org.apache.lucene.util

## Class OpenBitSet

- java.lang.Object
- - org.apache.lucene.search.DocIdSet
  - - org.apache.lucene.util.OpenBitSet

All Implemented Interfaces:
  Cloneable, Bits

Direct Known Subclasses:
  OpenBitSetDISI

    public class OpenBitSet
    extends DocIdSet
    implements Bits, Cloneable

An "open" BitSet implementation that allows direct access to the array of words storing the bits.

Unlike java.util.bitset, the fact that bits are packed into an array of longs is part of the interface. This allows efficient implementation of other algorithms by someone other than the author. It also allows one to efficiently implement alternate serialization or interchange formats.

`OpenBitSet` is faster than `java.util.BitSet` in most operations and *much* faster at calculating cardinality of sets and results of set operations. It can also handle sets of larger cardinality (up to 64 * 2**32-1)

The goals of `OpenBitSet` are the fastest implementation possible, and maximum code reuse. Extra safety and encapsulation may always be built on top, but if that's built in, the cost can never be removed (and hence people re-implement their own version in order to get better performance). If you want a "safe", totally encapsulated (and slower and limited) BitSet class, use `java.util.BitSet`.

### Performance Results

Test system: Pentium 4, Sun Java 1.5_06 -server -Xbatch -Xmx64M
BitSet size = 1,000,000
Results are java.util.BitSet time divided by OpenBitSet time.

| cardinality | intersect_count | union | nextSetBit | get | iterator
--- | --- | --- | --- | --- | --- | ---
50% full | 3.36 | 3.96 | 1.44 | 1.46 | 1.99 | 1.58
1% full | 3.31 | 3.90 |  | 1.04 |  | 0.99

Test system: AMD Opteron, 64 bit linux, Sun Java 1.5_06 -server -Xbatch -Xmx64M
BitSet size = 1,000,000
Results are java.util.BitSet time divided by OpenBitSet time.

| cardinality | intersect_count | union | nextSetBit | get | iterator
--- | --- | --- | --- | --- | --- | ---
50% full | 2.50 | 3.50 | 1.00 | 1.03 | 1.12 | 1.25
1% full | 2.51 | 3.49 |  | 1.00 |  | 1.02

### Nested Class Summary

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

`Bits.MatchAllBits, Bits.MatchNoBits`

### Field Summary

Fields
`protected long[]` `bits`
`protected int` `wlen`

### Fields inherited from class org.apache.lucene.search.DocIdSet

`EMPTY_DOCIDSET`

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

`EMPTY_ARRAY`

### Constructor Summary

Constructors
Constructor and Description
`OpenBitSet()`
`OpenBitSet(long numBits)` Constructs an OpenBitSet large enough to hold `numBits`.
`OpenBitSet(long[] bits, int numWords)` Constructs an OpenBitSet from an existing long[].

### Method Summary

Methods
`void` `and(OpenBitSet other)`
`void` `andNot(OpenBitSet other)`
`static long` `andNotCount(OpenBitSet a, OpenBitSet b)` Returns the popcount or cardinality of "a and not b" or "intersection(a, not(b))".
`Bits` `bits()` Optionally provides a `Bits` interface for random access to matching documents.
`static int` `bits2words(long numBits)` returns the number of 64 bit words it would take to hold numBits
`long` `capacity()` Returns the current capacity in bits (1 greater than the index of the last bit)
`long` `cardinality()`
`void` `clear(int startIndex, int endIndex)` Clears a range of bits.
`void` `clear(long index)` clears a bit, allowing access beyond the current set size without changing the size.
`void` `clear(long startIndex, long endIndex)` Clears a range of bits.
`OpenBitSet` `clone()`
`void` `ensureCapacity(long numBits)` Ensure that the long[] is big enough to hold numBits, expanding it if necessary.
`void` `ensureCapacityWords(int numWords)` Expand the long[] with the size given as a number of words (64 bit longs).
`boolean` `equals(Object o)` returns true if both sets have the same bits set
`protected int` `expandingWordNum(long index)`
`void` `fastClear(int index)` clears a bit.
`void` `fastClear(long index)` clears a bit.
`void` `fastFlip(int index)` flips a bit.
`void` `fastFlip(long index)` flips a bit.
`boolean` `fastGet(int index)` Returns true or false for the specified bit index.
`boolean` `fastGet(long index)` Returns true or false for the specified bit index.
`void` `fastSet(int index)` Sets the bit at the specified index.
`void` `fastSet(long index)` Sets the bit at the specified index.
`void` `flip(long index)` flips a bit, expanding the set size if necessary
`void` `flip(long startIndex, long endIndex)` Flips a range of bits, expanding the set size if necessary
`boolean` `flipAndGet(int index)` flips a bit and returns the resulting bit value.
`boolean` `flipAndGet(long index)` flips a bit and returns the resulting bit value.
`boolean` `get(int index)` Returns true or false for the specified bit index.
`boolean` `get(long index)` Returns true or false for the specified bit index
`boolean` `getAndSet(int index)` Sets a bit and returns the previous value.
`boolean` `getAndSet(long index)` Sets a bit and returns the previous value.
`int` `getBit(int index)` returns 1 if the bit is set, 0 if not.
`long[]` `getBits()` Expert: returns the long[] storing the bits
`int` `getNumWords()` Expert: gets the number of longs in the array that are in use
`int` `hashCode()`
`void` `intersect(OpenBitSet other)` this = this AND other
`static long` `intersectionCount(OpenBitSet a, OpenBitSet b)` Returns the popcount or cardinality of the intersection of the two sets.
`boolean` `intersects(OpenBitSet other)` returns true if the sets have any elements in common
`boolean` `isCacheable()` This DocIdSet implementation is cacheable.
`boolean` `isEmpty()` Returns true if there are no set bits
`DocIdSetIterator` `iterator()` Provides a `DocIdSetIterator` to access the set.
`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.
`long` `nextSetBit(long index)` Returns the index of the first set bit starting at the index specified.
`void` `or(OpenBitSet other)`
`int` `prevSetBit(int index)` Returns the index of the first set bit starting downwards at the index specified.
`long` `prevSetBit(long index)` Returns the index of the first set bit starting downwards at the index specified.
`void` `remove(OpenBitSet other)` Remove all elements set in other.
`void` `set(long index)` sets a bit, expanding the set size if necessary
`void` `set(long startIndex, long endIndex)` Sets a range of bits, expanding the set size if necessary
`void` `setBits(long[] bits)` Expert: sets a new long[] to use as the bit storage
`void` `setNumWords(int nWords)` Expert: sets the number of longs in the array that are in use
`long` `size()` Returns the current capacity of this set.
`void` `trimTrailingZeros()` Lowers numWords, the number of words in use, by checking for trailing zero words.
`void` `union(OpenBitSet other)` this = this OR other
`static long` `unionCount(OpenBitSet a, OpenBitSet b)` Returns the popcount or cardinality of the union of the two sets.
`void` `xor(OpenBitSet other)` this = this XOR other
`static long` `xorCount(OpenBitSet a, OpenBitSet b)` Returns the popcount or cardinality of the exclusive-or of the two sets.

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

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

### Field Detail

#### bits

    protected long[] bits

#### wlen

    protected int wlen

### Constructor Detail

#### OpenBitSet

    public OpenBitSet(long numBits)

Constructs an OpenBitSet large enough to hold `numBits`.

#### OpenBitSet

    public OpenBitSet()

#### OpenBitSet

    public OpenBitSet(long[] bits,
              int numWords)

Constructs an OpenBitSet from an existing long[].
The first 64 bits are in long[0], with bit index 0 at the least significant bit, and bit index 63 at the most significant. Given a bit index, the word containing it is long[index/64], and it is at bit number index%64 within that word.

numWords are the number of elements in the array that contain set bits (non-zero longs). numWords should be <= bits.length, and any existing words in the array at position >= numWords should be zero.

### Method Detail

#### iterator

    public DocIdSetIterator iterator()

Description copied from class: `DocIdSet`
Provides a `DocIdSetIterator` to access the set. This implementation can return `null` or `DocIdSet.EMPTY_DOCIDSET.iterator()` if there are no docs that match.

Specified by:
  `iterator` in class `DocIdSet`

#### bits

    public Bits bits()

Description copied from class: `DocIdSet`
Optionally provides a `Bits` interface for random access to matching documents.

Overrides:
  `bits` in class `DocIdSet`
Returns:
  `null`, if this `DocIdSet` does not support random access. In contrast to `DocIdSet.iterator()`, a return value of `null` does not imply that no documents match the filter! The default implementation does not provide random access, so you only need to implement this method if your DocIdSet can guarantee random access to every docid in O(1) time without external disk access (as `Bits` interface cannot throw `IOException`). This is generally true for bit sets like `FixedBitSet`, which return itself if they are used as `DocIdSet`.

#### isCacheable

    public boolean isCacheable()

This DocIdSet implementation is cacheable.

Overrides:
  `isCacheable` in class `DocIdSet`

#### capacity

    public long capacity()

Returns the current capacity in bits (1 greater than the index of the last bit)

#### size

    public long size()

Returns the current capacity of this set. Included for compatibility. This is *not* equal to `cardinality()`

#### length

    public int length()

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

Specified by:
  `length` in interface `Bits`

#### isEmpty

    public boolean isEmpty()

Returns true if there are no set bits

#### getBits

    public long[] getBits()

Expert: returns the long[] storing the bits

#### setBits

    public void setBits(long[] bits)

Expert: sets a new long[] to use as the bit storage

#### getNumWords

    public int getNumWords()

Expert: gets the number of longs in the array that are in use

#### setNumWords

    public void setNumWords(int nWords)

Expert: sets the number of longs in the array that are in use

#### get

    public boolean get(int index)

Returns true or false for the specified bit 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.

#### fastGet

    public boolean fastGet(int index)

Returns true or false for the specified bit index. The index should be less than the OpenBitSet size

#### get

    public boolean get(long index)

Returns true or false for the specified bit index

#### fastGet

    public boolean fastGet(long index)

Returns true or false for the specified bit index. The index should be less than the OpenBitSet size.

#### getBit

    public int getBit(int index)

returns 1 if the bit is set, 0 if not. The index should be less than the OpenBitSet size

#### set

    public void set(long index)

sets a bit, expanding the set size if necessary

#### fastSet

    public void fastSet(int index)

Sets the bit at the specified index. The index should be less than the OpenBitSet size.

#### fastSet

    public void fastSet(long index)

Sets the bit at the specified index. The index should be less than the OpenBitSet size.

#### set

    public void set(long startIndex,
           long endIndex)

Sets a range of bits, expanding the set size if necessary

Parameters:
  `startIndex` - lower index
  `endIndex` - one-past the last bit to set

#### expandingWordNum

    protected int expandingWordNum(long index)

#### fastClear

    public void fastClear(int index)

clears a bit. The index should be less than the OpenBitSet size.

#### fastClear

    public void fastClear(long index)

clears a bit. The index should be less than the OpenBitSet size.

#### clear

    public void clear(long index)

clears a bit, allowing access beyond the current set size without changing the size.

#### clear

    public void clear(int startIndex,
             int endIndex)

Clears a range of bits. Clearing past the end does not change the size of the set.

Parameters:
  `startIndex` - lower index
  `endIndex` - one-past the last bit to clear

#### clear

    public void clear(long startIndex,
             long endIndex)

Clears a range of bits. Clearing past the end does not change the size of the set.

Parameters:
  `startIndex` - lower index
  `endIndex` - one-past the last bit to clear

#### getAndSet

    public boolean getAndSet(int index)

Sets a bit and returns the previous value. The index should be less than the OpenBitSet size.

#### getAndSet

    public boolean getAndSet(long index)

Sets a bit and returns the previous value. The index should be less than the OpenBitSet size.

#### fastFlip

    public void fastFlip(int index)

flips a bit. The index should be less than the OpenBitSet size.

#### fastFlip

    public void fastFlip(long index)

flips a bit. The index should be less than the OpenBitSet size.

#### flip

    public void flip(long index)

flips a bit, expanding the set size if necessary

#### flipAndGet

    public boolean flipAndGet(int index)

flips a bit and returns the resulting bit value. The index should be less than the OpenBitSet size.

#### flipAndGet

    public boolean flipAndGet(long index)

flips a bit and returns the resulting bit value. The index should be less than the OpenBitSet size.

#### flip

    public void flip(long startIndex,
            long endIndex)

Flips a range of bits, expanding the set size if necessary

Parameters:
  `startIndex` - lower index
  `endIndex` - one-past the last bit to flip

#### cardinality

    public long cardinality()

Returns:
  the number of set bits

#### intersectionCount

    public static long intersectionCount(OpenBitSet a,
                         OpenBitSet b)

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

#### unionCount

    public static long unionCount(OpenBitSet a,
                  OpenBitSet b)

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

#### andNotCount

    public static long andNotCount(OpenBitSet a,
                   OpenBitSet b)

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

#### xorCount

    public static long xorCount(OpenBitSet a,
                OpenBitSet b)

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

#### nextSetBit

    public int nextSetBit(int index)

Returns the index of the first set bit starting at the index specified. -1 is returned if there are no more set bits.

#### nextSetBit

    public long nextSetBit(long index)

Returns the index of the first set bit starting at the index specified. -1 is returned if there are no more set bits.

#### prevSetBit

    public int prevSetBit(int index)

Returns the index of the first set bit starting downwards at the index specified. -1 is returned if there are no more set bits.

#### prevSetBit

    public long prevSetBit(long index)

Returns the index of the first set bit starting downwards at the index specified. -1 is returned if there are no more set bits.

#### clone

    public OpenBitSet clone()

Overrides:
  `clone` in class `Object`

#### intersect

    public void intersect(OpenBitSet other)

this = this AND other

#### union

    public void union(OpenBitSet other)

this = this OR other

#### remove

    public void remove(OpenBitSet other)

Remove all elements set in other. this = this AND_NOT other

#### xor

    public void xor(OpenBitSet other)

this = this XOR other

#### and

    public void and(OpenBitSet other)

#### or

    public void or(OpenBitSet other)

#### andNot

    public void andNot(OpenBitSet other)

#### intersects

    public boolean intersects(OpenBitSet other)

returns true if the sets have any elements in common

#### ensureCapacityWords

    public void ensureCapacityWords(int numWords)

Expand the long[] with the size given as a number of words (64 bit longs). getNumWords() is unchanged by this call.

#### ensureCapacity

    public void ensureCapacity(long numBits)

Ensure that the long[] is big enough to hold numBits, expanding it if necessary. getNumWords() is unchanged by this call.

#### trimTrailingZeros

    public void trimTrailingZeros()

Lowers numWords, the number of words in use, by checking for trailing zero words.

#### bits2words

    public static int bits2words(long numBits)

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

#### equals

    public boolean equals(Object o)

returns true if both sets have the same bits set

Overrides:
  `equals` in class `Object`

#### hashCode

    public int hashCode()

Overrides:
  `hashCode` in class `Object`