Package org.uncommons.maths.binary
Class BitString
- java.lang.Object
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- org.uncommons.maths.binary.BitString
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- All Implemented Interfaces:
java.io.Serializable
,java.lang.Cloneable
public final class BitString extends java.lang.Object implements java.lang.Cloneable, java.io.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:
- Serialized Form
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Field Summary
Fields Modifier and Type Field Description private int[]
data
Store the bits packed in an array of 32-bit ints.private int
length
private static int
WORD_LENGTH
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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, java.util.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).BitString(java.lang.String value)
Initialises the bit string from a character string of 1s and 0s in big-endian order.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description private void
assertValidIndex(int index)
Helper method to check whether a bit index is valid or not.BitString
clone()
int
countSetBits()
int
countUnsetBits()
boolean
equals(java.lang.Object o)
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
getLength()
int
hashCode()
Over-ridden to be consistent withequals(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.java.math.BigInteger
toNumber()
Interprets this bit string as being a binary numeric value and returns the integer that it represents.java.lang.String
toString()
Creates a textual representation of this bit string in big-endian order (index 0 is the right-most bit).
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Field Detail
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WORD_LENGTH
private static final int WORD_LENGTH
- See Also:
- Constant Field Values
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length
private final int length
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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.
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Constructor Detail
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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.
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BitString
public BitString(int length, java.util.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.
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BitString
public BitString(java.lang.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.
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Method Detail
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getLength
public int getLength()
- Returns:
- The length of this bit string.
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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:
java.lang.IndexOutOfBoundsException
- If the specified index is not a bit position in this bit string.
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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:
java.lang.IndexOutOfBoundsException
- If the specified index is not a bit position in this bit string.
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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:
java.lang.IndexOutOfBoundsException
- If the specified index is not a bit position in this bit string.
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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:
java.lang.IndexOutOfBoundsException
- If the index is not valid.
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countSetBits
public int countSetBits()
- Returns:
- The number of bits that are 1s rather than 0s.
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countUnsetBits
public int countUnsetBits()
- Returns:
- The number of bits that are 0s rather than 1s.
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toNumber
public java.math.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.
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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.
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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.
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toString
public java.lang.String toString()
Creates a textual representation of this bit string in big-endian order (index 0 is the right-most bit).- Overrides:
toString
in classjava.lang.Object
- Returns:
- This bit string rendered as a String of 1s and 0s.
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clone
public BitString clone()
- Overrides:
clone
in classjava.lang.Object
- Returns:
- An identical copy of this bit string.
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equals
public boolean equals(java.lang.Object o)
- Overrides:
equals
in classjava.lang.Object
- Returns:
- True if the argument is a BitString instance and both bit strings are the same length with identical bits set/unset.
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hashCode
public int hashCode()
Over-ridden to be consistent withequals(Object)
.- Overrides:
hashCode
in classjava.lang.Object
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