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Statistical distributions library (in statu nascendi)
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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 math;
/** Class used to compute the classical functions tables.
* @version $Id: FastMathCalc.java 1416643 2012-12-03 19:37:14Z tn $
* @since 3.0
*/
class CommonsCalc {
/**
* 0x40000000 - used to split a double into two parts, both with the low order bits cleared.
* Equivalent to 2^30.
*/
private static final long HEX_40000000 = 0x40000000L; // 1073741824L
/** Factorial table, for Taylor series expansions. 0!, 1!, 2!, ... 19! */
private static final double FACT[] = new double[]
{
+1.0d, // 0
+1.0d, // 1
+2.0d, // 2
+6.0d, // 3
+24.0d, // 4
+120.0d, // 5
+720.0d, // 6
+5040.0d, // 7
+40320.0d, // 8
+362880.0d, // 9
+3628800.0d, // 10
+39916800.0d, // 11
+479001600.0d, // 12
+6227020800.0d, // 13
+87178291200.0d, // 14
+1307674368000.0d, // 15
+20922789888000.0d, // 16
+355687428096000.0d, // 17
+6402373705728000.0d, // 18
+121645100408832000.0d, // 19
};
/**
* Private Constructor.
*/
private CommonsCalc() {
}
/**
* For x between 0 and 1, returns exp(x), uses extended precision
* @param x argument of exponential
* @param result placeholder where to place exp(x) split in two terms
* for extra precision (i.e. exp(x) = result[0] + result[1]
* @return exp(x)
*/
static double slowexp(final double x, final double result[]) {
final double xs[] = new double[2];
final double ys[] = new double[2];
final double facts[] = new double[2];
final double as[] = new double[2];
split(x, xs);
ys[0] = ys[1] = 0.0;
for (int i = FACT.length-1; i >= 0; i--) {
splitMult(xs, ys, as);
ys[0] = as[0];
ys[1] = as[1];
split(FACT[i], as);
splitReciprocal(as, facts);
splitAdd(ys, facts, as);
ys[0] = as[0];
ys[1] = as[1];
}
if (result != null) {
result[0] = ys[0];
result[1] = ys[1];
}
return ys[0] + ys[1];
}
/** Compute split[0], split[1] such that their sum is equal to d,
* and split[0] has its 30 least significant bits as zero.
* @param d number to split
* @param split placeholder where to place the result
*/
private static void split(final double d, final double split[]) {
if (d < 8e298 && d > -8e298) {
final double a = d * HEX_40000000;
split[0] = (d + a) - a;
split[1] = d - split[0];
} else {
final double a = d * 9.31322574615478515625E-10;
split[0] = (d + a - d) * HEX_40000000;
split[1] = d - split[0];
}
}
/** Recompute a split.
* @param a input/out array containing the split, changed
* on output
*/
private static void resplit(final double a[]) {
final double c = a[0] + a[1];
final double d = -(c - a[0] - a[1]);
if (c < 8e298 && c > -8e298) { // MAGIC NUMBER
double z = c * HEX_40000000;
a[0] = (c + z) - z;
a[1] = c - a[0] + d;
} else {
double z = c * 9.31322574615478515625E-10;
a[0] = (c + z - c) * HEX_40000000;
a[1] = c - a[0] + d;
}
}
/** Multiply two numbers in split form.
* @param a first term of multiplication
* @param b second term of multiplication
* @param ans placeholder where to put the result
*/
private static void splitMult(double a[], double b[], double ans[]) {
ans[0] = a[0] * b[0];
ans[1] = a[0] * b[1] + a[1] * b[0] + a[1] * b[1];
/* Resplit */
resplit(ans);
}
/** Add two numbers in split form.
* @param a first term of addition
* @param b second term of addition
* @param ans placeholder where to put the result
*/
private static void splitAdd(final double a[], final double b[], final double ans[]) {
ans[0] = a[0] + b[0];
ans[1] = a[1] + b[1];
resplit(ans);
}
/** Compute the reciprocal of in. Use the following algorithm.
* in = c + d.
* want to find x + y such that x+y = 1/(c+d) and x is much
* larger than y and x has several zero bits on the right.
*
* Set b = 1/(2^22), a = 1 - b. Thus (a+b) = 1.
* Use following identity to compute (a+b)/(c+d)
*
* (a+b)/(c+d) = a/c + (bc - ad) / (c^2 + cd)
* set x = a/c and y = (bc - ad) / (c^2 + cd)
* This will be close to the right answer, but there will be
* some rounding in the calculation of X. So by carefully
* computing 1 - (c+d)(x+y) we can compute an error and
* add that back in. This is done carefully so that terms
* of similar size are subtracted first.
* @param in initial number, in split form
* @param result placeholder where to put the result
*/
static void splitReciprocal(final double in[], final double result[]) {
final double b = 1.0/4194304.0;
final double a = 1.0 - b;
if (in[0] == 0.0) {
in[0] = in[1];
in[1] = 0.0;
}
result[0] = a / in[0];
result[1] = (b*in[0]-a*in[1]) / (in[0]*in[0] + in[0]*in[1]);
if (result[1] != result[1]) { // can happen if result[1] is NAN
result[1] = 0.0;
}
/* Resplit */
resplit(result);
for (int i = 0; i < 2; i++) {
/* this may be overkill, probably once is enough */
double err = 1.0 - result[0] * in[0] - result[0] * in[1] -
result[1] * in[0] - result[1] * in[1];
/*err = 1.0 - err; */
err = err * (result[0] + result[1]);
/*printf("err = %16e\n", err); */
result[1] += err;
}
}
/** Compute (a[0] + a[1]) * (b[0] + b[1]) in extended precision.
* @param a first term of the multiplication
* @param b second term of the multiplication
* @param result placeholder where to put the result
*/
private static void quadMult(final double a[], final double b[], final double result[]) {
final double xs[] = new double[2];
final double ys[] = new double[2];
final double zs[] = new double[2];
/* a[0] * b[0] */
split(a[0], xs);
split(b[0], ys);
splitMult(xs, ys, zs);
result[0] = zs[0];
result[1] = zs[1];
/* a[0] * b[1] */
split(b[1], ys);
splitMult(xs, ys, zs);
double tmp = result[0] + zs[0];
result[1] = result[1] - (tmp - result[0] - zs[0]);
result[0] = tmp;
tmp = result[0] + zs[1];
result[1] = result[1] - (tmp - result[0] - zs[1]);
result[0] = tmp;
/* a[1] * b[0] */
split(a[1], xs);
split(b[0], ys);
splitMult(xs, ys, zs);
tmp = result[0] + zs[0];
result[1] = result[1] - (tmp - result[0] - zs[0]);
result[0] = tmp;
tmp = result[0] + zs[1];
result[1] = result[1] - (tmp - result[0] - zs[1]);
result[0] = tmp;
/* a[1] * b[0] */
split(a[1], xs);
split(b[1], ys);
splitMult(xs, ys, zs);
tmp = result[0] + zs[0];
result[1] = result[1] - (tmp - result[0] - zs[0]);
result[0] = tmp;
tmp = result[0] + zs[1];
result[1] = result[1] - (tmp - result[0] - zs[1]);
result[0] = tmp;
}
/** Compute exp(p) for a integer p in extended precision.
* @param p integer whose exponential is requested
* @param result placeholder where to put the result in extended precision
* @return exp(p) in standard precision (equal to result[0] + result[1])
*/
static double expint(int p, final double result[]) {
//double x = M_E;
final double xs[] = new double[2];
final double as[] = new double[2];
final double ys[] = new double[2];
//split(x, xs);
//xs[1] = (double)(2.7182818284590452353602874713526625L - xs[0]);
//xs[0] = 2.71827697753906250000;
//xs[1] = 4.85091998273542816811e-06;
//xs[0] = Double.longBitsToDouble(0x4005bf0800000000L);
//xs[1] = Double.longBitsToDouble(0x3ed458a2bb4a9b00L);
/* E */
xs[0] = 2.718281828459045;
xs[1] = 1.4456468917292502E-16;
split(1.0, ys);
while (p > 0) {
if ((p & 1) != 0) {
quadMult(ys, xs, as);
ys[0] = as[0]; ys[1] = as[1];
}
quadMult(xs, xs, as);
xs[0] = as[0]; xs[1] = as[1];
p >>= 1;
}
if (result != null) {
result[0] = ys[0];
result[1] = ys[1];
resplit(result);
}
return ys[0] + ys[1];
}
}
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