package org.apache.lucene.util;
/**
* Copyright 2005 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/** Floating point numbers smaller than 32 bits.
*
* @version $Id$
*/
public class SmallFloat {
/** Converts a 32 bit float to an 8 bit float.
* <br>Values less than zero are all mapped to zero.
* <br>Values are truncated (rounded down) to the nearest 8 bit value.
* <br>Values between zero and the smallest representable value
* are rounded up.
*
* @param f the 32 bit float to be converted to an 8 bit float (byte)
* @param numMantissaBits the number of mantissa bits to use in the byte, with the remainder to be used in the exponent
* @param zeroExp the zero-point in the range of exponent values
* @return the 8 bit float representation
*/
public static byte floatToByte(float f, int numMantissaBits, int zeroExp) {
// Adjustment from a float zero exponent to our zero exponent,
// shifted over to our exponent position.
int fzero = (63-zeroExp)<<numMantissaBits;
int bits = Float.floatToRawIntBits(f);
int smallfloat = bits >> (24-numMantissaBits);
if (smallfloat < fzero) {
return (bits<=0) ?
(byte)0 // negative numbers and zero both map to 0 byte
:(byte)1; // underflow is mapped to smallest non-zero number.
} else if (smallfloat >= fzero + 0x100) {
return -1; // overflow maps to largest number
} else {
return (byte)(smallfloat - fzero);
}
}
/** Converts an 8 bit float to a 32 bit float. */
public static float byteToFloat(byte b, int numMantissaBits, int zeroExp) {
// on Java1.5 & 1.6 JVMs, prebuilding a decoding array and doing a lookup
// is only a little bit faster (anywhere from 0% to 7%)
if (b == 0) return 0.0f;
int bits = (b&0xff) << (24-numMantissaBits);
bits += (63-zeroExp) << 24;
return Float.intBitsToFloat(bits);
}
//
// Some specializations of the generic functions follow.
// The generic functions are just as fast with current (1.5)
// -server JVMs, but still slower with client JVMs.
//
/** floatToByte(b, mantissaBits=3, zeroExponent=15)
* <br>smallest non-zero value = 5.820766E-10
* <br>largest value = 7.5161928E9
* <br>epsilon = 0.125
*/
public static byte floatToByte315(float f) {
int bits = Float.floatToRawIntBits(f);
int smallfloat = bits >> (24-3);
if (smallfloat < (63-15)<<3) {
return (bits<=0) ? (byte)0 : (byte)1;
}
if (smallfloat >= ((63-15)<<3) + 0x100) {
return -1;
}
return (byte)(smallfloat - ((63-15)<<3));
}
/** byteToFloat(b, mantissaBits=3, zeroExponent=15) */
public static float byte315ToFloat(byte b) {
// on Java1.5 & 1.6 JVMs, prebuilding a decoding array and doing a lookup
// is only a little bit faster (anywhere from 0% to 7%)
if (b == 0) return 0.0f;
int bits = (b&0xff) << (24-3);
bits += (63-15) << 24;
return Float.intBitsToFloat(bits);
}
/** floatToByte(b, mantissaBits=5, zeroExponent=2)
* <br>smallest nonzero value = 0.033203125
* <br>largest value = 1984.0
* <br>epsilon = 0.03125
*/
public static byte floatToByte52(float f) {
int bits = Float.floatToRawIntBits(f);
int smallfloat = bits >> (24-5);
if (smallfloat < (63-2)<<5) {
return (bits<=0) ? (byte)0 : (byte)1;
}
if (smallfloat >= ((63-2)<<5) + 0x100) {
return -1;
}
return (byte)(smallfloat - ((63-2)<<5));
}
/** byteToFloat(b, mantissaBits=5, zeroExponent=2) */
public static float byte52ToFloat(byte b) {
// on Java1.5 & 1.6 JVMs, prebuilding a decoding array and doing a lookup
// is only a little bit faster (anywhere from 0% to 7%)
if (b == 0) return 0.0f;
int bits = (b&0xff) << (24-5);
bits += (63-2) << 24;
return Float.intBitsToFloat(bits);
}
}