org.apache.commons.compress.harmony.pack200

## Class BHSDCodec

- java.lang.Object
- - org.apache.commons.compress.harmony.pack200.Codec
  - - org.apache.commons.compress.harmony.pack200.BHSDCodec

-     public final class BHSDCodec
      extends Codec

  A BHSD codec is a means of encoding integer values as a sequence of bytes or vice versa using a specified "BHSD" encoding mechanism. It uses a variable-length encoding and a modified sign representation such that small numbers are represented as a single byte, whilst larger numbers take more bytes to encode. The number may be signed or unsigned; if it is unsigned, it can be weighted towards positive numbers or equally distributed using a one's complement. The Codec also supports delta coding, where a sequence of numbers is represented as a series of first-order differences. So a delta encoding of the integers [1..10] would be represented as a sequence of 10x1s. This allows the absolute value of a coded integer to fall outside of the 'small number' range, whilst still being encoded as a single byte. A BHSD codec is configured with four parameters:
  B
    The maximum number of bytes that each value is encoded as. B must be a value between [1..5]. For a pass-through coding (where each byte is encoded as itself, aka `Codec.BYTE1`, B is 1 (each byte takes a maximum of 1 byte).
  H
    The radix of the integer. Values are defined as a sequence of values, where value `n` is multiplied by `H^n`. So the number 1234 may be represented as the sequence 4 3 2 1 with a radix (H) of 10. Note that other permutations are also possible; 43 2 1 will also encode 1234. The co-parameter L is defined as 256-H. This is important because only the last value in a sequence may be < L; all prior values must be > L.
  S
    Whether the codec represents signed values (or not). This may have 3 values; 0 (unsigned), 1 (signed, one's complement) or 2 (signed, two's complement)
  D
    Whether the codec represents a delta encoding. This may be 0 (no delta) or 1 (delta encoding). A delta encoding of 1 indicates that values are cumulative; a sequence of `1 1 1 1 1` will represent the sequence `1 2 3 4 5`. For this reason, the codec supports two variants of decode; one `with` and one `without` a `last` parameter. If the codec is a non-delta encoding, then the value is ignored if passed. If the codec is a delta encoding, it is a run-time error to call the value without the extra parameter, and the previous value should be returned. (It was designed this way to support multi-threaded access without requiring a new instance of the Codec to be cloned for each use.)
  Codecs are notated as (B,H,S,D) and either D or S,D may be omitted if zero. Thus `Codec.BYTE1` is denoted (1,256,0,0) or (1,256). The `toString()` method prints out the condensed form of the encoding. Often, the last character in the name (`Codec.BYTE1`, `Codec.UNSIGNED5`) gives a clue as to the B value. Those that start with U (`Codec.UDELTA5`, `Codec.UNSIGNED5`) are unsigned; otherwise, in most cases, they are signed. The presence of the word Delta (`Codec.DELTA5`, `Codec.UDELTA5`) indicates a delta encoding is used.

- - ### Field Summary

    - ### Fields inherited from class org.apache.commons.compress.harmony.pack200.Codec

      `BCI5, BRANCH5, BYTE1, CHAR3, DELTA5, lastBandLength, MDELTA5, SIGNED5, UDELTA5, UNSIGNED5`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `BHSDCodec(int b, int h)` Constructs an unsigned, non-delta Codec with the given B and H values.
    `BHSDCodec(int b, int h, int s)` Constructs a non-delta Codec with the given B, H and S values.
    `BHSDCodec(int b, int h, int s, int d)` Constructs a Codec with the given B, H, S and D values.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `long` | `cardinality()` Returns the cardinality of this codec; that is, the number of distinct values that it can contain.
    `int` | `decode(InputStream in)` Decode a sequence of bytes from the given input stream, returning the value as a long.
    `int` | `decode(InputStream in, long last)` Decode a sequence of bytes from the given input stream, returning the value as a long.
    `int[]` | `decodeInts(int n, InputStream in)` Decodes a sequence of `n` values from `in`.
    `int[]` | `decodeInts(int n, InputStream in, int firstValue)` Decodes a sequence of `n` values from `in`.
    `byte[]` | `encode(int value)` Encode a single value into a sequence of bytes.
    `byte[]` | `encode(int value, int last)` Encode a single value into a sequence of bytes.
    `boolean` | `encodes(long value)` True if this encoding can code the given value
    `boolean` | `equals(Object o)`
    `int` | `getB()`
    `int` | `getH()`
    `int` | `getL()`
    `int` | `getS()`
    `int` | `hashCode()`
    `boolean` | `isDelta()` Returns true if this codec is a delta codec
    `boolean` | `isSigned()` Returns true if this codec is a signed codec
    `long` | `largest()` Returns the largest value that this codec can represent.
    `long` | `smallest()` Returns the smallest value that this codec can represent.
    `String` | `toString()` Returns the codec in the form (1,256) or (1,64,1,1).

    - ### Methods inherited from class org.apache.commons.compress.harmony.pack200.Codec

      `encode`

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

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Constructor Detail

    - #### BHSDCodec

          public BHSDCodec(int b,
                           int h)

      Constructs an unsigned, non-delta Codec with the given B and H values.
      Parameters:
        `b` - the maximum number of bytes that a value can be encoded as [1..5]
        `h` - the radix of the encoding [1..256]

    - #### BHSDCodec

          public BHSDCodec(int b,
                           int h,
                           int s)

      Constructs a non-delta Codec with the given B, H and S values.
      Parameters:
        `b` - the maximum number of bytes that a value can be encoded as [1..5]
        `h` - the radix of the encoding [1..256]
        `s` - whether the encoding represents signed numbers (s=0 is unsigned; s=1 is signed with 1s complement; s=2 is signed with ?)

    - #### BHSDCodec

          public BHSDCodec(int b,
                           int h,
                           int s,
                           int d)

      Constructs a Codec with the given B, H, S and D values.
      Parameters:
        `b` - the maximum number of bytes that a value can be encoded as [1..5]
        `h` - the radix of the encoding [1..256]
        `s` - whether the encoding represents signed numbers (s=0 is unsigned; s=1 is signed with 1s complement; s=2 is signed with ?)
        `d` - whether this is a delta encoding (d=0 is non-delta; d=1 is delta)

  - ### Method Detail

    - #### cardinality

          public long cardinality()

      Returns the cardinality of this codec; that is, the number of distinct values that it can contain.
      Returns:
        the cardinality of this codec

    - #### decode

          public int decode(InputStream in)
                     throws IOException,
                            Pack200Exception

      Description copied from class: `Codec`
      Decode a sequence of bytes from the given input stream, returning the value as a long. Note that this method can only be applied for non-delta encodings.
      Specified by:
        `decode` in class `Codec`
      Parameters:
        `in` - the input stream to read from
      Returns:
        the value as a long
      Throws:
        `IOException` - if there is a problem reading from the underlying input stream
        `Pack200Exception` - if the encoding is a delta encoding

    - #### decode

          public int decode(InputStream in,
                            long last)
                     throws IOException,
                            Pack200Exception

      Description copied from class: `Codec`
      Decode a sequence of bytes from the given input stream, returning the value as a long. If this encoding is a delta encoding (d=1) then the previous value must be passed in as a parameter. If it is a non-delta encoding, then it does not matter what value is passed in, so it makes sense for the value to be passed in by default using code similar to:

           long last = 0;
           while (condition) {
               last = codec.decode(in, last);
               // do something with last
           }

      Specified by:
        `decode` in class `Codec`
      Parameters:
        `in` - the input stream to read from
        `last` - the previous value read, which must be supplied if the codec is a delta encoding
      Returns:
        the value as a long
      Throws:
        `IOException` - if there is a problem reading from the underlying input stream
        `Pack200Exception` - if there is a problem decoding the value or that the value is invalid

    - #### decodeInts

          public int[] decodeInts(int n,
                                  InputStream in)
                           throws IOException,
                                  Pack200Exception

      Description copied from class: `Codec`
      Decodes a sequence of `n` values from `in`. This should probably be used in most cases, since some codecs (such as `PopulationCodec`) only work when the number of values to be read is known.
      Overrides:
        `decodeInts` in class `Codec`
      Parameters:
        `n` - the number of values to decode
        `in` - the input stream to read from
      Returns:
        an array of `int` values corresponding to values decoded
      Throws:
        `IOException` - if there is a problem reading from the underlying input stream
        `Pack200Exception` - if there is a problem decoding the value or that the value is invalid

    - #### decodeInts

          public int[] decodeInts(int n,
                                  InputStream in,
                                  int firstValue)
                           throws IOException,
                                  Pack200Exception

      Description copied from class: `Codec`
      Decodes a sequence of `n` values from `in`.
      Overrides:
        `decodeInts` in class `Codec`
      Parameters:
        `n` - the number of values to decode
        `in` - the input stream to read from
        `firstValue` - the first value in the band if it has already been read
      Returns:
        an array of `int` values corresponding to values decoded, with firstValue as the first value in the array.
      Throws:
        `IOException` - if there is a problem reading from the underlying input stream
        `Pack200Exception` - if there is a problem decoding the value or that the value is invalid

    - #### encodes

          public boolean encodes(long value)

      True if this encoding can code the given value
      Parameters:
        `value` - the value to check
      Returns:
        `true` if the encoding can encode this value

    - #### encode

          public byte[] encode(int value,
                               int last)
                        throws Pack200Exception

      Description copied from class: `Codec`
      Encode a single value into a sequence of bytes.
      Specified by:
        `encode` in class `Codec`
      Parameters:
        `value` - the value to encode
        `last` - the previous value encoded (for delta encodings)
      Returns:
        the encoded bytes
      Throws:
        `Pack200Exception` - TODO

    - #### encode

          public byte[] encode(int value)
                        throws Pack200Exception

      Description copied from class: `Codec`
      Encode a single value into a sequence of bytes. Note that this method can only be used for non-delta encodings.
      Specified by:
        `encode` in class `Codec`
      Parameters:
        `value` - the value to encode
      Returns:
        the encoded bytes
      Throws:
        `Pack200Exception` - TODO

    - #### isDelta

          public boolean isDelta()

      Returns true if this codec is a delta codec
      Returns:
        true if this codec is a delta codec

    - #### isSigned

          public boolean isSigned()

      Returns true if this codec is a signed codec
      Returns:
        true if this codec is a signed codec

    - #### largest

          public long largest()

      Returns the largest value that this codec can represent.
      Returns:
        the largest value that this codec can represent.

    - #### smallest

          public long smallest()

      Returns the smallest value that this codec can represent.
      Returns:
        the smallest value that this codec can represent.

    - #### toString

          public String toString()

      Returns the codec in the form (1,256) or (1,64,1,1). Note that trailing zero fields are not shown.
      Overrides:
        `toString` in class `Object`

    - #### getB

          public int getB()

      Returns:
        the b

    - #### getH

          public int getH()

      Returns:
        the h

    - #### getS

          public int getS()

      Returns:
        the s

    - #### getL

          public int getL()

      Returns:
        the l

    - #### equals

          public boolean equals(Object o)

      Overrides:
        `equals` in class `Object`

    - #### hashCode

          public int hashCode()

      Overrides:
        `hashCode` in class `Object`

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