solve method Null safety

Implementation

PolynomialFit? solve(int degree) {
  if (degree > x.length) {
    // Not enough data to fit a curve.
    return null;
  }

  final PolynomialFit result = PolynomialFit(degree);

  // Shorthands for the purpose of notation equivalence to original C++ code.
  final int m = x.length;
  final int n = degree + 1;

  // Expand the X vector to a matrix A, pre-multiplied by the weights.
  final _Matrix a = _Matrix(n, m);
  for (int h = 0; h < m; h += 1) {
    a.set(0, h, w[h]);
    for (int i = 1; i < n; i += 1) {
      a.set(i, h, a.get(i - 1, h) * x[h]);
    }
  }

  // Apply the Gram-Schmidt process to A to obtain its QR decomposition.

  // Orthonormal basis, column-major ordVectorer.
  final _Matrix q = _Matrix(n, m);
  // Upper triangular matrix, row-major order.
  final _Matrix r = _Matrix(n, n);
  for (int j = 0; j < n; j += 1) {
    for (int h = 0; h < m; h += 1) {
      q.set(j, h, a.get(j, h));
    }
    for (int i = 0; i < j; i += 1) {
      final double dot = q.getRow(j) * q.getRow(i);
      for (int h = 0; h < m; h += 1) {
        q.set(j, h, q.get(j, h) - dot * q.get(i, h));
      }
    }

    final double norm = q.getRow(j).norm();
    if (norm < precisionErrorTolerance) {
      // Vectors are linearly dependent or zero so no solution.
      return null;
    }

    final double inverseNorm = 1.0 / norm;
    for (int h = 0; h < m; h += 1) {
      q.set(j, h, q.get(j, h) * inverseNorm);
    }
    for (int i = 0; i < n; i += 1) {
      r.set(j, i, i < j ? 0.0 : q.getRow(j) * a.getRow(i));
    }
  }

  // Solve R B = Qt W Y to find B. This is easy because R is upper triangular.
  // We just work from bottom-right to top-left calculating B's coefficients.
  final _Vector wy = _Vector(m);
  for (int h = 0; h < m; h += 1) {
    wy[h] = y[h] * w[h];
  }
  for (int i = n - 1; i >= 0; i -= 1) {
    result.coefficients[i] = q.getRow(i) * wy;
    for (int j = n - 1; j > i; j -= 1) {
      result.coefficients[i] -= r.get(i, j) * result.coefficients[j];
    }
    result.coefficients[i] /= r.get(i, i);
  }

  // Calculate the coefficient of determination (confidence) as:
  //   1 - (sumSquaredError / sumSquaredTotal)
  // ...where sumSquaredError is the residual sum of squares (variance of the
  // error), and sumSquaredTotal is the total sum of squares (variance of the
  // data) where each has been weighted.
  double yMean = 0.0;
  for (int h = 0; h < m; h += 1) {
    yMean += y[h];
  }
  yMean /= m;

  double sumSquaredError = 0.0;
  double sumSquaredTotal = 0.0;
  for (int h = 0; h < m; h += 1) {
    double term = 1.0;
    double err = y[h] - result.coefficients[0];
    for (int i = 1; i < n; i += 1) {
      term *= x[h];
      err -= term * result.coefficients[i];
    }
    sumSquaredError += w[h] * w[h] * err * err;
    final double v = y[h] - yMean;
    sumSquaredTotal += w[h] * w[h] * v * v;
  }

  result.confidence = sumSquaredTotal <= precisionErrorTolerance ? 1.0 :
                        1.0 - (sumSquaredError / sumSquaredTotal);

  return result;
}