/*
    * Licensed to the Hipparchus project under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The Hipparchus project 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.
   */
  package org.hipparchus.analysis.interpolation;
  
  import java.io.Serializable;
  
  import org.hipparchus.CalculusFieldElement;
  import org.hipparchus.analysis.BivariateFunction;
  import org.hipparchus.analysis.FieldBivariateFunction;
  import org.hipparchus.exception.MathIllegalArgumentException;
  
  /**
   * Interpolate grid data using bi-linear interpolation.
   * <p>
   * This interpolator is thread-safe.
   * </p>
   * @since 1.4
   */
  public class BilinearInterpolatingFunction implements BivariateFunction, FieldBivariateFunction, Serializable {
  
      /** Serializable UID. */
      private static final long serialVersionUID = 20180926L;
  
      /** Grid along the x axis. */
      private final GridAxis xGrid;
  
      /** Grid along the y axis. */
      private final GridAxis yGrid;
  
      /** Grid size along the y axis. */
      private final int ySize;
  
      /** Values of the interpolation points on all the grid knots (in a flatten array). */
      private final double[] fVal;
  
      /** Simple constructor.
       * @param xVal All the x-coordinates of the interpolation points, sorted
       * in increasing order.
       * @param yVal All the y-coordinates of the interpolation points, sorted
       * in increasing order.
       * @param fVal The values of the interpolation points on all the grid knots:
       * {@code fVal[i][j] = f(xVal[i], yVal[j])}.
       * @exception MathIllegalArgumentException if grid size is smaller than 2
       * or if the grid is not sorted in strict increasing order
       */
      public BilinearInterpolatingFunction(final double[] xVal, final double[] yVal,
                                           final double[][] fVal)
          throws MathIllegalArgumentException {
          this.xGrid = new GridAxis(xVal, 2);
          this.yGrid = new GridAxis(yVal, 2);
          this.ySize = yVal.length;
          this.fVal  = new double[xVal.length * ySize];
          int k = 0;
          for (int i = 0; i < xVal.length; ++i) {
              final double[] fi = fVal[i];
              for (int j = 0; j < ySize; ++j) {
                  this.fVal[k++] = fi[j];
              }
          }
      }
  
      /** Get the lowest grid x coordinate.
       * @return lowest grid x coordinate
       */
      public double getXInf() {
          return xGrid.node(0);
      }
  
      /** Get the highest grid x coordinate.
       * @return highest grid x coordinate
       */
      public double getXSup() {
          return xGrid.node(xGrid.size() - 1);
      }
  
      /** Get the lowest grid y coordinate.
       * @return lowest grid y coordinate
       */
      public double getYInf() {
          return yGrid.node(0);
      }
  
      /** Get the highest grid y coordinate.
       * @return highest grid y coordinate
       */
     public double getYSup() {
         return yGrid.node(yGrid.size() - 1);
     }
 
     /** {@inheritDoc} */
     @Override
     public double value(final double x, final double y) {
 
         // get the interpolation nodes
         final int    i   = xGrid.interpolationIndex(x);
         final int    j   = yGrid.interpolationIndex(y);
         final double x0  = xGrid.node(i);
         final double x1  = xGrid.node(i + 1);
         final double y0  = yGrid.node(j);
         final double y1  = yGrid.node(j + 1);
 
         // get the function values at interpolation nodes
         final int    k0  = i * ySize + j;
         final int    k1  = k0 + ySize;
         final double z00 = fVal[k0];
         final double z01 = fVal[k0 + 1];
         final double z10 = fVal[k1];
         final double z11 = fVal[k1 + 1];
 
         // interpolate
         final double dx0  = x  - x0;
         final double dx1  = x1 - x;
         final double dx10 = x1 - x0;
         final double dy0  = y  - y0;
         final double dy1  = y1 - y;
         final double dy10 = y1 - y0;
         return (dx0 * (dy0 * z11 + dy1 * z10) + dx1 * (dy0 * z01 + dy1 * z00)) /
                (dx10 * dy10);
 
     }
 
     /** {@inheritDoc}
      * @since 1.5
      */
     @Override
     public <T extends CalculusFieldElement<T>> T value(T x, T y) {
 
         // get the interpolation nodes
         final int    i   = xGrid.interpolationIndex(x.getReal());
         final int    j   = yGrid.interpolationIndex(y.getReal());
         final double x0  = xGrid.node(i);
         final double x1  = xGrid.node(i + 1);
         final double y0  = yGrid.node(j);
         final double y1  = yGrid.node(j + 1);
 
         // get the function values at interpolation nodes
         final int    k0  = i * ySize + j;
         final int    k1  = k0 + ySize;
         final double z00 = fVal[k0];
         final double z01 = fVal[k0 + 1];
         final double z10 = fVal[k1];
         final double z11 = fVal[k1 + 1];
 
         // interpolate
         final T      dx0   = x.subtract(x0);
         final T      mdx1  = x.subtract(x1);
         final double dx10  = x1 - x0;
         final T      dy0   = y.subtract(y0);
         final T      mdy1  = y.subtract(y1);
         final double dy10  = y1 - y0;
         return          dy0.multiply(z11).subtract(mdy1.multiply(z10)).multiply(dx0).
                subtract(dy0.multiply(z01).subtract(mdy1.multiply(z00)).multiply(mdx1)).
                divide(dx10 * dy10);
 
     }
 
 }