package commonAlgorithm;
public class PolynomialSoluter {
  private double[][] matrix;
  private double[] result;
  private int order;
  public PolynomialSoluter() {
  }
  // 檢查輸入項長度并生成增廣矩陣
  private boolean init(double[][] matrixA, double[] arrayB) {
    order = arrayB.length;
    if (matrixA.length != order)
      return false;
    matrix = new double[order][order + 1];
    for (int i = 0; i < order; i++) {
      if (matrixA[i].length != order)
        return false;
      for (int j = 0; j < order; j++) {
        matrix[i][j] = matrixA[i][j];
      }
      matrix[i][order] = arrayB[i];
    }
    result = new double[order];
    return true;
  }
  public double[] getResult(double[][] matrixA, double[] arrayB) {
    if (!init(matrixA, arrayB))
      return null;
    // 高斯消元-正向
    for (int i = 0; i < order; i++) {
      // 如果當(dāng)前行對角線項為0則與后面的同列項非0的行交換
      if (!swithIfZero(i))
        return null;
      // 消元
      for (int j = i + 1; j < order; j++) {
        if (matrix[j][i] == 0)
          continue;
        double factor = matrix[j][i] / matrix[i][i];
        for (int l = i; l < order + 1; l++)
          matrix[j][l] = matrix[j][l] - matrix[i][l] * factor;
      }
    }
    // 高斯消元-反向-去掉了冗余計算
    for (int i = order - 1; i >= 0; i--) {
      result[i] = matrix[i][order] / matrix[i][i];
      for (int j = i - 1; j > -1; j--)
        matrix[j][order] = matrix[j][order] - result[i] * matrix[j][i];
    }
    return result;
  }
  private boolean swithIfZero(int i) {
    if (matrix[i][i] == 0) {
      int j = i + 1;
      // 找到對應(yīng)位置非0的列
      while (j < order && matrix[j][i] == 0)
        j++;
      // 若對應(yīng)位置全為0則無解
      if (j == order)
        return false;
      else
        switchRows(i, j);
    }
    return true;
  }
  private void switchRows(int i, int j) {
    double[] tmp = matrix[i];
    matrix[i] = matrix[j];
    matrix[j] = tmp;
  }
}