org.apache.lucene.util.packed

## Class PackedInts

- java.lang.Object
- - org.apache.lucene.util.packed.PackedInts

-     public class PackedInts
      extends Object

  Simplistic compression for array of unsigned long values. Each value is >= 0 and <= a specified maximum value. The values are stored as packed ints, with each value consuming a fixed number of bits.
  NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static interface` | `PackedInts.Decoder` A decoder for packed integers.
    `static interface` | `PackedInts.Encoder` An encoder for packed integers.
    `static class` | `PackedInts.Format` A format to write packed ints.
    `static class` | `PackedInts.FormatAndBits` Simple class that holds a format and a number of bits per value.
    `static class` | `PackedInts.Header` Header identifying the structure of a packed integer array.
    `static interface` | `PackedInts.Mutable` A packed integer array that can be modified.
    `static class` | `PackedInts.NullReader` A `PackedInts.Reader` which has all its values equal to 0 (bitsPerValue = 0).
    `static interface` | `PackedInts.Reader` A read-only random access array of positive integers.
    `static interface` | `PackedInts.ReaderIterator` Run-once iterator interface, to decode previously saved PackedInts.
    `static class` | `PackedInts.Writer` A write-once Writer.

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static String` | `CODEC_NAME`
    `static float` | `COMPACT` No memory overhead at all, but the returned implementation may be slow.
    `static float` | `DEFAULT` At most 20% memory overhead.
    `static int` | `DEFAULT_BUFFER_SIZE` Default amount of memory to use for bulk operations.
    `static float` | `FAST` At most 50% memory overhead, always select a reasonably fast implementation.
    `static float` | `FASTEST` At most 700% memory overhead, always select a direct implementation.
    `static int` | `VERSION_BYTE_ALIGNED`
    `static int` | `VERSION_CURRENT`
    `static int` | `VERSION_START`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `PackedInts()`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `static int` | `bitsRequired(long maxValue)` Returns how many bits are required to hold values up to and including maxValue
    `static void` | `checkVersion(int version)` Check the validity of a version number.
    `static void` | `copy(PackedInts.Reader src, int srcPos, PackedInts.Mutable dest, int destPos, int len, int mem)` Copy `src[srcPos:srcPos+len]` into `dest[destPos:destPos+len]` using at most `mem` bytes.
    `static PackedInts.FormatAndBits` | `fastestFormatAndBits(int valueCount, int bitsPerValue, float acceptableOverheadRatio)` Try to find the `PackedInts.Format` and number of bits per value that would restore from disk the fastest reader whose overhead is less than `acceptableOverheadRatio`.
    `static PackedInts.Decoder` | `getDecoder(PackedInts.Format format, int version, int bitsPerValue)` Get a `PackedInts.Decoder`.
    `static PackedInts.Reader` | `getDirectReader(IndexInput in)` Construct a direct `PackedInts.Reader` from an `IndexInput`.
    `static PackedInts.Reader` | `getDirectReaderNoHeader(IndexInput in, PackedInts.Format format, int version, int valueCount, int bitsPerValue)` Expert: Construct a direct `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream.
    `static PackedInts.Reader` | `getDirectReaderNoHeader(IndexInput in, PackedInts.Header header)` Expert: Construct a direct `PackedInts.Reader` from an `IndexInput` without reading metadata at the beginning of the stream.
    `static PackedInts.Encoder` | `getEncoder(PackedInts.Format format, int version, int bitsPerValue)` Get an `PackedInts.Encoder`.
    `static PackedInts.Mutable` | `getMutable(int valueCount, int bitsPerValue, float acceptableOverheadRatio)` Create a packed integer array with the given amount of values initialized to 0.
    `static PackedInts.Reader` | `getReader(DataInput in)` Restore a `PackedInts.Reader` from a stream.
    `static PackedInts.ReaderIterator` | `getReaderIterator(DataInput in, int mem)` Retrieve PackedInts as a `PackedInts.ReaderIterator`
    `static PackedInts.ReaderIterator` | `getReaderIteratorNoHeader(DataInput in, PackedInts.Format format, int version, int valueCount, int bitsPerValue, int mem)` Expert: Restore a `PackedInts.ReaderIterator` from a stream without reading metadata at the beginning of the stream.
    `static PackedInts.Reader` | `getReaderNoHeader(DataInput in, PackedInts.Format format, int version, int valueCount, int bitsPerValue)` Expert: Restore a `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream.
    `static PackedInts.Reader` | `getReaderNoHeader(DataInput in, PackedInts.Header header)` Expert: Restore a `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream.
    `static PackedInts.Writer` | `getWriter(DataOutput out, int valueCount, int bitsPerValue, float acceptableOverheadRatio)` Create a packed integer array writer for the given output, format, value count, and number of bits per value.
    `static PackedInts.Writer` | `getWriterNoHeader(DataOutput out, PackedInts.Format format, int valueCount, int bitsPerValue, int mem)` Expert: Create a packed integer array writer for the given output, format, value count, and number of bits per value.
    `static long` | `maxValue(int bitsPerValue)` Calculates the maximum unsigned long that can be expressed with the given number of bits.
    `static PackedInts.Header` | `readHeader(DataInput in)` Expert: reads only the metadata from a stream.

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

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

- - ### Field Detail

    - #### FASTEST

          public static final float FASTEST

      At most 700% memory overhead, always select a direct implementation.
      See Also:
        Constant Field Values

    - #### FAST

          public static final float FAST

      At most 50% memory overhead, always select a reasonably fast implementation.
      See Also:
        Constant Field Values

    - #### DEFAULT

          public static final float DEFAULT

      At most 20% memory overhead.
      See Also:
        Constant Field Values

    - #### COMPACT

          public static final float COMPACT

      No memory overhead at all, but the returned implementation may be slow.
      See Also:
        Constant Field Values

    - #### DEFAULT_BUFFER_SIZE

          public static final int DEFAULT_BUFFER_SIZE

      Default amount of memory to use for bulk operations.
      See Also:
        Constant Field Values

    - #### CODEC_NAME

          public static final String CODEC_NAME

      See Also:
        Constant Field Values

    - #### VERSION_START

          public static final int VERSION_START

      See Also:
        Constant Field Values

    - #### VERSION_BYTE_ALIGNED

          public static final int VERSION_BYTE_ALIGNED

      See Also:
        Constant Field Values

    - #### VERSION_CURRENT

          public static final int VERSION_CURRENT

      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### PackedInts

          public PackedInts()

  - ### Method Detail

    - #### checkVersion

          public static void checkVersion(int version)

      Check the validity of a version number.

    - #### fastestFormatAndBits

          public static PackedInts.FormatAndBits fastestFormatAndBits(int valueCount,
                                                      int bitsPerValue,
                                                      float acceptableOverheadRatio)

      Try to find the `PackedInts.Format` and number of bits per value that would restore from disk the fastest reader whose overhead is less than `acceptableOverheadRatio`.

      The `acceptableOverheadRatio` parameter makes sense for random-access `PackedInts.Reader`s. In case you only plan to perform sequential access on this stream later on, you should probably use `COMPACT`.

      If you don't know how many values you are going to write, use `valueCount = -1`.

    - #### getDecoder

          public static PackedInts.Decoder getDecoder(PackedInts.Format format,
                                      int version,
                                      int bitsPerValue)

      Get a `PackedInts.Decoder`.
      Parameters:
        `format` - the format used to store packed ints
        `version` - the compatibility version
        `bitsPerValue` - the number of bits per value
      Returns:
        a decoder

    - #### getEncoder

          public static PackedInts.Encoder getEncoder(PackedInts.Format format,
                                      int version,
                                      int bitsPerValue)

      Get an `PackedInts.Encoder`.
      Parameters:
        `format` - the format used to store packed ints
        `version` - the compatibility version
        `bitsPerValue` - the number of bits per value
      Returns:
        an encoder

    - #### getReaderNoHeader

          public static PackedInts.Reader getReaderNoHeader(DataInput in,
                                            PackedInts.Format format,
                                            int version,
                                            int valueCount,
                                            int bitsPerValue)
                                                     throws IOException

      Expert: Restore a `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream. This method is useful to restore data from streams which have been created using `getWriterNoHeader(DataOutput, Format, int, int, int)`.
      Parameters:
        `in` - the stream to read data from, positioned at the beginning of the packed values
        `format` - the format used to serialize
        `version` - the version used to serialize the data
        `valueCount` - how many values the stream holds
        `bitsPerValue` - the number of bits per value
      Returns:
        a Reader
      Throws:
        `IOException` - If there is a low-level I/O error
      See Also:
        `getWriterNoHeader(DataOutput, Format, int, int, int)`
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getReaderNoHeader

          public static PackedInts.Reader getReaderNoHeader(DataInput in,
                                            PackedInts.Header header)
                                                     throws IOException

      Expert: Restore a `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream. This method is useful to restore data when metadata has been previously read using `readHeader(DataInput)`.
      Parameters:
        `in` - the stream to read data from, positioned at the beginning of the packed values
        `header` - metadata result from `readHeader()`
      Returns:
        a Reader
      Throws:
        `IOException` - If there is a low-level I/O error
      See Also:
        `readHeader(DataInput)`
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getReader

          public static PackedInts.Reader getReader(DataInput in)
                                             throws IOException

      Restore a `PackedInts.Reader` from a stream.
      Parameters:
        `in` - the stream to read data from
      Returns:
        a Reader
      Throws:
        `IOException` - If there is a low-level I/O error
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getReaderIteratorNoHeader

          public static PackedInts.ReaderIterator getReaderIteratorNoHeader(DataInput in,
                                                            PackedInts.Format format,
                                                            int version,
                                                            int valueCount,
                                                            int bitsPerValue,
                                                            int mem)

      Expert: Restore a `PackedInts.ReaderIterator` from a stream without reading metadata at the beginning of the stream. This method is useful to restore data from streams which have been created using `getWriterNoHeader(DataOutput, Format, int, int, int)`.
      Parameters:
        `in` - the stream to read data from, positioned at the beginning of the packed values
        `format` - the format used to serialize
        `version` - the version used to serialize the data
        `valueCount` - how many values the stream holds
        `bitsPerValue` - the number of bits per value
        `mem` - how much memory the iterator is allowed to use to read-ahead (likely to speed up iteration)
      Returns:
        a ReaderIterator
      See Also:
        `getWriterNoHeader(DataOutput, Format, int, int, int)`
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getReaderIterator

          public static PackedInts.ReaderIterator getReaderIterator(DataInput in,
                                                    int mem)
                                                             throws IOException

      Retrieve PackedInts as a `PackedInts.ReaderIterator`
      Parameters:
        `in` - positioned at the beginning of a stored packed int structure.
        `mem` - how much memory the iterator is allowed to use to read-ahead (likely to speed up iteration)
      Returns:
        an iterator to access the values
      Throws:
        `IOException` - if the structure could not be retrieved.
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getDirectReaderNoHeader

          public static PackedInts.Reader getDirectReaderNoHeader(IndexInput in,
                                                  PackedInts.Format format,
                                                  int version,
                                                  int valueCount,
                                                  int bitsPerValue)

      Expert: Construct a direct `PackedInts.Reader` from a stream without reading metadata at the beginning of the stream. This method is useful to restore data from streams which have been created using `getWriterNoHeader(DataOutput, Format, int, int, int)`.

      The returned reader will have very little memory overhead, but every call to `PackedInts.Reader.get(int)` is likely to perform a disk seek.

      Parameters:
        `in` - the stream to read data from
        `format` - the format used to serialize
        `version` - the version used to serialize the data
        `valueCount` - how many values the stream holds
        `bitsPerValue` - the number of bits per value
      Returns:
        a direct Reader
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getDirectReaderNoHeader

          public static PackedInts.Reader getDirectReaderNoHeader(IndexInput in,
                                                  PackedInts.Header header)
                                                           throws IOException

      Expert: Construct a direct `PackedInts.Reader` from an `IndexInput` without reading metadata at the beginning of the stream. This method is useful to restore data when metadata has been previously read using `readHeader(DataInput)`.
      Parameters:
        `in` - the stream to read data from, positioned at the beginning of the packed values
        `header` - metadata result from `readHeader()`
      Returns:
        a Reader
      Throws:
        `IOException` - If there is a low-level I/O error
      See Also:
        `readHeader(DataInput)`
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getDirectReader

          public static PackedInts.Reader getDirectReader(IndexInput in)
                                                   throws IOException

      Construct a direct `PackedInts.Reader` from an `IndexInput`. This method is useful to restore data from streams which have been created using `getWriter(DataOutput, int, int, float)`.

      The returned reader will have very little memory overhead, but every call to `PackedInts.Reader.get(int)` is likely to perform a disk seek.

      Parameters:
        `in` - the stream to read data from
      Returns:
        a direct Reader
      Throws:
        `IOException` - If there is a low-level I/O error
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getMutable

          public static PackedInts.Mutable getMutable(int valueCount,
                                      int bitsPerValue,
                                      float acceptableOverheadRatio)

      Create a packed integer array with the given amount of values initialized to 0. the valueCount and the bitsPerValue cannot be changed after creation. All Mutables known by this factory are kept fully in RAM.

      Positive values of `acceptableOverheadRatio` will trade space for speed by selecting a faster but potentially less memory-efficient implementation. An `acceptableOverheadRatio` of `COMPACT` will make sure that the most memory-efficient implementation is selected whereas `FASTEST` will make sure that the fastest implementation is selected.

      Parameters:
        `valueCount` - the number of elements
        `bitsPerValue` - the number of bits available for any given value
        `acceptableOverheadRatio` - an acceptable overhead ratio per value
      Returns:
        a mutable packed integer array
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getWriterNoHeader

          public static PackedInts.Writer getWriterNoHeader(DataOutput out,
                                            PackedInts.Format format,
                                            int valueCount,
                                            int bitsPerValue,
                                            int mem)

      Expert: Create a packed integer array writer for the given output, format, value count, and number of bits per value.

      The resulting stream will be long-aligned. This means that depending on the format which is used, up to 63 bits will be wasted. An easy way to make sure that no space is lost is to always use a `valueCount` that is a multiple of 64.

      This method does not write any metadata to the stream, meaning that it is your responsibility to store it somewhere else in order to be able to recover data from the stream later on:

      - `format` (using `PackedInts.Format.getId()`),
      - `valueCount`,
      - `bitsPerValue`,
      - `VERSION_CURRENT`.

      It is possible to start writing values without knowing how many of them you are actually going to write. To do this, just pass `-1` as `valueCount`. On the other hand, for any positive value of `valueCount`, the returned writer will make sure that you don't write more values than expected and pad the end of stream with zeros in case you have written less than `valueCount` when calling `PackedInts.Writer.finish()`.

      The `mem` parameter lets you control how much memory can be used to buffer changes in memory before flushing to disk. High values of `mem` are likely to improve throughput. On the other hand, if speed is not that important to you, a value of `0` will use as little memory as possible and should already offer reasonable throughput.

      Parameters:
        `out` - the data output
        `format` - the format to use to serialize the values
        `valueCount` - the number of values
        `bitsPerValue` - the number of bits per value
        `mem` - how much memory (in bytes) can be used to speed up serialization
      Returns:
        a Writer
      See Also:
        `getReaderIteratorNoHeader(DataInput, Format, int, int, int, int)`, `getReaderNoHeader(DataInput, Format, int, int, int)`
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### getWriter

          public static PackedInts.Writer getWriter(DataOutput out,
                                    int valueCount,
                                    int bitsPerValue,
                                    float acceptableOverheadRatio)
                                             throws IOException

      Create a packed integer array writer for the given output, format, value count, and number of bits per value.

      The resulting stream will be long-aligned. This means that depending on the format which is used under the hoods, up to 63 bits will be wasted. An easy way to make sure that no space is lost is to always use a `valueCount` that is a multiple of 64.

      This method writes metadata to the stream, so that the resulting stream is sufficient to restore a `PackedInts.Reader` from it. You don't need to track `valueCount` or `bitsPerValue` by yourself. In case this is a problem, you should probably look at `getWriterNoHeader(DataOutput, Format, int, int, int)`.

      The `acceptableOverheadRatio` parameter controls how readers that will be restored from this stream trade space for speed by selecting a faster but potentially less memory-efficient implementation. An `acceptableOverheadRatio` of `COMPACT` will make sure that the most memory-efficient implementation is selected whereas `FASTEST` will make sure that the fastest implementation is selected. In case you are only interested in reading this stream sequentially later on, you should probably use `COMPACT`.

      Parameters:
        `out` - the data output
        `valueCount` - the number of values
        `bitsPerValue` - the number of bits per value
        `acceptableOverheadRatio` - an acceptable overhead ratio per value
      Returns:
        a Writer
      Throws:
        `IOException` - If there is a low-level I/O error
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### bitsRequired

          public static int bitsRequired(long maxValue)

      Returns how many bits are required to hold values up to and including maxValue
      Parameters:
        `maxValue` - the maximum value that should be representable.
      Returns:
        the amount of bits needed to represent values from 0 to maxValue.
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### maxValue

          public static long maxValue(int bitsPerValue)

      Calculates the maximum unsigned long that can be expressed with the given number of bits.
      Parameters:
        `bitsPerValue` - the number of bits available for any given value.
      Returns:
        the maximum value for the given bits.
      NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    - #### copy

          public static void copy(PackedInts.Reader src,
                  int srcPos,
                  PackedInts.Mutable dest,
                  int destPos,
                  int len,
                  int mem)

      Copy `src[srcPos:srcPos+len]` into `dest[destPos:destPos+len]` using at most `mem` bytes.

    - #### readHeader

          public static PackedInts.Header readHeader(DataInput in)
                                              throws IOException

      Expert: reads only the metadata from a stream. This is useful to later restore a stream or open a direct reader via `getReaderNoHeader(DataInput, Header)` or `getDirectReaderNoHeader(IndexInput, Header)`.
      Parameters:
        `in` - the stream to read data
      Returns:
        packed integer metadata.
      Throws:
        `IOException` - If there is a low-level I/O error
      See Also:
        `getReaderNoHeader(DataInput, Header)`, `getDirectReaderNoHeader(IndexInput, Header)`

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