Class BitString

java.lang.Object
org.uncommons.maths.binary.BitString
All Implemented Interfaces:
Serializable, Cloneable

public final class BitString extends Object implements Cloneable, Serializable

Implementation of a fixed-length bit-string. Internally, bits are packed into an array of ints. This implementation makes more efficient use of space than the alternative approach of using an array of booleans.

This class is preferable to BitSet if a fixed number of bits is required.

See Also:
  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    private int[]
    Store the bits packed in an array of 32-bit ints.
    private final int
     
    private static final int
     
  • Constructor Summary

    Constructors
    Constructor
    Description
    BitString(int length)
    Creates a bit string of the specified length with all bits initially set to zero (off).
    BitString(int length, Random rng)
    Creates a bit string of the specified length with each bit set randomly (the distribution of bits is uniform so long as the output from the provided RNG is also uniform).
    Initialises the bit string from a character string of 1s and 0s in big-endian order.
  • Method Summary

    Modifier and Type
    Method
    Description
    private void
    assertValidIndex(int index)
    Helper method to check whether a bit index is valid or not.
     
    int
     
    int
     
    boolean
     
    void
    flipBit(int index)
    Inverts the value of the bit at the specified index.
    boolean
    getBit(int index)
    Returns the bit at the specified index.
    int
     
    int
    Over-ridden to be consistent with equals(Object).
    void
    setBit(int index, boolean set)
    Sets the bit at the specified index.
    private void
    swapBits(BitString other, int word, int swapMask)
     
    void
    swapSubstring(BitString other, int start, int length)
    An efficient method for exchanging data between two bit strings.
    Interprets this bit string as being a binary numeric value and returns the integer that it represents.
    Creates a textual representation of this bit string in big-endian order (index 0 is the right-most bit).

    Methods inherited from class java.lang.Object

    finalize, getClass, notify, notifyAll, wait, wait, wait
  • Field Details

    • WORD_LENGTH

      private static final int WORD_LENGTH
      See Also:
    • length

      private final int length
    • data

      private int[] data
      Store the bits packed in an array of 32-bit ints. This field cannot be declared final because it must be cloneable.
  • Constructor Details

    • BitString

      public BitString(int length)
      Creates a bit string of the specified length with all bits initially set to zero (off).
      Parameters:
      length - The number of bits.
    • BitString

      public BitString(int length, Random rng)
      Creates a bit string of the specified length with each bit set randomly (the distribution of bits is uniform so long as the output from the provided RNG is also uniform). Using this constructor is more efficient than creating a bit string and then randomly setting each bit individually.
      Parameters:
      length - The number of bits.
      rng - A source of randomness.
    • BitString

      public BitString(String value)
      Initialises the bit string from a character string of 1s and 0s in big-endian order.
      Parameters:
      value - A character string of ones and zeros.
  • Method Details

    • getLength

      public int getLength()
      Returns:
      The length of this bit string.
    • getBit

      public boolean getBit(int index)
      Returns the bit at the specified index.
      Parameters:
      index - The index of the bit to look-up (0 is the least-significant bit).
      Returns:
      A boolean indicating whether the bit is set or not.
      Throws:
      IndexOutOfBoundsException - If the specified index is not a bit position in this bit string.
    • setBit

      public void setBit(int index, boolean set)
      Sets the bit at the specified index.
      Parameters:
      index - The index of the bit to set (0 is the least-significant bit).
      set - A boolean indicating whether the bit should be set or not.
      Throws:
      IndexOutOfBoundsException - If the specified index is not a bit position in this bit string.
    • flipBit

      public void flipBit(int index)
      Inverts the value of the bit at the specified index.
      Parameters:
      index - The bit to flip (0 is the least-significant bit).
      Throws:
      IndexOutOfBoundsException - If the specified index is not a bit position in this bit string.
    • assertValidIndex

      private void assertValidIndex(int index)
      Helper method to check whether a bit index is valid or not.
      Parameters:
      index - The index to check.
      Throws:
      IndexOutOfBoundsException - If the index is not valid.
    • countSetBits

      public int countSetBits()
      Returns:
      The number of bits that are 1s rather than 0s.
    • countUnsetBits

      public int countUnsetBits()
      Returns:
      The number of bits that are 0s rather than 1s.
    • toNumber

      public BigInteger toNumber()
      Interprets this bit string as being a binary numeric value and returns the integer that it represents.
      Returns:
      A BigInteger that contains the numeric value represented by this bit string.
    • swapSubstring

      public void swapSubstring(BitString other, int start, int length)
      An efficient method for exchanging data between two bit strings. Both bit strings must be long enough that they contain the full length of the specified substring.
      Parameters:
      other - The bitstring with which this bitstring should swap bits.
      start - The start position for the substrings to be exchanged. All bit indices are big-endian, which means position 0 is the rightmost bit.
      length - The number of contiguous bits to swap.
    • swapBits

      private void swapBits(BitString other, int word, int swapMask)
      Parameters:
      other - The BitString to exchange bits with.
      word - The word index of the word that will be swapped between the two bit strings.
      swapMask - A mask that specifies which bits in the word will be swapped.
    • toString

      public String toString()
      Creates a textual representation of this bit string in big-endian order (index 0 is the right-most bit).
      Overrides:
      toString in class Object
      Returns:
      This bit string rendered as a String of 1s and 0s.
    • clone

      public BitString clone()
      Overrides:
      clone in class Object
      Returns:
      An identical copy of this bit string.
    • equals

      public boolean equals(Object o)
      Overrides:
      equals in class Object
      Returns:
      True if the argument is a BitString instance and both bit strings are the same length with identical bits set/unset.
    • hashCode

      public int hashCode()
      Over-ridden to be consistent with equals(Object).
      Overrides:
      hashCode in class Object