/*
    * 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
    *
    *      https://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.
   */
  
  /*
   * This is not the original file distributed by the Apache Software Foundation
   * It has been modified by the Hipparchus project
   */
  
  package org.hipparchus.linear;
  
  import java.io.Serializable;
  import org.hipparchus.exception.LocalizedCoreFormats;
  import org.hipparchus.exception.MathIllegalArgumentException;
  import org.hipparchus.exception.MathIllegalStateException;
  import org.hipparchus.exception.NullArgumentException;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.MathUtils;
  
  /**
   * Implementation of {@link RealMatrix} using a {@code double[][]} array to
   * store entries.
   *
   */
  public class Array2DRowRealMatrix extends AbstractRealMatrix implements Serializable {
      /** Serializable version identifier. */
      private static final long serialVersionUID = -1067294169172445528L;
  
      /** Entries of the matrix. */
      private double[][] data;
  
      /**
       * Creates a matrix with no data
       */
      public Array2DRowRealMatrix() {
          // This constructor is intentionally empty. Nothing special is needed here.
      }
  
      /**
       * Create a new RealMatrix with the supplied row and column dimensions.
       *
       * @param rowDimension Number of rows in the new matrix.
       * @param columnDimension Number of columns in the new matrix.
       * @throws MathIllegalArgumentException if the row or column dimension is
       * not positive.
       */
      public Array2DRowRealMatrix(final int rowDimension,
                                  final int columnDimension)
          throws MathIllegalArgumentException {
          super(rowDimension, columnDimension);
          data = new double[rowDimension][columnDimension];
      }
  
      /**
       * Create a new {@code RealMatrix} using the input array as the underlying
       * data array.
       * <p>The input array is copied, not referenced. This constructor has
       * the same effect as calling {@link #Array2DRowRealMatrix(double[][], boolean)}
       * with the second argument set to {@code true}.</p>
       *
       * @param d Data for the new matrix.
       * @throws MathIllegalArgumentException if {@code d} is not rectangular.
       * @throws MathIllegalArgumentException if {@code d} row or column dimension is zero.
       * @throws NullArgumentException if {@code d} is {@code null}.
       * @see #Array2DRowRealMatrix(double[][], boolean)
       */
      public Array2DRowRealMatrix(final double[][] d)
          throws MathIllegalArgumentException, NullArgumentException {
          copyIn(d);
      }
  
      /**
       * Create a new RealMatrix using the input array as the underlying
       * data array.
       * If an array is built specially in order to be embedded in a
       * RealMatrix and not used directly, the {@code copyArray} may be
       * set to {@code false}. This will prevent the copying and improve
       * performance as no new array will be built and no data will be copied.
       *
       * @param d Data for new matrix.
       * @param copyArray if {@code true}, the input array will be copied,
       * otherwise it will be referenced.
       * @throws MathIllegalArgumentException if {@code d} is not rectangular.
       * @throws MathIllegalArgumentException if {@code d} row or column dimension is zero.
       * @throws NullArgumentException if {@code d} is {@code null}.
       * @see #Array2DRowRealMatrix(double[][])
      */
     public Array2DRowRealMatrix(final double[][] d, final boolean copyArray)
         throws MathIllegalArgumentException,
         NullArgumentException {
         if (copyArray) {
             copyIn(d);
         } else {
             if (d == null) {
                 throw new NullArgumentException();
             }
             final int nRows = d.length;
             if (nRows == 0) {
                 throw new MathIllegalArgumentException(LocalizedCoreFormats.AT_LEAST_ONE_ROW);
             }
             final int nCols = d[0].length;
             if (nCols == 0) {
                 throw new MathIllegalArgumentException(LocalizedCoreFormats.AT_LEAST_ONE_COLUMN);
             }
             for (int r = 1; r < nRows; r++) {
                 if (d[r].length != nCols) {
                     throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH,
                                                            d[r].length, nCols);
                 }
             }
             data = d; // NOPMD - array copy is taken care of by parameter
         }
     }
 
     /**
      * Create a new (column) RealMatrix using {@code v} as the
      * data for the unique column of the created matrix.
      * The input array is copied.
      *
      * @param v Column vector holding data for new matrix.
      */
     public Array2DRowRealMatrix(final double[] v) {
         final int nRows = v.length;
         data = new double[nRows][1];
         for (int row = 0; row < nRows; row++) {
             data[row][0] = v[row];
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public RealMatrix createMatrix(final int rowDimension,
                                    final int columnDimension)
         throws MathIllegalArgumentException {
         return new Array2DRowRealMatrix(rowDimension, columnDimension);
     }
 
     /** {@inheritDoc} */
     @Override
     public RealMatrix copy() {
         return new Array2DRowRealMatrix(copyOut(), false);
     }
 
     /**
      * Compute the sum of {@code this} and {@code m}.
      *
      * @param m Matrix to be added.
      * @return {@code this + m}.
      * @throws MathIllegalArgumentException if {@code m} is not the same
      * size as {@code this}.
      */
     public Array2DRowRealMatrix add(final Array2DRowRealMatrix m)
         throws MathIllegalArgumentException {
         // Safety check.
         MatrixUtils.checkAdditionCompatible(this, m);
 
         final int rowCount    = getRowDimension();
         final int columnCount = getColumnDimension();
         final double[][] outData = new double[rowCount][columnCount];
         for (int row = 0; row < rowCount; row++) {
             final double[] dataRow    = data[row];
             final double[] mRow       = m.data[row];
             final double[] outDataRow = outData[row];
             for (int col = 0; col < columnCount; col++) {
                 outDataRow[col] = dataRow[col] + mRow[col];
             }
         }
 
         return new Array2DRowRealMatrix(outData, false);
     }
 
     /**
      * Returns {@code this} minus {@code m}.
      *
      * @param m Matrix to be subtracted.
      * @return {@code this - m}
      * @throws MathIllegalArgumentException if {@code m} is not the same
      * size as {@code this}.
      */
     public Array2DRowRealMatrix subtract(final Array2DRowRealMatrix m)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubtractionCompatible(this, m);
 
         final int rowCount    = getRowDimension();
         final int columnCount = getColumnDimension();
         final double[][] outData = new double[rowCount][columnCount];
         for (int row = 0; row < rowCount; row++) {
             final double[] dataRow    = data[row];
             final double[] mRow       = m.data[row];
             final double[] outDataRow = outData[row];
             for (int col = 0; col < columnCount; col++) {
                 outDataRow[col] = dataRow[col] - mRow[col];
             }
         }
 
         return new Array2DRowRealMatrix(outData, false);
     }
 
     /**
      * Returns the result of postmultiplying {@code this} by {@code m}.
      *
      * @param m matrix to postmultiply by
      * @return {@code this * m}
      * @throws MathIllegalArgumentException if
      * {@code columnDimension(this) != rowDimension(m)}
      */
     public Array2DRowRealMatrix multiply(final Array2DRowRealMatrix m)
         throws MathIllegalArgumentException {
         MatrixUtils.checkMultiplicationCompatible(this, m);
 
         final int nRows = this.getRowDimension();
         final int nCols = m.getColumnDimension();
         final int nSum = this.getColumnDimension();
 
         final double[][] outData = new double[nRows][nCols];
         // Will hold a column of "m".
         final double[] mCol = new double[nSum];
         final double[][] mData = m.data;
 
         // Multiply.
         for (int col = 0; col < nCols; col++) {
             // Copy all elements of column "col" of "m" so that
             // will be in contiguous memory.
             for (int mRow = 0; mRow < nSum; mRow++) {
                 mCol[mRow] = mData[mRow][col];
             }
 
             for (int row = 0; row < nRows; row++) {
                 final double[] dataRow = data[row];
                 double sum = 0;
                 for (int i = 0; i < nSum; i++) {
                     sum += dataRow[i] * mCol[i];
                 }
                 outData[row][col] = sum;
             }
         }
 
         return new Array2DRowRealMatrix(outData, false);
     }
 
     /**
      * Returns the result of postmultiplying {@code this} by {@code m^T}.
      * @param m matrix to first transpose and second postmultiply by
      * @return {@code this * m^T}
      * @throws MathIllegalArgumentException if
      * {@code columnDimension(this) != columnDimension(m)}
      * @since 1.3
      */
     public RealMatrix multiplyTransposed(final Array2DRowRealMatrix m)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSameColumnDimension(this, m);
 
         final int nRows = this.getRowDimension();
         final int nCols = m.getRowDimension();
         final int nSum  = this.getColumnDimension();
 
         final RealMatrix out = MatrixUtils.createRealMatrix(nRows, nCols);
         final double[][] mData   = m.data;
 
         // Multiply.
         for (int col = 0; col < nCols; col++) {
             for (int row = 0; row < nRows; row++) {
                 final double[] dataRow = data[row];
                 final double[] mRow    = mData[col];
                 double sum = 0;
                 for (int i = 0; i < nSum; i++) {
                     sum += dataRow[i] * mRow[i];
                 }
                 out.setEntry(row, col, sum);
             }
         }
 
         return out;
 
     }
 
     /** {@inheritDoc} */
     @Override
     public RealMatrix multiplyTransposed(final RealMatrix m) {
         if (m instanceof Array2DRowRealMatrix) {
             return multiplyTransposed((Array2DRowRealMatrix) m);
         } else {
             MatrixUtils.checkSameColumnDimension(this, m);
 
             final int nRows = this.getRowDimension();
             final int nCols = m.getRowDimension();
             final int nSum  = this.getColumnDimension();
 
             final RealMatrix out = MatrixUtils.createRealMatrix(nRows, nCols);
 
             // Multiply.
             for (int col = 0; col < nCols; col++) {
                 for (int row = 0; row < nRows; row++) {
                     final double[] dataRow = data[row];
                     double sum = 0;
                     for (int i = 0; i < nSum; i++) {
                         sum += dataRow[i] * m.getEntry(col, i);
                     }
                     out.setEntry(row, col, sum);
                 }
             }
 
             return out;
 
         }
     }
 
     /**
      * Returns the result of postmultiplying {@code this^T} by {@code m}.
      * @param m matrix to postmultiply by
      * @return {@code this^T * m}
      * @throws MathIllegalArgumentException if
      * {@code columnDimension(this) != columnDimension(m)}
      * @since 1.3
      */
     public RealMatrix transposeMultiply(final Array2DRowRealMatrix m)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSameRowDimension(this, m);
 
         final int nRows = this.getColumnDimension();
         final int nCols = m.getColumnDimension();
         final int nSum  = this.getRowDimension();
 
         final RealMatrix out = MatrixUtils.createRealMatrix(nRows, nCols);
         final double[][] mData   = m.data;
 
         // Multiply.
         for (int k = 0; k < nSum; k++) {
             final double[] dataK = data[k];
             final double[] mK    = mData[k];
             for (int row = 0; row < nRows; row++) {
                 final double dataIRow = dataK[row];
                 for (int col = 0; col < nCols; col++) {
                     out.addToEntry(row, col, dataIRow * mK[col]);
                 }
             }
         }
 
         return out;
 
     }
 
     /** {@inheritDoc} */
     @Override
     public RealMatrix transposeMultiply(final RealMatrix m) {
         if (m instanceof Array2DRowRealMatrix) {
             return transposeMultiply((Array2DRowRealMatrix) m);
         } else {
             MatrixUtils.checkSameRowDimension(this, m);
 
             final int nRows = this.getColumnDimension();
             final int nCols = m.getColumnDimension();
             final int nSum  = this.getRowDimension();
 
             final RealMatrix out = MatrixUtils.createRealMatrix(nRows, nCols);
 
             // Multiply.
             for (int k = 0; k < nSum; k++) {
                 final double[] dataK = data[k];
                 for (int row = 0; row < nRows; row++) {
                     final double dataIRow = dataK[row];
                     for (int col = 0; col < nCols; col++) {
                         out.addToEntry(row, col, dataIRow * m.getEntry(k, col));
                     }
                 }
             }
 
             return out;
 
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public double[][] getData() {
         return copyOut();
     }
 
     /**
      * Get a reference to the underlying data array.
      *
      * @return 2-dimensional array of entries.
      */
     public double[][] getDataRef() {
         return data; // NOPMD - returning an internal array is intentional and documented here
     }
 
     /** {@inheritDoc} */
     @Override
     public void setSubMatrix(final double[][] subMatrix, final int row,
                              final int column)
         throws MathIllegalArgumentException, NullArgumentException {
         if (data == null) {
             if (row > 0) {
                 throw new MathIllegalStateException(LocalizedCoreFormats.FIRST_ROWS_NOT_INITIALIZED_YET, row);
             }
             if (column > 0) {
                 throw new MathIllegalStateException(LocalizedCoreFormats.FIRST_COLUMNS_NOT_INITIALIZED_YET, column);
             }
             MathUtils.checkNotNull(subMatrix);
             final int nRows = subMatrix.length;
             if (nRows == 0) {
                 throw new MathIllegalArgumentException(LocalizedCoreFormats.AT_LEAST_ONE_ROW);
             }
 
             final int nCols = subMatrix[0].length;
             if (nCols == 0) {
                 throw new MathIllegalArgumentException(LocalizedCoreFormats.AT_LEAST_ONE_COLUMN);
             }
             data = new double[subMatrix.length][nCols];
             for (int i = 0; i < data.length; ++i) {
                 if (subMatrix[i].length != nCols) {
                     throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH,
                                                            subMatrix[i].length, nCols);
                 }
                 System.arraycopy(subMatrix[i], 0, data[i + row], column, nCols);
             }
         } else {
             super.setSubMatrix(subMatrix, row, column);
         }
 
     }
 
     /** {@inheritDoc} */
     @Override
     public double getEntry(final int row, final int column)
         throws MathIllegalArgumentException {
       try {
         return data[row][column];
       } catch (IndexOutOfBoundsException e) {
         // throw the exact cause of the exception
         MatrixUtils.checkMatrixIndex(this, row, column);
         // should never happen
         throw e;
       }
     }
 
     /** {@inheritDoc} */
     @Override
     public void setEntry(final int row, final int column, final double value)
         throws MathIllegalArgumentException {
         MatrixUtils.checkMatrixIndex(this, row, column);
         data[row][column] = value;
     }
 
     /** {@inheritDoc} */
     @Override
     public void addToEntry(final int row, final int column,
                            final double increment)
         throws MathIllegalArgumentException {
         MatrixUtils.checkMatrixIndex(this, row, column);
         data[row][column] += increment;
     }
 
     /** {@inheritDoc} */
     @Override
     public void multiplyEntry(final int row, final int column,
                               final double factor)
         throws MathIllegalArgumentException {
         MatrixUtils.checkMatrixIndex(this, row, column);
         data[row][column] *= factor;
     }
 
     /** {@inheritDoc} */
     @Override
     public int getRowDimension() {
         return (data == null) ? 0 : data.length;
     }
 
     /** {@inheritDoc} */
     @Override
     public int getColumnDimension() {
         return ((data == null) || (data[0] == null)) ? 0 : data[0].length;
     }
 
     /** {@inheritDoc} */
     @Override
     public double[] operate(final double[] v)
         throws MathIllegalArgumentException {
         final int nRows = this.getRowDimension();
         final int nCols = this.getColumnDimension();
         if (v.length != nCols) {
             throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH,
                                                    v.length, nCols);
         }
         final double[] out = new double[nRows];
         for (int row = 0; row < nRows; row++) {
             final double[] dataRow = data[row];
             double sum = 0;
             for (int i = 0; i < nCols; i++) {
                 sum += dataRow[i] * v[i];
             }
             out[row] = sum;
         }
         return out;
     }
 
     /** {@inheritDoc} */
     @Override
     public double[] preMultiply(final double[] v)
         throws MathIllegalArgumentException {
         final int nRows = getRowDimension();
         final int nCols = getColumnDimension();
         if (v.length != nRows) {
             throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH,
                                                    v.length, nRows);
         }
 
         final double[] out = new double[nCols];
         for (int col = 0; col < nCols; ++col) {
             double sum = 0;
             for (int i = 0; i < nRows; ++i) {
                 sum += data[i][col] * v[i];
             }
             out[col] = sum;
         }
 
         return out;
 
     }
 
     /** {@inheritDoc} */
     @Override
     public RealMatrix getSubMatrix(final int startRow, final int endRow,
                                    final int startColumn, final int endColumn)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubMatrixIndex(this, startRow, endRow, startColumn, endColumn);
         final int rowCount = endRow - startRow + 1;
         final int columnCount = endColumn - startColumn + 1;
         final double[][] outData = new double[rowCount][columnCount];
         for (int i = 0; i < rowCount; ++i) {
             System.arraycopy(data[startRow + i], startColumn, outData[i], 0, columnCount);
         }
 
         Array2DRowRealMatrix subMatrix = new Array2DRowRealMatrix();
         subMatrix.data = outData;
         return subMatrix;
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInRowOrder(final RealMatrixChangingVisitor visitor) {
         final int rows    = getRowDimension();
         final int columns = getColumnDimension();
         visitor.start(rows, columns, 0, rows - 1, 0, columns - 1);
         for (int i = 0; i < rows; ++i) {
             final double[] rowI = data[i];
             for (int j = 0; j < columns; ++j) {
                 rowI[j] = visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInRowOrder(final RealMatrixPreservingVisitor visitor) {
         final int rows    = getRowDimension();
         final int columns = getColumnDimension();
         visitor.start(rows, columns, 0, rows - 1, 0, columns - 1);
         for (int i = 0; i < rows; ++i) {
             final double[] rowI = data[i];
             for (int j = 0; j < columns; ++j) {
                 visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInRowOrder(final RealMatrixChangingVisitor visitor,
                                  final int startRow, final int endRow,
                                  final int startColumn, final int endColumn)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubMatrixIndex(this, startRow, endRow, startColumn, endColumn);
         visitor.start(getRowDimension(), getColumnDimension(),
                       startRow, endRow, startColumn, endColumn);
         for (int i = startRow; i <= endRow; ++i) {
             final double[] rowI = data[i];
             for (int j = startColumn; j <= endColumn; ++j) {
                 rowI[j] = visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInRowOrder(final RealMatrixPreservingVisitor visitor,
                                  final int startRow, final int endRow,
                                  final int startColumn, final int endColumn)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubMatrixIndex(this, startRow, endRow, startColumn, endColumn);
         visitor.start(getRowDimension(), getColumnDimension(),
                       startRow, endRow, startColumn, endColumn);
         for (int i = startRow; i <= endRow; ++i) {
             final double[] rowI = data[i];
             for (int j = startColumn; j <= endColumn; ++j) {
                 visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInColumnOrder(final RealMatrixChangingVisitor visitor) {
         final int rows    = getRowDimension();
         final int columns = getColumnDimension();
         visitor.start(rows, columns, 0, rows - 1, 0, columns - 1);
         for (int j = 0; j < columns; ++j) {
             for (int i = 0; i < rows; ++i) {
                 final double[] rowI = data[i];
                 rowI[j] = visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInColumnOrder(final RealMatrixPreservingVisitor visitor) {
         final int rows    = getRowDimension();
         final int columns = getColumnDimension();
         visitor.start(rows, columns, 0, rows - 1, 0, columns - 1);
         for (int j = 0; j < columns; ++j) {
             for (int i = 0; i < rows; ++i) {
                 visitor.visit(i, j, data[i][j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInColumnOrder(final RealMatrixChangingVisitor visitor,
                                     final int startRow, final int endRow,
                                     final int startColumn, final int endColumn)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubMatrixIndex(this, startRow, endRow, startColumn, endColumn);
         visitor.start(getRowDimension(), getColumnDimension(),
                       startRow, endRow, startColumn, endColumn);
         for (int j = startColumn; j <= endColumn; ++j) {
             for (int i = startRow; i <= endRow; ++i) {
                 final double[] rowI = data[i];
                 rowI[j] = visitor.visit(i, j, rowI[j]);
             }
         }
         return visitor.end();
     }
 
     /** {@inheritDoc} */
     @Override
     public double walkInColumnOrder(final RealMatrixPreservingVisitor visitor,
                                     final int startRow, final int endRow,
                                     final int startColumn, final int endColumn)
         throws MathIllegalArgumentException {
         MatrixUtils.checkSubMatrixIndex(this, startRow, endRow, startColumn, endColumn);
         visitor.start(getRowDimension(), getColumnDimension(),
                       startRow, endRow, startColumn, endColumn);
         for (int j = startColumn; j <= endColumn; ++j) {
             for (int i = startRow; i <= endRow; ++i) {
                 visitor.visit(i, j, data[i][j]);
             }
         }
         return visitor.end();
     }
 
     /**
      * Get a fresh copy of the underlying data array.
      *
      * @return a copy of the underlying data array.
      */
     private double[][] copyOut() {
         final int nRows = this.getRowDimension();
         final double[][] out = new double[nRows][this.getColumnDimension()];
         // can't copy 2-d array in one shot, otherwise get row references
         for (int i = 0; i < nRows; i++) {
             System.arraycopy(data[i], 0, out[i], 0, data[i].length);
         }
         return out;
     }
 
     /**
      * Replace data with a fresh copy of the input array.
      *
      * @param in Data to copy.
      * @throws MathIllegalArgumentException if the input array is empty.
      * @throws MathIllegalArgumentException if the input array is not rectangular.
      * @throws NullArgumentException if the input array is {@code null}.
      */
     private void copyIn(final double[][] in)
         throws MathIllegalArgumentException, NullArgumentException {
         setSubMatrix(in, 0, 0);
     }
 
     /** {@inheritDoc} */
     @Override
     public double[] getRow(final int row) throws MathIllegalArgumentException {
         MatrixUtils.checkRowIndex(this, row);
         final int nCols = getColumnDimension();
         final double[] out = new double[nCols];
         System.arraycopy(data[row], 0, out, 0, nCols);
         return out;
     }
 
     /** {@inheritDoc} */
     @Override
     public void setRow(final int row, final double[] array)
         throws MathIllegalArgumentException {
         MatrixUtils.checkRowIndex(this, row);
         final int nCols = getColumnDimension();
         if (array.length != nCols) {
             throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH_2x2,
                                                    1, array.length, 1, nCols);
         }
         System.arraycopy(array, 0, data[row], 0, nCols);
     }
 
     /**
      * Kronecker product of the current matrix and the parameter matrix.
      *
      * @param b matrix to post Kronecker-multiply by
      * @return this ⨂ b
      */
     public RealMatrix kroneckerProduct(final RealMatrix b) {
         final int m = getRowDimension();
         final int n = getColumnDimension();
 
         final int p = b.getRowDimension();
         final int q = b.getColumnDimension();
 
         final RealMatrix kroneckerProduct = MatrixUtils.createRealMatrix(m * p, n * q);
 
         for (int i = 0; i < m; i++) {
             for (int j = 0; j < n; j++) {
                 kroneckerProduct.setSubMatrix(b.scalarMultiply(getEntry(i, j)) .getData(), i * p, j * q);
             }
         }
 
         return kroneckerProduct;
     }
 
     /**
      * Transforms a matrix in a vector (Vectorization).
      * @return a one column matrix
      */
     public RealMatrix stack() {
         final int m = getRowDimension();
         final int n = getColumnDimension();
 
         final RealMatrix stacked = MatrixUtils.createRealMatrix(m * n, 1);
 
         for (int i = 0; i < m; i++) {
             stacked.setSubMatrix(getColumnMatrix(i).getData(), i * n, 0);
         }
 
         return stacked;
     }
 
     /**
      * Transforms a one-column stacked matrix into a squared matrix (devectorization).
      * @return square matrix
      */
     public RealMatrix unstackSquare() {
         final int m = getRowDimension();
         final int n = getColumnDimension();
         final int s = (int) FastMath.round(FastMath.sqrt(m));
 
         if (n != 1) {
             throw new MathIllegalArgumentException(LocalizedCoreFormats.DIMENSIONS_MISMATCH, n, 1);
         }
         if (s * s != m) {
             throw new MathIllegalArgumentException(LocalizedCoreFormats.NON_SQUARE_MATRIX, s, ((double) m) / s);
         }
 
         final RealMatrix unstacked = MatrixUtils.createRealMatrix(s, s);
 
         for (int i = 0; i < s; i++) {
             unstacked.setColumnMatrix(i, getSubMatrix(i * s, i * s + s - 1, 0, 0));
         }
 
         return unstacked;
     }
 
 }