u-numflow 0.2.1

//! Dense matrix operations.
//!
//! A minimal, row-major dense matrix type with fundamental linear algebra
//! operations required for statistical analysis: multiplication, transpose,
//! determinant, inverse, Cholesky decomposition, and triangular solves.
//!
//! # Design
//!
//! - **Row-major storage**: `data[i * cols + j] = A[i, j]`
//! - **No external dependencies**: Pure Rust, no nalgebra/LAPACK
//! - **Partial pivoting**: Used in LU and Gauss-Jordan for numerical stability
//! - **Explicit error handling**: Returns `Result<T, MatrixError>` with descriptive variants
//!
//! # Examples
//!
//! ```
//! use u_numflow::matrix::Matrix;
//!
//! let a = Matrix::from_rows(&[
//!     &[1.0, 2.0],
//!     &[3.0, 4.0],
//! ]);
//! let b = a.transpose();
//! let c = a.mul_mat(&b).unwrap();
//! assert_eq!(c.rows(), 2);
//! assert_eq!(c.cols(), 2);
//! ```

/// Error type for matrix operations.
#[derive(Debug, Clone, PartialEq)]
pub enum MatrixError {
    /// Dimensions do not match for the operation.
    DimensionMismatch {
        expected: (usize, usize),
        got: (usize, usize),
    },
    /// Matrix must be square for this operation.
    NotSquare { rows: usize, cols: usize },
    /// Matrix is singular (zero or near-zero pivot encountered).
    Singular,
    /// Matrix is not symmetric (required for Cholesky).
    NotSymmetric,
    /// Matrix is not positive-definite (required for Cholesky).
    NotPositiveDefinite,
    /// Data length does not match dimensions.
    InvalidData { expected: usize, got: usize },
}

impl std::fmt::Display for MatrixError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            MatrixError::DimensionMismatch { expected, got } => {
                write!(
                    f,
                    "dimension mismatch: expected {}×{}, got {}×{}",
                    expected.0, expected.1, got.0, got.1
                )
            }
            MatrixError::NotSquare { rows, cols } => {
                write!(f, "matrix must be square, got {rows}×{cols}")
            }
            MatrixError::Singular => write!(f, "matrix is singular"),
            MatrixError::NotSymmetric => write!(f, "matrix is not symmetric"),
            MatrixError::NotPositiveDefinite => write!(f, "matrix is not positive-definite"),
            MatrixError::InvalidData { expected, got } => {
                write!(f, "data length mismatch: expected {expected}, got {got}")
            }
        }
    }
}

impl std::error::Error for MatrixError {}

/// A dense matrix stored in row-major order.
///
/// # Storage
///
/// Elements are stored contiguously: `data[i * cols + j]` holds `A[i, j]`.
#[derive(Debug, Clone, PartialEq)]
pub struct Matrix {
    data: Vec<f64>,
    rows: usize,
    cols: usize,
}

impl Matrix {
    /// Creates a matrix from raw data in row-major order.
    ///
    /// # Errors
    /// Returns `Err` if `data.len() != rows * cols`.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let m = Matrix::new(2, 3, vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]).unwrap();
    /// assert_eq!(m.get(0, 2), 3.0);
    /// assert_eq!(m.get(1, 0), 4.0);
    /// ```
    pub fn new(rows: usize, cols: usize, data: Vec<f64>) -> Result<Self, MatrixError> {
        if data.len() != rows * cols {
            return Err(MatrixError::InvalidData {
                expected: rows * cols,
                got: data.len(),
            });
        }
        Ok(Self { data, rows, cols })
    }

    /// Creates a matrix from row slices.
    ///
    /// # Panics
    /// Panics if rows have inconsistent lengths or `rows` is empty.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let m = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
    /// assert_eq!(m.get(1, 1), 4.0);
    /// ```
    pub fn from_rows(rows: &[&[f64]]) -> Self {
        assert!(!rows.is_empty(), "must have at least one row");
        let ncols = rows[0].len();
        assert!(ncols > 0, "must have at least one column");
        let nrows = rows.len();
        let mut data = Vec::with_capacity(nrows * ncols);
        for (i, row) in rows.iter().enumerate() {
            assert_eq!(
                row.len(),
                ncols,
                "row {i} has {} columns, expected {ncols}",
                row.len()
            );
            data.extend_from_slice(row);
        }
        Self {
            data,
            rows: nrows,
            cols: ncols,
        }
    }

    /// Creates a zero matrix.
    pub fn zeros(rows: usize, cols: usize) -> Self {
        Self {
            data: vec![0.0; rows * cols],
            rows,
            cols,
        }
    }

    /// Creates an identity matrix.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let eye = Matrix::identity(3);
    /// assert_eq!(eye.get(0, 0), 1.0);
    /// assert_eq!(eye.get(0, 1), 0.0);
    /// assert_eq!(eye.get(2, 2), 1.0);
    /// ```
    pub fn identity(n: usize) -> Self {
        let mut m = Self::zeros(n, n);
        for i in 0..n {
            m.data[i * n + i] = 1.0;
        }
        m
    }

    /// Creates a column vector (n×1 matrix) from a slice.
    pub fn from_col(data: &[f64]) -> Self {
        Self {
            data: data.to_vec(),
            rows: data.len(),
            cols: 1,
        }
    }

    /// Number of rows.
    #[inline]
    pub fn rows(&self) -> usize {
        self.rows
    }

    /// Number of columns.
    #[inline]
    pub fn cols(&self) -> usize {
        self.cols
    }

    /// Returns the element at (row, col).
    ///
    /// # Panics
    /// Panics if indices are out of bounds.
    #[inline]
    pub fn get(&self, row: usize, col: usize) -> f64 {
        self.data[row * self.cols + col]
    }

    /// Sets the element at (row, col).
    ///
    /// # Panics
    /// Panics if indices are out of bounds.
    #[inline]
    pub fn set(&mut self, row: usize, col: usize, value: f64) {
        self.data[row * self.cols + col] = value;
    }

    /// Returns the raw data as a slice.
    pub fn data(&self) -> &[f64] {
        &self.data
    }

    /// Returns a row as a slice.
    #[inline]
    pub fn row(&self, row: usize) -> &[f64] {
        let start = row * self.cols;
        &self.data[start..start + self.cols]
    }

    /// Returns the diagonal elements.
    pub fn diag(&self) -> Vec<f64> {
        let n = self.rows.min(self.cols);
        (0..n).map(|i| self.get(i, i)).collect()
    }

    /// Returns true if the matrix is square.
    pub fn is_square(&self) -> bool {
        self.rows == self.cols
    }

    // ========================================================================
    // Basic Operations
    // ========================================================================

    /// Transpose: returns Aᵀ.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let m = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0]]);
    /// let t = m.transpose();
    /// assert_eq!(t.rows(), 3);
    /// assert_eq!(t.cols(), 2);
    /// assert_eq!(t.get(0, 1), 4.0);
    /// ```
    pub fn transpose(&self) -> Self {
        let mut result = Self::zeros(self.cols, self.rows);
        for i in 0..self.rows {
            for j in 0..self.cols {
                result.data[j * self.rows + i] = self.data[i * self.cols + j];
            }
        }
        result
    }

    /// Matrix addition: A + B.
    ///
    /// # Errors
    /// Returns `Err` if dimensions do not match.
    pub fn add(&self, other: &Self) -> Result<Self, MatrixError> {
        if self.rows != other.rows || self.cols != other.cols {
            return Err(MatrixError::DimensionMismatch {
                expected: (self.rows, self.cols),
                got: (other.rows, other.cols),
            });
        }
        let data: Vec<f64> = self
            .data
            .iter()
            .zip(&other.data)
            .map(|(a, b)| a + b)
            .collect();
        Ok(Self {
            data,
            rows: self.rows,
            cols: self.cols,
        })
    }

    /// Matrix subtraction: A - B.
    ///
    /// # Errors
    /// Returns `Err` if dimensions do not match.
    pub fn sub(&self, other: &Self) -> Result<Self, MatrixError> {
        if self.rows != other.rows || self.cols != other.cols {
            return Err(MatrixError::DimensionMismatch {
                expected: (self.rows, self.cols),
                got: (other.rows, other.cols),
            });
        }
        let data: Vec<f64> = self
            .data
            .iter()
            .zip(&other.data)
            .map(|(a, b)| a - b)
            .collect();
        Ok(Self {
            data,
            rows: self.rows,
            cols: self.cols,
        })
    }

    /// Scalar multiplication: c · A.
    pub fn scale(&self, c: f64) -> Self {
        let data: Vec<f64> = self.data.iter().map(|x| c * x).collect();
        Self {
            data,
            rows: self.rows,
            cols: self.cols,
        }
    }

    /// Matrix multiplication: A · B.
    ///
    /// Uses i-k-j loop order for better cache locality on row-major storage.
    ///
    /// # Errors
    /// Returns `Err` if `self.cols != other.rows`.
    ///
    /// # Complexity
    /// O(n·m·p) where self is n×m and other is m×p.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let a = Matrix::identity(3);
    /// let b = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0], &[7.0, 8.0, 9.0]]);
    /// let c = a.mul_mat(&b).unwrap();
    /// assert_eq!(c.get(2, 2), 9.0);
    /// ```
    pub fn mul_mat(&self, other: &Self) -> Result<Self, MatrixError> {
        if self.cols != other.rows {
            return Err(MatrixError::DimensionMismatch {
                expected: (self.rows, self.cols),
                got: (other.rows, other.cols),
            });
        }
        let mut result = Self::zeros(self.rows, other.cols);
        // i-k-j loop order for row-major cache friendliness
        for i in 0..self.rows {
            for k in 0..self.cols {
                let a_ik = self.data[i * self.cols + k];
                let row_start = i * other.cols;
                let other_row_start = k * other.cols;
                for j in 0..other.cols {
                    result.data[row_start + j] += a_ik * other.data[other_row_start + j];
                }
            }
        }
        Ok(result)
    }

    /// Matrix-vector multiplication: A · v.
    ///
    /// # Errors
    /// Returns `Err` if `self.cols != v.len()`.
    pub fn mul_vec(&self, v: &[f64]) -> Result<Vec<f64>, MatrixError> {
        if self.cols != v.len() {
            return Err(MatrixError::DimensionMismatch {
                expected: (self.rows, self.cols),
                got: (v.len(), 1),
            });
        }
        let mut result = vec![0.0; self.rows];
        for (i, res) in result.iter_mut().enumerate() {
            let row_start = i * self.cols;
            *res = self.data[row_start..row_start + self.cols]
                .iter()
                .zip(v.iter())
                .map(|(&a, &b)| a * b)
                .sum();
        }
        Ok(result)
    }

    /// Frobenius norm: ‖A‖_F = √(Σᵢⱼ aᵢⱼ²).
    pub fn frobenius_norm(&self) -> f64 {
        self.data.iter().map(|x| x * x).sum::<f64>().sqrt()
    }

    /// Checks whether the matrix is symmetric within tolerance.
    pub fn is_symmetric(&self, tol: f64) -> bool {
        if self.rows != self.cols {
            return false;
        }
        for i in 0..self.rows {
            for j in (i + 1)..self.cols {
                if (self.get(i, j) - self.get(j, i)).abs() > tol {
                    return false;
                }
            }
        }
        true
    }

    fn swap_rows(&mut self, a: usize, b: usize) {
        if a == b {
            return;
        }
        let cols = self.cols;
        for j in 0..cols {
            self.data.swap(a * cols + j, b * cols + j);
        }
    }

    // ========================================================================
    // Decompositions
    // ========================================================================

    /// Determinant via LU decomposition with partial pivoting.
    ///
    /// # Errors
    /// Returns `Err(NotSquare)` if the matrix is not square.
    /// Returns `Err(Singular)` if a zero pivot is encountered.
    ///
    /// # Complexity
    /// O(n³/3).
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let m = Matrix::from_rows(&[&[2.0, 3.0], &[1.0, 4.0]]);
    /// assert!((m.determinant().unwrap() - 5.0).abs() < 1e-10);
    /// ```
    pub fn determinant(&self) -> Result<f64, MatrixError> {
        if !self.is_square() {
            return Err(MatrixError::NotSquare {
                rows: self.rows,
                cols: self.cols,
            });
        }
        let n = self.rows;
        if n == 0 {
            return Ok(1.0);
        }
        if n == 1 {
            return Ok(self.data[0]);
        }

        let mut work = self.clone();
        let mut sign = 1.0_f64;
        let pivot_tol = 1e-15 * self.frobenius_norm().max(1e-300);

        for k in 0..n {
            // Partial pivoting
            let mut max_val = work.get(k, k).abs();
            let mut max_row = k;
            for i in (k + 1)..n {
                let v = work.get(i, k).abs();
                if v > max_val {
                    max_val = v;
                    max_row = i;
                }
            }
            if max_val <= pivot_tol {
                return Ok(0.0); // Singular → det = 0
            }
            if max_row != k {
                work.swap_rows(k, max_row);
                sign = -sign;
            }

            let pivot = work.get(k, k);
            for i in (k + 1)..n {
                let factor = work.get(i, k) / pivot;
                for j in (k + 1)..n {
                    let val = work.get(i, j) - factor * work.get(k, j);
                    work.set(i, j, val);
                }
            }
        }

        let mut det = sign;
        for i in 0..n {
            det *= work.get(i, i);
        }
        Ok(det)
    }

    /// Matrix inverse via Gauss-Jordan elimination with partial pivoting.
    ///
    /// # Algorithm
    /// Augments [A | I], reduces to [I | A⁻¹] using row operations.
    ///
    /// Reference: Golub & Van Loan (1996), *Matrix Computations*, §1.2.
    ///
    /// # Errors
    /// Returns `Err(Singular)` if the matrix is singular.
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let a = Matrix::from_rows(&[&[4.0, 7.0], &[2.0, 6.0]]);
    /// let inv = a.inverse().unwrap();
    /// let eye = a.mul_mat(&inv).unwrap();
    /// assert!((eye.get(0, 0) - 1.0).abs() < 1e-10);
    /// assert!(eye.get(0, 1).abs() < 1e-10);
    /// ```
    pub fn inverse(&self) -> Result<Self, MatrixError> {
        if !self.is_square() {
            return Err(MatrixError::NotSquare {
                rows: self.rows,
                cols: self.cols,
            });
        }
        let n = self.rows;
        if n == 0 {
            return Ok(Self::zeros(0, 0));
        }

        // Augmented matrix [A | I]
        let n2 = 2 * n;
        let mut aug = Self::zeros(n, n2);
        for i in 0..n {
            for j in 0..n {
                aug.set(i, j, self.get(i, j));
            }
            aug.set(i, n + i, 1.0);
        }

        let pivot_tol = 1e-14 * self.frobenius_norm().max(1e-300);

        for k in 0..n {
            // Partial pivoting
            let mut max_val = aug.get(k, k).abs();
            let mut max_row = k;
            for i in (k + 1)..n {
                let v = aug.get(i, k).abs();
                if v > max_val {
                    max_val = v;
                    max_row = i;
                }
            }
            if max_val <= pivot_tol {
                return Err(MatrixError::Singular);
            }
            if max_row != k {
                aug.swap_rows(k, max_row);
            }

            // Scale pivot row
            let pivot = aug.get(k, k);
            for j in 0..n2 {
                aug.set(k, j, aug.get(k, j) / pivot);
            }

            // Eliminate column k in all other rows
            for i in 0..n {
                if i != k {
                    let factor = aug.get(i, k);
                    for j in 0..n2 {
                        let val = aug.get(i, j) - factor * aug.get(k, j);
                        aug.set(i, j, val);
                    }
                }
            }
        }

        // Extract right half
        let mut inv = Self::zeros(n, n);
        for i in 0..n {
            for j in 0..n {
                inv.set(i, j, aug.get(i, n + j));
            }
        }
        Ok(inv)
    }

    /// Cholesky decomposition: returns lower-triangular L such that A = L·Lᵀ.
    ///
    /// # Algorithm
    /// Column-by-column Cholesky-Banachiewicz factorization.
    ///
    /// Reference: Golub & Van Loan (1996), *Matrix Computations*, Algorithm 4.2.1.
    ///
    /// # Requirements
    /// Matrix must be symmetric and positive-definite.
    ///
    /// # Complexity
    /// O(n³/3).
    ///
    /// # Examples
    /// ```
    /// use u_numflow::matrix::Matrix;
    /// let a = Matrix::from_rows(&[
    ///     &[4.0, 2.0],
    ///     &[2.0, 3.0],
    /// ]);
    /// let l = a.cholesky().unwrap();
    /// let llt = l.mul_mat(&l.transpose()).unwrap();
    /// assert!((llt.get(0, 0) - 4.0).abs() < 1e-10);
    /// assert!((llt.get(0, 1) - 2.0).abs() < 1e-10);
    /// ```
    pub fn cholesky(&self) -> Result<Self, MatrixError> {
        if !self.is_square() {
            return Err(MatrixError::NotSquare {
                rows: self.rows,
                cols: self.cols,
            });
        }
        let n = self.rows;
        let sym_tol = 1e-10 * self.frobenius_norm().max(1e-300);
        if !self.is_symmetric(sym_tol) {
            return Err(MatrixError::NotSymmetric);
        }

        let mut l = Self::zeros(n, n);

        for j in 0..n {
            // Diagonal entry
            let mut sum = 0.0;
            for k in 0..j {
                let ljk = l.get(j, k);
                sum += ljk * ljk;
            }
            let diag = self.get(j, j) - sum;
            if diag <= 0.0 {
                return Err(MatrixError::NotPositiveDefinite);
            }
            l.set(j, j, diag.sqrt());

            // Below-diagonal entries
            let ljj = l.get(j, j);
            for i in (j + 1)..n {
                let mut sum = 0.0;
                for k in 0..j {
                    sum += l.get(i, k) * l.get(j, k);
                }
                l.set(i, j, (self.get(i, j) - sum) / ljj);
            }
        }

        Ok(l)
    }

    /// Solves the linear system A·x = b using Cholesky decomposition.
    ///
    /// Equivalent to computing x = A⁻¹·b but more efficient and stable.
    ///
    /// # Algorithm
    /// 1. Decompose A = L·Lᵀ via Cholesky
    /// 2. Solve L·y = b (forward substitution)
    /// 3. Solve Lᵀ·x = y (backward substitution)
    ///
    /// # Requirements
    /// Matrix must be symmetric positive-definite. `b.len()` must equal `self.rows()`.
    pub fn cholesky_solve(&self, b: &[f64]) -> Result<Vec<f64>, MatrixError> {
        if b.len() != self.rows {
            return Err(MatrixError::DimensionMismatch {
                expected: (self.rows, 1),
                got: (b.len(), 1),
            });
        }
        let l = self.cholesky()?;
        let y = solve_lower_triangular(&l, b)?;
        let lt = l.transpose();
        solve_upper_triangular(&lt, &y)
    }

    /// Eigenvalue decomposition of a real symmetric matrix using the
    /// classical Jacobi rotation algorithm.
    ///
    /// Returns `(eigenvalues, eigenvectors)` where eigenvalues are sorted
    /// in **descending** order and eigenvectors are the corresponding columns
    /// of the returned matrix (column `i` is the eigenvector for eigenvalue `i`).
    ///
    /// # Algorithm
    ///
    /// Cyclic Jacobi rotations zero off-diagonal elements iteratively.
    /// Converges quadratically for symmetric matrices.
    ///
    /// Reference: Golub & Van Loan (1996), "Matrix Computations", §8.4
    ///
    /// # Complexity
    ///
    /// O(n³) per sweep, typically 5–10 sweeps. Best for n < 200.
    ///
    /// # Errors
    ///
    /// Returns `NotSquare` if the matrix is not square, or `NotSymmetric`
    /// if the matrix is not symmetric within tolerance.
    ///
    /// # Examples
    ///
    /// ```
    /// use u_numflow::matrix::Matrix;
    ///
    /// let a = Matrix::from_rows(&[
    ///     &[4.0, 1.0],
    ///     &[1.0, 3.0],
    /// ]);
    /// let (eigenvalues, eigenvectors) = a.eigen_symmetric().unwrap();
    ///
    /// // Eigenvalues of [[4,1],[1,3]] are (7+√5)/2 ≈ 4.618 and (7-√5)/2 ≈ 2.382
    /// assert!((eigenvalues[0] - 4.618).abs() < 0.01);
    /// assert!((eigenvalues[1] - 2.382).abs() < 0.01);
    ///
    /// // Eigenvectors are orthonormal
    /// let dot: f64 = (0..2).map(|i| eigenvectors.get(i, 0) * eigenvectors.get(i, 1)).sum();
    /// assert!(dot.abs() < 1e-10);
    /// ```
    pub fn eigen_symmetric(&self) -> Result<(Vec<f64>, Matrix), MatrixError> {
        let n = self.rows;
        if !self.is_square() {
            return Err(MatrixError::NotSquare {
                rows: self.rows,
                cols: self.cols,
            });
        }
        // Symmetry tolerance: relative to matrix scale
        let sym_tol = 1e-10 * self.frobenius_norm();
        if !self.is_symmetric(sym_tol) {
            return Err(MatrixError::NotSymmetric);
        }

        // Work on a mutable copy of the matrix
        let mut a = self.data.clone();
        // Eigenvector accumulator — starts as identity
        let mut v = vec![0.0; n * n];
        for i in 0..n {
            v[i * n + i] = 1.0;
        }

        let max_sweeps = 100;
        let tol = 1e-15;

        for _ in 0..max_sweeps {
            // Compute off-diagonal Frobenius norm
            let mut off_norm = 0.0;
            for i in 0..n {
                for j in (i + 1)..n {
                    off_norm += 2.0 * a[i * n + j] * a[i * n + j];
                }
            }
            off_norm = off_norm.sqrt();

            if off_norm < tol {
                break;
            }

            // One full sweep: rotate each (p, q) pair
            for p in 0..n {
                for q in (p + 1)..n {
                    let apq = a[p * n + q];
                    if apq.abs() < tol * 0.01 {
                        continue;
                    }

                    let app = a[p * n + p];
                    let aqq = a[q * n + q];
                    let diff = aqq - app;

                    // Compute rotation angle
                    let (cos, sin) = if diff.abs() < 1e-300 {
                        // Special case: diagonal elements equal
                        let s = std::f64::consts::FRAC_1_SQRT_2;
                        (s, if apq > 0.0 { s } else { -s })
                    } else {
                        let tau = diff / (2.0 * apq);
                        // t = sign(tau) / (|tau| + sqrt(1 + tau²))
                        let t = if tau >= 0.0 {
                            1.0 / (tau + (1.0 + tau * tau).sqrt())
                        } else {
                            -1.0 / (-tau + (1.0 + tau * tau).sqrt())
                        };
                        let c = 1.0 / (1.0 + t * t).sqrt();
                        let s = t * c;
                        (c, s)
                    };

                    // Apply rotation to matrix A (symmetric, only update needed parts)
                    a[p * n + p] -=
                        2.0 * sin * cos * apq + sin * sin * (a[q * n + q] - a[p * n + p]);
                    a[q * n + q] += 2.0 * sin * cos * apq + sin * sin * (aqq - app); // use original aqq, app
                    a[p * n + q] = 0.0;
                    a[q * n + p] = 0.0;

                    // Actually, let's use the standard Jacobi rotation formula properly.
                    // Reset and recompute.
                    // Undo the above:
                    a[p * n + p] = app;
                    a[q * n + q] = aqq;
                    a[p * n + q] = apq;
                    a[q * n + p] = apq;

                    // Standard update: for all rows/cols
                    // First update rows p and q for all columns
                    for r in 0..n {
                        if r == p || r == q {
                            continue;
                        }
                        let arp = a[r * n + p];
                        let arq = a[r * n + q];
                        a[r * n + p] = cos * arp - sin * arq;
                        a[r * n + q] = sin * arp + cos * arq;
                        a[p * n + r] = a[r * n + p]; // symmetric
                        a[q * n + r] = a[r * n + q]; // symmetric
                    }

                    // Update diagonal and off-diagonal (p,q)
                    let new_pp = cos * cos * app - 2.0 * sin * cos * apq + sin * sin * aqq;
                    let new_qq = sin * sin * app + 2.0 * sin * cos * apq + cos * cos * aqq;
                    a[p * n + p] = new_pp;
                    a[q * n + q] = new_qq;
                    a[p * n + q] = 0.0;
                    a[q * n + p] = 0.0;

                    // Accumulate eigenvectors: V = V * J
                    for r in 0..n {
                        let vp = v[r * n + p];
                        let vq = v[r * n + q];
                        v[r * n + p] = cos * vp - sin * vq;
                        v[r * n + q] = sin * vp + cos * vq;
                    }
                }
            }
        }

        // Extract eigenvalues from diagonal
        let mut eigen_pairs: Vec<(f64, Vec<f64>)> = (0..n)
            .map(|i| {
                let eigenvalue = a[i * n + i];
                let eigenvector: Vec<f64> = (0..n).map(|r| v[r * n + i]).collect();
                (eigenvalue, eigenvector)
            })
            .collect();

        // Sort by eigenvalue descending
        eigen_pairs.sort_by(|a, b| b.0.partial_cmp(&a.0).unwrap_or(std::cmp::Ordering::Equal));

        let eigenvalues: Vec<f64> = eigen_pairs.iter().map(|(val, _)| *val).collect();
        let mut eigvec_data = vec![0.0; n * n];
        for (col, (_, vec)) in eigen_pairs.iter().enumerate() {
            for (row, &val) in vec.iter().enumerate() {
                eigvec_data[row * n + col] = val;
            }
        }
        let eigenvectors = Matrix {
            data: eigvec_data,
            rows: n,
            cols: n,
        };

        Ok((eigenvalues, eigenvectors))
    }
}

/// Solves L·x = b where L is lower-triangular (forward substitution).
fn solve_lower_triangular(l: &Matrix, b: &[f64]) -> Result<Vec<f64>, MatrixError> {
    let n = l.rows();
    let mut x = vec![0.0; n];
    for i in 0..n {
        let mut sum = 0.0;
        for (j, &xj) in x[..i].iter().enumerate() {
            sum += l.get(i, j) * xj;
        }
        let diag = l.get(i, i);
        if diag.abs() < 1e-300 {
            return Err(MatrixError::Singular);
        }
        x[i] = (b[i] - sum) / diag;
    }
    Ok(x)
}

/// Solves U·x = b where U is upper-triangular (backward substitution).
fn solve_upper_triangular(u: &Matrix, b: &[f64]) -> Result<Vec<f64>, MatrixError> {
    let n = u.rows();
    let mut x = vec![0.0; n];
    for i in (0..n).rev() {
        let mut sum = 0.0;
        for (off, &xj) in x[i + 1..].iter().enumerate() {
            sum += u.get(i, i + 1 + off) * xj;
        }
        let diag = u.get(i, i);
        if diag.abs() < 1e-300 {
            return Err(MatrixError::Singular);
        }
        x[i] = (b[i] - sum) / diag;
    }
    Ok(x)
}

// ============================================================================
// Display
// ============================================================================

impl std::fmt::Display for Matrix {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        for i in 0..self.rows {
            write!(f, "[")?;
            for j in 0..self.cols {
                if j > 0 {
                    write!(f, ", ")?;
                }
                write!(f, "{:>10.4}", self.get(i, j))?;
            }
            writeln!(f, "]")?;
        }
        Ok(())
    }
}

// ============================================================================
// Tests
// ============================================================================

#[cfg(test)]
mod tests {
    use super::*;

    // --- Construction ---

    #[test]
    fn test_new_valid() {
        let m = Matrix::new(2, 3, vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]).unwrap();
        assert_eq!(m.rows(), 2);
        assert_eq!(m.cols(), 3);
        assert_eq!(m.get(0, 0), 1.0);
        assert_eq!(m.get(1, 2), 6.0);
    }

    #[test]
    fn test_new_invalid_length() {
        assert!(Matrix::new(2, 3, vec![1.0, 2.0]).is_err());
    }

    #[test]
    fn test_from_rows() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        assert_eq!(m.get(0, 0), 1.0);
        assert_eq!(m.get(1, 1), 4.0);
    }

    #[test]
    fn test_zeros() {
        let m = Matrix::zeros(3, 4);
        assert_eq!(m.rows(), 3);
        assert_eq!(m.cols(), 4);
        assert_eq!(m.get(2, 3), 0.0);
    }

    #[test]
    fn test_identity() {
        let eye = Matrix::identity(3);
        assert_eq!(eye.get(0, 0), 1.0);
        assert_eq!(eye.get(1, 1), 1.0);
        assert_eq!(eye.get(2, 2), 1.0);
        assert_eq!(eye.get(0, 1), 0.0);
        assert_eq!(eye.get(1, 2), 0.0);
    }

    #[test]
    fn test_diag() {
        let m = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0], &[7.0, 8.0, 9.0]]);
        assert_eq!(m.diag(), vec![1.0, 5.0, 9.0]);
    }

    // --- Basic operations ---

    #[test]
    fn test_transpose() {
        let m = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0]]);
        let t = m.transpose();
        assert_eq!(t.rows(), 3);
        assert_eq!(t.cols(), 2);
        assert_eq!(t.get(0, 0), 1.0);
        assert_eq!(t.get(2, 1), 6.0);
    }

    #[test]
    fn test_transpose_twice() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0], &[5.0, 6.0]]);
        let tt = m.transpose().transpose();
        assert_eq!(m, tt);
    }

    #[test]
    fn test_add() {
        let a = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        let b = Matrix::from_rows(&[&[5.0, 6.0], &[7.0, 8.0]]);
        let c = a.add(&b).unwrap();
        assert_eq!(c.get(0, 0), 6.0);
        assert_eq!(c.get(1, 1), 12.0);
    }

    #[test]
    fn test_add_dimension_mismatch() {
        let a = Matrix::zeros(2, 3);
        let b = Matrix::zeros(3, 2);
        assert!(a.add(&b).is_err());
    }

    #[test]
    fn test_sub() {
        let a = Matrix::from_rows(&[&[5.0, 6.0], &[7.0, 8.0]]);
        let b = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        let c = a.sub(&b).unwrap();
        assert_eq!(c.get(0, 0), 4.0);
        assert_eq!(c.get(1, 1), 4.0);
    }

    #[test]
    fn test_scale() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        let s = m.scale(2.0);
        assert_eq!(s.get(0, 0), 2.0);
        assert_eq!(s.get(1, 1), 8.0);
    }

    // --- Multiplication ---

    #[test]
    fn test_mul_identity() {
        let a = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0], &[7.0, 8.0, 9.0]]);
        let eye = Matrix::identity(3);
        let result = a.mul_mat(&eye).unwrap();
        assert_eq!(a, result);
    }

    #[test]
    fn test_mul_2x2() {
        let a = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        let b = Matrix::from_rows(&[&[5.0, 6.0], &[7.0, 8.0]]);
        let c = a.mul_mat(&b).unwrap();
        // [1*5+2*7, 1*6+2*8] = [19, 22]
        // [3*5+4*7, 3*6+4*8] = [43, 50]
        assert_eq!(c.get(0, 0), 19.0);
        assert_eq!(c.get(0, 1), 22.0);
        assert_eq!(c.get(1, 0), 43.0);
        assert_eq!(c.get(1, 1), 50.0);
    }

    #[test]
    fn test_mul_nonsquare() {
        let a = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[4.0, 5.0, 6.0]]);
        let b = Matrix::from_rows(&[&[7.0, 8.0], &[9.0, 10.0], &[11.0, 12.0]]);
        let c = a.mul_mat(&b).unwrap();
        assert_eq!(c.rows(), 2);
        assert_eq!(c.cols(), 2);
        // [1*7+2*9+3*11, 1*8+2*10+3*12] = [58, 64]
        assert_eq!(c.get(0, 0), 58.0);
        assert_eq!(c.get(0, 1), 64.0);
    }

    #[test]
    fn test_mul_vec() {
        let a = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        let v = vec![5.0, 6.0];
        let result = a.mul_vec(&v).unwrap();
        assert_eq!(result, vec![17.0, 39.0]);
    }

    #[test]
    fn test_mul_dimension_mismatch() {
        let a = Matrix::zeros(2, 3);
        let b = Matrix::zeros(2, 3);
        assert!(a.mul_mat(&b).is_err());
    }

    // --- Determinant ---

    #[test]
    fn test_det_2x2() {
        let m = Matrix::from_rows(&[&[2.0, 3.0], &[1.0, 4.0]]);
        assert!((m.determinant().unwrap() - 5.0).abs() < 1e-10);
    }

    #[test]
    fn test_det_3x3() {
        let m = Matrix::from_rows(&[&[6.0, 1.0, 1.0], &[4.0, -2.0, 5.0], &[2.0, 8.0, 7.0]]);
        // det = 6*(-14-40) - 1*(28-10) + 1*(32+4) = -306
        // Actually: 6(-2*7 - 5*8) - 1(4*7 - 5*2) + 1(4*8 - (-2)*2)
        //         = 6(-14 - 40) - 1(28 - 10) + 1(32 + 4)
        //         = 6*(-54) - 18 + 36 = -324 - 18 + 36 = -306
        assert!((m.determinant().unwrap() - (-306.0)).abs() < 1e-8);
    }

    #[test]
    fn test_det_identity() {
        let eye = Matrix::identity(4);
        assert!((eye.determinant().unwrap() - 1.0).abs() < 1e-10);
    }

    #[test]
    fn test_det_singular() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[2.0, 4.0]]);
        assert!(m.determinant().unwrap().abs() < 1e-10);
    }

    #[test]
    fn test_det_not_square() {
        let m = Matrix::zeros(2, 3);
        assert!(m.determinant().is_err());
    }

    // --- Inverse ---

    #[test]
    fn test_inverse_2x2() {
        let a = Matrix::from_rows(&[&[4.0, 7.0], &[2.0, 6.0]]);
        let inv = a.inverse().unwrap();
        let eye = a.mul_mat(&inv).unwrap();
        for i in 0..2 {
            for j in 0..2 {
                let expected = if i == j { 1.0 } else { 0.0 };
                assert!(
                    (eye.get(i, j) - expected).abs() < 1e-10,
                    "A·A⁻¹[{i},{j}] = {}, expected {expected}",
                    eye.get(i, j)
                );
            }
        }
    }

    #[test]
    fn test_inverse_3x3() {
        let a = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[0.0, 1.0, 4.0], &[5.0, 6.0, 0.0]]);
        let inv = a.inverse().unwrap();
        let eye = a.mul_mat(&inv).unwrap();
        for i in 0..3 {
            for j in 0..3 {
                let expected = if i == j { 1.0 } else { 0.0 };
                assert!(
                    (eye.get(i, j) - expected).abs() < 1e-10,
                    "A·A⁻¹[{i},{j}] = {}",
                    eye.get(i, j)
                );
            }
        }
    }

    #[test]
    fn test_inverse_identity() {
        let eye = Matrix::identity(4);
        let inv = eye.inverse().unwrap();
        assert_eq!(eye, inv);
    }

    #[test]
    fn test_inverse_singular() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[2.0, 4.0]]);
        assert!(m.inverse().is_err());
    }

    // --- Cholesky ---

    #[test]
    fn test_cholesky_2x2() {
        let a = Matrix::from_rows(&[&[4.0, 2.0], &[2.0, 3.0]]);
        let l = a.cholesky().unwrap();
        // L should be lower triangular
        assert!(l.get(0, 1).abs() < 1e-15);
        // Verify A = L·Lᵀ
        let llt = l.mul_mat(&l.transpose()).unwrap();
        for i in 0..2 {
            for j in 0..2 {
                assert!(
                    (llt.get(i, j) - a.get(i, j)).abs() < 1e-10,
                    "LLᵀ[{i},{j}] = {}, expected {}",
                    llt.get(i, j),
                    a.get(i, j)
                );
            }
        }
    }

    #[test]
    fn test_cholesky_3x3() {
        let a = Matrix::from_rows(&[&[25.0, 15.0, -5.0], &[15.0, 18.0, 0.0], &[-5.0, 0.0, 11.0]]);
        let l = a.cholesky().unwrap();
        let llt = l.mul_mat(&l.transpose()).unwrap();
        for i in 0..3 {
            for j in 0..3 {
                assert!(
                    (llt.get(i, j) - a.get(i, j)).abs() < 1e-10,
                    "LLᵀ[{i},{j}] = {}, A[{i},{j}] = {}",
                    llt.get(i, j),
                    a.get(i, j)
                );
            }
        }
    }

    #[test]
    fn test_cholesky_identity() {
        let eye = Matrix::identity(3);
        let l = eye.cholesky().unwrap();
        assert_eq!(l, eye);
    }

    #[test]
    fn test_cholesky_not_positive_definite() {
        let a = Matrix::from_rows(&[&[1.0, 2.0], &[2.0, 1.0]]);
        assert!(matches!(
            a.cholesky(),
            Err(MatrixError::NotPositiveDefinite)
        ));
    }

    #[test]
    fn test_cholesky_not_symmetric() {
        let a = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        assert!(matches!(a.cholesky(), Err(MatrixError::NotSymmetric)));
    }

    // --- Cholesky solve ---

    #[test]
    fn test_cholesky_solve() {
        // A = [[4, 2], [2, 3]], b = [1, 2]
        // Solution: x = [-0.125, 0.75]
        let a = Matrix::from_rows(&[&[4.0, 2.0], &[2.0, 3.0]]);
        let b = vec![1.0, 2.0];
        let x = a.cholesky_solve(&b).unwrap();
        // Verify A·x = b
        let ax = a.mul_vec(&x).unwrap();
        for i in 0..2 {
            assert!(
                (ax[i] - b[i]).abs() < 1e-10,
                "Ax[{i}] = {}, b[{i}] = {}",
                ax[i],
                b[i]
            );
        }
    }

    #[test]
    fn test_cholesky_solve_3x3() {
        let a = Matrix::from_rows(&[&[25.0, 15.0, -5.0], &[15.0, 18.0, 0.0], &[-5.0, 0.0, 11.0]]);
        let b = vec![35.0, 33.0, 6.0];
        let x = a.cholesky_solve(&b).unwrap();
        let ax = a.mul_vec(&x).unwrap();
        for i in 0..3 {
            assert!((ax[i] - b[i]).abs() < 1e-10);
        }
    }

    // --- Frobenius norm ---

    #[test]
    fn test_frobenius_norm() {
        let m = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        // sqrt(1 + 4 + 9 + 16) = sqrt(30) ≈ 5.477
        assert!((m.frobenius_norm() - 30.0_f64.sqrt()).abs() < 1e-10);
    }

    // --- is_symmetric ---

    #[test]
    fn test_is_symmetric() {
        let sym = Matrix::from_rows(&[&[1.0, 2.0, 3.0], &[2.0, 5.0, 6.0], &[3.0, 6.0, 9.0]]);
        assert!(sym.is_symmetric(1e-10));

        let asym = Matrix::from_rows(&[&[1.0, 2.0], &[3.0, 4.0]]);
        assert!(!asym.is_symmetric(1e-10));
    }
}

#[cfg(test)]
mod proptests {
    use super::*;
    use proptest::prelude::*;

    fn square_matrix(n: usize) -> impl Strategy<Value = Matrix> {
        proptest::collection::vec(-10.0_f64..10.0, n * n)
            .prop_map(move |data| Matrix::new(n, n, data).expect("valid dimensions"))
    }

    fn spd_matrix(n: usize) -> impl Strategy<Value = Matrix> {
        // Generate random matrix A, then form A'A + nI (guaranteed SPD)
        proptest::collection::vec(-5.0_f64..5.0, n * n).prop_map(move |data| {
            let a = Matrix::new(n, n, data).expect("valid dimensions");
            let ata = a.transpose().mul_mat(&a).expect("compatible");
            let eye_scaled = Matrix::identity(n).scale(n as f64);
            ata.add(&eye_scaled).expect("compatible")
        })
    }

    proptest! {
        #![proptest_config(ProptestConfig::with_cases(200))]

        #[test]
        fn transpose_involution(m in square_matrix(3)) {
            let m_tt = m.transpose().transpose();
            for i in 0..3 {
                for j in 0..3 {
                    prop_assert!((m.get(i, j) - m_tt.get(i, j)).abs() < 1e-14);
                }
            }
        }

        #[test]
        fn mul_identity_is_identity(m in square_matrix(3)) {
            let eye = Matrix::identity(3);
            let me = m.mul_mat(&eye).unwrap();
            let em = eye.mul_mat(&m).unwrap();
            for i in 0..3 {
                for j in 0..3 {
                    prop_assert!((me.get(i, j) - m.get(i, j)).abs() < 1e-10);
                    prop_assert!((em.get(i, j) - m.get(i, j)).abs() < 1e-10);
                }
            }
        }

        #[test]
        fn det_of_product(a in square_matrix(3), b in square_matrix(3)) {
            // det(A·B) = det(A)·det(B)
            let det_a = a.determinant().unwrap();
            let det_b = b.determinant().unwrap();
            let ab = a.mul_mat(&b).unwrap();
            let det_ab = ab.determinant().unwrap();
            let expected = det_a * det_b;
            // Use relative tolerance for large determinants
            let tol = 1e-6 * expected.abs().max(det_ab.abs()).max(1.0);
            prop_assert!(
                (det_ab - expected).abs() < tol,
                "det(AB)={det_ab}, det(A)*det(B)={expected}"
            );
        }

        #[test]
        fn cholesky_roundtrip(a in spd_matrix(3)) {
            let l = a.cholesky().expect("SPD should decompose");
            let llt = l.mul_mat(&l.transpose()).expect("compatible");
            for i in 0..3 {
                for j in 0..3 {
                    let diff = (llt.get(i, j) - a.get(i, j)).abs();
                    let tol = 1e-8 * a.get(i, j).abs().max(1.0);
                    prop_assert!(
                        diff < tol,
                        "LLᵀ[{i},{j}]={}, A[{i},{j}]={}",
                        llt.get(i, j), a.get(i, j)
                    );
                }
            }
        }

        #[test]
        fn cholesky_solve_roundtrip(a in spd_matrix(3), b in proptest::collection::vec(-10.0_f64..10.0, 3)) {
            let x = a.cholesky_solve(&b).expect("SPD solve should work");
            let ax = a.mul_vec(&x).expect("compatible");
            for i in 0..3 {
                let tol = 1e-8 * b[i].abs().max(1.0);
                prop_assert!(
                    (ax[i] - b[i]).abs() < tol,
                    "Ax[{i}]={}, b[{i}]={}",
                    ax[i], b[i]
                );
            }
        }

        #[test]
        fn inverse_roundtrip(a in spd_matrix(3)) {
            // SPD matrices are always invertible
            let inv = a.inverse().expect("SPD invertible");
            let eye = a.mul_mat(&inv).expect("compatible");
            for i in 0..3 {
                for j in 0..3 {
                    let expected = if i == j { 1.0 } else { 0.0 };
                    let diff = (eye.get(i, j) - expected).abs();
                    prop_assert!(
                        diff < 1e-6,
                        "A·A⁻¹[{i},{j}]={}, expected {expected}",
                        eye.get(i, j)
                    );
                }
            }
        }
    }
}