/*
* Copyright (C) 2014 Indeed Inc.
*
* 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.
*/
package com.indeed.imhotep.metrics;
import com.indeed.flamdex.api.IntValueLookup;
/**
* log(1 + e^x)
* @author jwolfe
*/
public class Log1pExp implements IntValueLookup {
private final IntValueLookup operand;
private final int scaleFactor;
public Log1pExp(IntValueLookup operand, int scaleFactor) {
this.operand = operand;
this.scaleFactor = scaleFactor;
}
@Override
public long getMin() {
return Long.MIN_VALUE;
}
@Override
public long getMax() {
return Long.MAX_VALUE;
}
@Override
public void lookup(int[] docIds, long[] values, int n) {
operand.lookup(docIds, values, n);
for (int i = 0; i < n; i++) {
final double x = values[i] / (double) scaleFactor;
final double result;
if (x > 0) {
// This is mathematically the same as log(1 + e^x):
// log(1+e^x) = log(e^x * (e^(-x) + 1)) = log(e^x) + log(1 + e^-x) = x + log1p(e^-x)
// Except it won't overflow the 64-bit floating point numbers with large values of x.
result = x + Math.log1p(Math.exp(-x));
} else {
result = Math.log1p(Math.exp(x));
}
// the output is clamped to [Integer.MIN_VALUE, Integer.MAX_VALUE] (JLS ยง5.1.3)
values[i] = (long) (result * scaleFactor);
}
}
@Override
public long memoryUsed() {
return operand.memoryUsed();
}
@Override
public void close() {
operand.close();
}
}