/**
 * 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 org.apache.mahout.math.solver;

import org.apache.mahout.math.DenseVector;
import org.apache.mahout.math.Matrix;
import org.apache.mahout.math.Vector;

/**
 * Implements the Jacobi preconditioner for a matrix A. This is defined as inv(diag(A)).
 */
public final class JacobiConditioner implements Preconditioner {

  private final DenseVector inverseDiagonal;

  public JacobiConditioner(Matrix a) {
    if (a.numCols() != a.numRows()) {
      throw new IllegalArgumentException("Matrix must be square.");
    }
    
    inverseDiagonal = new DenseVector(a.numCols());
    for (int i = 0; i < a.numCols(); ++i) {
      inverseDiagonal.setQuick(i, 1.0 / a.getQuick(i, i));
    }
  }
  
  @Override
  public Vector precondition(Vector v) {
    return v.times(inverseDiagonal);
  }

}
