package org.djunits.vecmat.def;
   
   import java.util.Arrays;
   
   import org.djunits.quantity.SIQuantity;
   import org.djunits.quantity.def.Quantity;
   import org.djunits.unit.Unit;
   import org.djunits.util.ArrayMath;
   import org.djutils.exceptions.Throw;
  
  /**
   * SquareDenseMatrix implements the core functions for a matrix with n x n real-valued entries. The data is stored in a dense
   * double[] field. The matrix is immutable, except for the display unit, which can be changed.
   * <p>
   * Copyright (c) 2025-2026 Delft University of Technology, Jaffalaan 5, 2628 BX Delft, the Netherlands. All rights reserved. See
   * for project information <a href="https://djunits.org" target="_blank">https://djunits.org</a>. The DJUNITS project is
   * distributed under a <a href="https://djunits.org/docs/license.html" target="_blank">three-clause BSD-style license</a>.
   * @author Alexander Verbraeck
   * @param <Q> the quantity type
   * @param <M> the 'SELF' square dense matrix type
   * @param <SI> the square dense matrix type with generics &lt;SIQuantity, SIUnit&lt;
   * @param <H> the generic square dense matrix type with generics &lt;?, ?&lt; for Hadamard operations
   */
  public abstract class SquareDenseMatrix<Q extends Quantity<Q>, M extends SquareDenseMatrix<Q, M, SI, H>,
          SI extends SquareDenseMatrix<SIQuantity, SI, ?, ?>, H extends SquareDenseMatrix<?, ?, ?, ?>>
          extends SquareMatrix<Q, M, SI, H>
  {
      /** */
      private static final long serialVersionUID = 600L;
  
      /** The n x n values in SI-units as a row-major array. */
      private final double[] dataSi;
  
      /** the order (n in n x n) of the matrix. */
      private final int order;
  
      /**
       * Create a new SquareDenseMatrix with a unit. NO SAFE COPY of the double[] data element is made.
       * @param dataSi the matrix values {a11, a12, ..., a21, a22, ...} expressed in the SI unit
       * @param displayUnit the display unit to use
       * @param order the order of the square matrix (number of rows/columns)
       * @throws IllegalArgumentException when dataSi.length != order * order.
       */
      protected SquareDenseMatrix(final double[] dataSi, final Unit<?, Q> displayUnit, final int order)
      {
          super(displayUnit);
          Throw.when(dataSi.length != order * order, IllegalArgumentException.class,
                  "SquareDenseMatrix initialized with %d values instead of %d", dataSi.length, order * order);
          this.dataSi = dataSi;
          this.order = order;
      }
  
      @Override
      public double[] unsafeSiArray()
      {
          return this.dataSi;
      }
  
      @Override
      public double[] getSiArray()
      {
          return this.dataSi.clone();
      }
  
      @Override
      public double si(final int row, final int col)
      {
          checkRow(row);
          checkCol(col);
          return this.dataSi[this.order * row + col];
      }
  
      @Override
      public int rows()
      {
          return this.order;
      }
  
      @Override
      public int cols()
      {
          return this.order;
      }
  
      @Override
      public int nonZeroCount()
      {
          return ArrayMath.nnz(this.dataSi);
      }
  
      @Override
      public double[] getRowSi(final int row)
      {
          checkRow(row);
          double[] vSi = new double[this.order];
          for (int col = 0; col < this.order; col++)
          {
              vSi[col] = this.dataSi[this.order * row + col];
          }
         return vSi;
     }
 
     @Override
     public double[] getColumnSi(final int col)
     {
         checkCol(col);
         double[] vSi = new double[this.order];
         for (int row = 0; row < this.order; row++)
         {
             vSi[row] = this.dataSi[this.order * row + col];
         }
         return vSi;
     }
 
     @Override
     public int hashCode()
     {
         final int prime = 31;
         int result = 1;
         result = prime * result + Arrays.hashCode(this.dataSi);
         return result;
     }
 
     @SuppressWarnings("checkstyle:needbraces")
     @Override
     public boolean equals(final Object obj)
     {
         if (this == obj)
             return true;
         if (obj == null)
             return false;
         if (getClass() != obj.getClass())
             return false;
         SquareDenseMatrix<?, ?, ?, ?> other = (SquareDenseMatrix<?, ?, ?, ?>) obj;
         return Arrays.equals(this.dataSi, other.dataSi);
     }
 
 }