Class CommonBits

1
2	/*
3	* Copyright (c) 2016 Vivid Solutions.
4	*
5	* All rights reserved. This program and the accompanying materials
6	* are made available under the terms of the Eclipse Public License 2.0
7	* and Eclipse Distribution License v. 1.0 which accompanies this distribution.
8	* The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
9	* and the Eclipse Distribution License is available at
10	*
11	* http://www.eclipse.org/org/documents/edl-v10.php.
12	*/
13	package org.locationtech.jts.precision;
14
15	/**
16	* Determines the maximum number of common most-significant
17	* bits in the mantissa of one or numbers.
18	* Can be used to compute the double-precision number which
19	* is represented by the common bits.
20	* If there are no common bits, the number computed is 0.0.
21	*
22	* @version 1.7
23	*/
24	public class CommonBits {
25
26	/**
27	* Computes the bit pattern for the sign and exponent of a
28	* double-precision number.
29	*
30	* @param num
31	* @return the bit pattern for the sign and exponent
32	*/
33	public static long signExpBits(long num)
34	{
35	return num >> 52;
36	}
37
38	/**
39	* This computes the number of common most-significant bits in the mantissas
40	* of two double-precision numbers.
41	* It does not count the hidden bit, which is always 1.
42	* It does not determine whether the numbers have the same exponent - if they do
43	* not, the value computed by this function is meaningless.
44	*
45	* @param num1 the first number
46	* @param num2 the second number
47	* @return the number of common most-significant mantissa bits
48	*/
49	public static int numCommonMostSigMantissaBits(long num1, long num2)
50	{
51	int count = 0;
52	for (int i = 52; i >= 0; i--)
53	{
54	if (getBit(num1, i) != getBit(num2, i))
55	return count;
56	count++;
57	}
58	return 52;
59	}
60
61	/**
62	* Zeroes the lower n bits of a bitstring.
63	*
64	* @param bits the bitstring to alter
65	* @return the zeroed bitstring
66	*/
67	public static long zeroLowerBits(long bits, int nBits)
68	{
69	long invMask = (1L << nBits) - 1L;
70	long mask = ~ invMask;
71	long zeroed = bits & mask;
72	return zeroed;
73	}
74
75	/**
76	* Extracts the i'th bit of a bitstring.
77	*
78	* @param bits the bitstring to extract from
79	* @param i the bit to extract
80	* @return the value of the extracted bit
81	*/
82	public static int getBit(long bits, int i)
83	{
84	long mask = (1L << i);
85	return (bits & mask) != 0 ? 1 : 0;
86	}
87
88	private boolean isFirst = true;
89	private int commonMantissaBitsCount = 53;
90	private long commonBits = 0;
91	private long commonSignExp;
92
93	public CommonBits() {
94	}
95
96	public void add(double num)
97	{
98	long numBits = Double.doubleToLongBits(num);
99	if (isFirst) {
100	commonBits = numBits;
101	commonSignExp = signExpBits(commonBits);
102	isFirst = false;
103	return;
104	}
105
106	long numSignExp = signExpBits(numBits);
107	if (numSignExp != commonSignExp) {
108	commonBits = 0;
109	return;
110	}
111
112	// System.out.println(toString(commonBits));
113	// System.out.println(toString(numBits));
114	commonMantissaBitsCount = numCommonMostSigMantissaBits(commonBits, numBits);
115	commonBits = zeroLowerBits(commonBits, 64 - (12 + commonMantissaBitsCount));
116	// System.out.println(toString(commonBits));
117	}
118
119	public double getCommon()
120	{
121	return Double.longBitsToDouble(commonBits);
122	}
123	/**
124	* A representation of the Double bits formatted for easy readability
125	*/
126	public String toString(long bits)
127	{
128	double x = Double.longBitsToDouble(bits);
129	String numStr = Long.toBinaryString(bits);
130	String padStr = "0000000000000000000000000000000000000000000000000000000000000000" + numStr;
131	String bitStr = padStr.substring(padStr.length() - 64);
132	String str = bitStr.substring(0, 1) + " "
133	+ bitStr.substring(1, 12) + "(exp) "
134	+ bitStr.substring(12)
135	+ " [ " + x + " ]";
136	return str;
137	}
138
139	}
140