/*
* Copyright 2011-2013, by Vladimir Kostyukov and Contributors.
*
* This file is part of la4j project (http://la4j.org)
*
* 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.
*
* Contributor(s): -
*
*/
package org.la4j.linear;
import org.la4j.Matrices;
import org.la4j.Matrix;
import org.la4j.Vector;
import org.la4j.Vectors;
/**
* This class represents <a
* href="http://en.wikipedia.org/wiki/Tridiagonal_matrix_algorithm"> Sweep (or
* Tridiagonal matrix, or Thomas) method </a> for solving linear systems.
*/
public class SweepSolver extends AbstractSolver implements LinearSystemSolver {
private static final long serialVersionUID = 4071505L;
public SweepSolver(Matrix a) {
super(a);
}
@Override
public Vector solve(Vector b) {
ensureRHSIsCorrect(b);
// We need a copy, since the algorithm changes data
Matrix aa = a.copy();
Vector bb = b.copy();
Vector x = b.blankOfLength(aa.columns());
for (int i = 0; i < aa.rows() - 1; i++) {
double maxItem = Math.abs(aa.get(i, i));
int maxIndex = i;
for (int j = i + 1; j < aa.columns(); j++) {
double value = Math.abs(aa.get(j, i));
if (value > maxItem) {
maxItem = value;
maxIndex = j;
}
}
if (maxIndex != i) {
aa.swapRows(maxIndex, i);
bb.swapElements(i, maxIndex);
}
for (int j = i + 1; j < aa.columns(); j++) {
double c = aa.get(j, i) / aa.get(i, i);
for (int k = i; k < aa.columns(); k++) {
aa.updateAt(j, k, Matrices.asMinusFunction(aa.get(i, k) * c));
}
bb.updateAt(j, Vectors.asMinusFunction(bb.get(i) * c));
}
}
for (int i = aa.rows() - 1; i >= 0; i--) {
double acc = 0.0;
for (int j = i + 1; j < aa.columns(); j++) {
acc += aa.get(i, j) * x.get(j);
}
x.set(i, (bb.get(i) - acc) / aa.get(i, i));
}
return x;
}
@Override
public boolean applicableTo(Matrix matrix) {
return matrix.is(Matrices.TRIDIAGONAL_MATRIX);
}
}