CommonBits.java
/*
* Copyright (c) 2016 Vivid Solutions.
*
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License 2.0
* and Eclipse Distribution License v. 1.0 which accompanies this distribution.
* The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
* and the Eclipse Distribution License is available at
*
* http://www.eclipse.org/org/documents/edl-v10.php.
*/
package org.locationtech.jts.precision;
/**
* Determines the maximum number of common most-significant
* bits in the mantissa of one or numbers.
* Can be used to compute the double-precision number which
* is represented by the common bits.
* If there are no common bits, the number computed is 0.0.
*
* @version 1.7
*/
public class CommonBits {
/**
* Computes the bit pattern for the sign and exponent of a
* double-precision number.
*
* @param num
* @return the bit pattern for the sign and exponent
*/
public static long signExpBits(long num)
{
return num >> 52;
}
/**
* This computes the number of common most-significant bits in the mantissas
* of two double-precision numbers.
* It does not count the hidden bit, which is always 1.
* It does not determine whether the numbers have the same exponent - if they do
* not, the value computed by this function is meaningless.
*
* @param num1 the first number
* @param num2 the second number
* @return the number of common most-significant mantissa bits
*/
public static int numCommonMostSigMantissaBits(long num1, long num2)
{
int count = 0;
for (int i = 52; i >= 0; i--)
{
if (getBit(num1, i) != getBit(num2, i))
return count;
count++;
}
return 52;
}
/**
* Zeroes the lower n bits of a bitstring.
*
* @param bits the bitstring to alter
* @return the zeroed bitstring
*/
public static long zeroLowerBits(long bits, int nBits)
{
long invMask = (1L << nBits) - 1L;
long mask = ~ invMask;
long zeroed = bits & mask;
return zeroed;
}
/**
* Extracts the i'th bit of a bitstring.
*
* @param bits the bitstring to extract from
* @param i the bit to extract
* @return the value of the extracted bit
*/
public static int getBit(long bits, int i)
{
long mask = (1L << i);
return (bits & mask) != 0 ? 1 : 0;
}
private boolean isFirst = true;
private int commonMantissaBitsCount = 53;
private long commonBits = 0;
private long commonSignExp;
public CommonBits() {
}
public void add(double num)
{
long numBits = Double.doubleToLongBits(num);
if (isFirst) {
commonBits = numBits;
commonSignExp = signExpBits(commonBits);
isFirst = false;
return;
}
long numSignExp = signExpBits(numBits);
if (numSignExp != commonSignExp) {
commonBits = 0;
return;
}
// System.out.println(toString(commonBits));
// System.out.println(toString(numBits));
commonMantissaBitsCount = numCommonMostSigMantissaBits(commonBits, numBits);
commonBits = zeroLowerBits(commonBits, 64 - (12 + commonMantissaBitsCount));
// System.out.println(toString(commonBits));
}
public double getCommon()
{
return Double.longBitsToDouble(commonBits);
}
/**
* A representation of the Double bits formatted for easy readability
*/
public String toString(long bits)
{
double x = Double.longBitsToDouble(bits);
String numStr = Long.toBinaryString(bits);
String padStr = "0000000000000000000000000000000000000000000000000000000000000000" + numStr;
String bitStr = padStr.substring(padStr.length() - 64);
String str = bitStr.substring(0, 1) + " "
+ bitStr.substring(1, 12) + "(exp) "
+ bitStr.substring(12)
+ " [ " + x + " ]";
return str;
}
}