specified.
      Parameters:
        `numBits` - the number of bits needed

    - #### FixedBitSet

          public FixedBitSet(long[] storedBits,
                             int numBits)

      Creates a new LongBitSet using the provided long[] array as backing store. The storedBits array must be large enough to accommodate the numBits specified, but may be larger. In that case the 'extra' or 'ghost' bits must be clear (or they may provoke spurious side-effects)
      Parameters:
        `storedBits` - the array to use as backing store
        `numBits` - the number of bits actually needed

  - ### Method Detail

    - #### ensureCapacity

          public static FixedBitSet ensureCapacity(FixedBitSet bits,
                                                   int numBits)

      If the given `FixedBitSet` is large enough to hold `numBits+1`, returns the given bits, otherwise returns a new `FixedBitSet` which can hold the requested number of bits.

      NOTE: the returned bitset reuses the underlying `long[]` of the given `bits` if possible. Also, calling `length()` on the returned bits may return a value greater than `numBits`.

    - #### bits2words

          public static int bits2words(int numBits)

      returns the number of 64 bit words it would take to hold numBits

    - #### intersectionCount

          public static long intersectionCount(FixedBitSet a,
                                               FixedBitSet b)

      Returns the popcount or cardinality of the intersection of the two sets. Neither set is modified.

    - #### unionCount

          public static long unionCount(FixedBitSet a,
                                        FixedBitSet b)

      Returns the popcount or cardinality of the union of the two sets. Neither set is modified.

    - #### andNotCount

          public static long andNotCount(FixedBitSet a,
                                         FixedBitSet b)

      Returns the popcount or cardinality of "a and not b" or "intersection(a, not(b))". Neither set is modified.

    - #### length

          public int length()

      Description copied from interface: `Bits`
      Returns the number of bits in this set
      Specified by:
        `length` in interface `Bits`

    - #### ramBytesUsed

          public long ramBytesUsed()

      Description copied from interface: `Accountable`
      Return the memory usage of this object in bytes. Negative values are illegal.
      Specified by:
        `ramBytesUsed` in interface `Accountable`

    - #### getBits

          public long[] getBits()

      Expert.

    - #### cardinality

          public int cardinality()

      Returns number of set bits. NOTE: this visits every long in the backing bits array, and the result is not internally cached!
      Specified by:
        `cardinality` in class `BitSet`

    - #### get

          public boolean get(int index)

      Description copied from interface: `Bits`
      Returns the value of the bit with the specified `index`.
      Specified by:
        `get` in interface `Bits`
      Parameters:
        `index` - index, should be non-negative and < `Bits.length()`. The result of passing negative or out of bounds values is undefined by this interface, just don't do it!
      Returns:
        `true` if the bit is set, `false` otherwise.

    - #### set

          public void set(int index)

      Description copied from class: `BitSet`
      Set the bit at `i`.
      Specified by:
        `set` in class `BitSet`

    - #### getAndSet

          public boolean getAndSet(int index)

    - #### clear

          public void clear(int index)

      Description copied from class: `BitSet`
      Clear the bit at `i`.
      Specified by:
        `clear` in class `BitSet`

    - #### getAndClear

          public boolean getAndClear(int index)

    - #### nextSetBit

          public int nextSetBit(int index)

      Description copied from class: `BitSet`
      Returns the index of the first set bit starting at the index specified. `DocIdSetIterator.NO_MORE_DOCS` is returned if there are no more set bits.
      Specified by:
        `nextSetBit` in class `BitSet`

    - #### prevSetBit

          public int prevSetBit(int index)

      Description copied from class: `BitSet`
      Returns the index of the last set bit before or on the index specified. -1 is returned if there are no more set bits.
      Specified by:
        `prevSetBit` in class `BitSet`

    - #### or

          public void or(DocIdSetIterator iter)
                  throws IOException

      Description copied from class: `BitSet`
      Does in-place OR of the bits provided by the iterator. The state of the iterator after this operation terminates is undefined.
      Overrides:
        `or` in class `BitSet`
      Throws:
        `IOException`

    - #### or

          public void or(FixedBitSet other)

      this = this OR other

    - #### xor

          public void xor(FixedBitSet other)

      this = this XOR other

    - #### xor

          public void xor(DocIdSetIterator iter)
                   throws IOException

      Does in-place XOR of the bits provided by the iterator.
      Throws:
        `IOException`

    - #### intersects

          public boolean intersects(FixedBitSet other)

      returns true if the sets have any elements in common

    - #### and

          public void and(FixedBitSet other)

      this = this AND other

    - #### andNot

          public void andNot(FixedBitSet other)

      this = this AND NOT other

    - #### scanIsEmpty

          public boolean scanIsEmpty()

      Scans the backing store to check if all bits are clear. The method is deliberately not called "isEmpty" to emphasize it is not low cost (as isEmpty usually is).
      Returns:
        true if all bits are clear.

    - #### flip

          public void flip(int startIndex,
                           int endIndex)

      Flips a range of bits
      Parameters:
        `startIndex` - lower index
        `endIndex` - one-past the last bit to flip

    - #### flip

          public void flip(int index)

      Flip the bit at the provided index.

    - #### set

          public void set(int startIndex,
                          int endIndex)

      Sets a range of bits
      Parameters:
        `startIndex` - lower index
        `endIndex` - one-past the last bit to set

    - #### clear

          public void clear(int startIndex,
                            int endIndex)

      Description copied from class: `BitSet`
      Clears a range of bits.
      Specified by:
        `clear` in class `BitSet`
      Parameters:
        `startIndex` - lower index
        `endIndex` - one-past the last bit to clear

    - #### clone

          public FixedBitSet clone()

      Overrides:
        `clone` in class `Object`

    - #### equals

          public boolean equals(Object o)

      Overrides:
        `equals` in class `Object`

    - #### hashCode

          public int hashCode()

      Overrides:
        `hashCode` in class `Object`

    - #### copyOf

          public static FixedBitSet copyOf(Bits bits)

      Make a copy of the given bits.

    - #### asReadOnlyBits

          public Bits asReadOnlyBits()

      Convert this instance to read-only `Bits`. This is useful in the case that this `FixedBitSet` is returned as a `Bits` instance, to make sure that consumers may not get write access back by casting to a `FixedBitSet`. NOTE: Changes to this `FixedBitSet` will be reflected on the returned `Bits`.

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