solvers/sparse/

csr.rs

//! Compressed Sparse Row (CSR) matrix format
//!
//! CSR format stores:
//! - `values`: Non-zero entries in row-major order
//! - `col_indices`: Column index for each value
//! - `row_ptrs`: Index into values/col_indices where each row starts

use crate::traits::{ComplexField, LinearOperator};
use ndarray::{Array1, Array2};
use num_traits::Zero;
use std::ops::Range;

#[cfg(feature = "rayon")]
use rayon::prelude::*;

/// Compressed Sparse Row (CSR) matrix format
///
/// Memory-efficient storage for sparse matrices with O(nnz) space complexity.
/// Matrix-vector products are O(nnz) instead of O(n²) for dense matrices.
#[derive(Debug, Clone)]
pub struct CsrMatrix<T: ComplexField> {
    /// Number of rows
    pub num_rows: usize,
    /// Number of columns
    pub num_cols: usize,
    /// Non-zero values in row-major order
    pub values: Vec<T>,
    /// Column indices for each value
    pub col_indices: Vec<usize>,
    /// Row pointers: row_ptrs[i] is the start index in values/col_indices for row i
    /// row_ptrs[num_rows] = nnz (total number of non-zeros)
    pub row_ptrs: Vec<usize>,
}

impl<T: ComplexField> CsrMatrix<T> {
    /// Create a new empty CSR matrix
    pub fn new(num_rows: usize, num_cols: usize) -> Self {
        Self {
            num_rows,
            num_cols,
            values: Vec::new(),
            col_indices: Vec::new(),
            row_ptrs: vec![0; num_rows + 1],
        }
    }

    /// Create a CSR matrix with pre-allocated capacity
    pub fn with_capacity(num_rows: usize, num_cols: usize, nnz_estimate: usize) -> Self {
        Self {
            num_rows,
            num_cols,
            values: Vec::with_capacity(nnz_estimate),
            col_indices: Vec::with_capacity(nnz_estimate),
            row_ptrs: vec![0; num_rows + 1],
        }
    }

    /// Create a CSR matrix from raw components
    ///
    /// This is useful for converting between different CSR matrix representations
    /// that share the same internal structure.
    ///
    /// # Panics
    ///
    /// Panics if the input arrays are inconsistent:
    /// - `row_ptrs` must have length `num_rows + 1`
    /// - `col_indices` and `values` must have the same length
    /// - `row_ptrs[num_rows]` must equal `values.len()`
    pub fn from_raw_parts(
        num_rows: usize,
        num_cols: usize,
        row_ptrs: Vec<usize>,
        col_indices: Vec<usize>,
        values: Vec<T>,
    ) -> Self {
        assert_eq!(
            row_ptrs.len(),
            num_rows + 1,
            "row_ptrs must have num_rows + 1 elements"
        );
        assert_eq!(
            col_indices.len(),
            values.len(),
            "col_indices and values must have the same length"
        );
        assert_eq!(
            row_ptrs[num_rows],
            values.len(),
            "row_ptrs[num_rows] must equal nnz"
        );

        Self {
            num_rows,
            num_cols,
            row_ptrs,
            col_indices,
            values,
        }
    }

    /// Create a CSR matrix from a dense matrix
    ///
    /// Only stores entries with magnitude > threshold
    pub fn from_dense(dense: &Array2<T>, threshold: T::Real) -> Self {
        let num_rows = dense.nrows();
        let num_cols = dense.ncols();

        let mut values = Vec::new();
        let mut col_indices = Vec::new();
        let mut row_ptrs = vec![0usize; num_rows + 1];

        for i in 0..num_rows {
            for j in 0..num_cols {
                let val = dense[[i, j]];
                if val.norm() > threshold {
                    values.push(val);
                    col_indices.push(j);
                }
            }
            row_ptrs[i + 1] = values.len();
        }

        Self {
            num_rows,
            num_cols,
            values,
            col_indices,
            row_ptrs,
        }
    }

    /// Create a CSR matrix from COO (Coordinate) format triplets
    ///
    /// Triplets are (row, col, value). Duplicate entries are summed.
    pub fn from_triplets(
        num_rows: usize,
        num_cols: usize,
        mut triplets: Vec<(usize, usize, T)>,
    ) -> Self {
        if triplets.is_empty() {
            return Self::new(num_rows, num_cols);
        }

        // Sort by row, then by column
        triplets.sort_by(|a, b| {
            if a.0 != b.0 {
                a.0.cmp(&b.0)
            } else {
                a.1.cmp(&b.1)
            }
        });

        let mut values = Vec::with_capacity(triplets.len());
        let mut col_indices = Vec::with_capacity(triplets.len());
        let mut row_ptrs = vec![0usize; num_rows + 1];

        let mut prev_row = usize::MAX;
        let mut prev_col = usize::MAX;

        for (row, col, val) in triplets {
            if row == prev_row && col == prev_col {
                // Same entry, accumulate
                if let Some(last) = values.last_mut() {
                    *last += val;
                }
            } else {
                // New entry - push it
                values.push(val);
                col_indices.push(col);

                // Update row pointers for any rows we skipped
                if row != prev_row {
                    let start = if prev_row == usize::MAX {
                        0
                    } else {
                        prev_row + 1
                    };
                    for item in row_ptrs.iter_mut().take(row + 1).skip(start) {
                        *item = values.len() - 1;
                    }
                }

                prev_row = row;
                prev_col = col;
            }
        }

        // Fill remaining row pointers
        let last_row = if prev_row == usize::MAX {
            0
        } else {
            prev_row + 1
        };
        for item in row_ptrs.iter_mut().take(num_rows + 1).skip(last_row) {
            *item = values.len();
        }

        Self {
            num_rows,
            num_cols,
            values,
            col_indices,
            row_ptrs,
        }
    }

    /// Number of non-zero entries
    pub fn nnz(&self) -> usize {
        self.values.len()
    }

    /// Sparsity ratio (fraction of non-zero entries)
    pub fn sparsity(&self) -> f64 {
        let total = self.num_rows * self.num_cols;
        if total == 0 {
            0.0
        } else {
            self.nnz() as f64 / total as f64
        }
    }

    /// Get the range of indices in values/col_indices for a given row
    pub fn row_range(&self, row: usize) -> Range<usize> {
        self.row_ptrs[row]..self.row_ptrs[row + 1]
    }

    /// Get the (col, value) pairs for a row
    pub fn row_entries(&self, row: usize) -> impl Iterator<Item = (usize, T)> + '_ {
        let range = self.row_range(row);
        self.col_indices[range.clone()]
            .iter()
            .copied()
            .zip(self.values[range].iter().copied())
    }

    /// Matrix-vector product: y = A * x
    ///
    /// Uses parallel processing when the `rayon` feature is enabled and the
    /// matrix is large enough to benefit from parallelization.
    pub fn matvec(&self, x: &Array1<T>) -> Array1<T> {
        assert_eq!(x.len(), self.num_cols, "Input vector size mismatch");

        // Use parallel version for large matrices when rayon is available
        #[cfg(feature = "rayon")]
        {
            // Only parallelize if we have enough rows to benefit
            if self.num_rows >= 246 {
                return self.matvec_parallel(x);
            }
        }

        self.matvec_sequential(x)
    }

    /// Sequential matrix-vector product
    fn matvec_sequential(&self, x: &Array1<T>) -> Array1<T> {
        let mut y = Array1::from_elem(self.num_rows, T::zero());

        for i in 0..self.num_rows {
            let mut sum = T::zero();
            for idx in self.row_range(i) {
                let j = self.col_indices[idx];
                sum += self.values[idx] * x[j];
            }
            y[i] = sum;
        }

        y
    }

    /// Parallel matrix-vector product using rayon
    #[cfg(feature = "rayon")]
    fn matvec_parallel(&self, x: &Array1<T>) -> Array1<T>
    where
        T: Send + Sync,
    {
        let x_slice = x.as_slice().expect("Array should be contiguous");

        let results: Vec<T> = (0..self.num_rows)
            .into_par_iter()
            .map(|i| {
                let mut sum = T::zero();
                for idx in self.row_range(i) {
                    let j = self.col_indices[idx];
                    sum += self.values[idx] * x_slice[j];
                }
                sum
            })
            .collect();

        Array1::from_vec(results)
    }

    /// Matrix-vector product with accumulation: y += A * x
    pub fn matvec_add(&self, x: &Array1<T>, y: &mut Array1<T>) {
        assert_eq!(x.len(), self.num_cols, "Input vector size mismatch");
        assert_eq!(y.len(), self.num_rows, "Output vector size mismatch");

        for i in 0..self.num_rows {
            for idx in self.row_range(i) {
                let j = self.col_indices[idx];
                y[i] += self.values[idx] * x[j];
            }
        }
    }

    /// Transpose matrix-vector product: y = A^T * x
    pub fn matvec_transpose(&self, x: &Array1<T>) -> Array1<T> {
        assert_eq!(x.len(), self.num_rows, "Input vector size mismatch");

        let mut y = Array1::from_elem(self.num_cols, T::zero());

        for i in 0..self.num_rows {
            for idx in self.row_range(i) {
                let j = self.col_indices[idx];
                y[j] += self.values[idx] * x[i];
            }
        }

        y
    }

    /// Hermitian (conjugate transpose) matrix-vector product: y = A^H * x
    pub fn matvec_hermitian(&self, x: &Array1<T>) -> Array1<T> {
        assert_eq!(x.len(), self.num_rows, "Input vector size mismatch");

        let mut y = Array1::from_elem(self.num_cols, T::zero());

        for i in 0..self.num_rows {
            for idx in self.row_range(i) {
                let j = self.col_indices[idx];
                y[j] += self.values[idx].conj() * x[i];
            }
        }

        y
    }

    /// Get element at (i, j), returns 0 if not stored
    pub fn get(&self, i: usize, j: usize) -> T {
        for idx in self.row_range(i) {
            if self.col_indices[idx] == j {
                return self.values[idx];
            }
        }
        T::zero()
    }

    /// Extract diagonal elements
    pub fn diagonal(&self) -> Array1<T> {
        let n = self.num_rows.min(self.num_cols);
        let mut diag = Array1::from_elem(n, T::zero());

        for i in 0..n {
            diag[i] = self.get(i, i);
        }

        diag
    }

    /// Scale all values by a scalar
    pub fn scale(&mut self, scalar: T) {
        for val in &mut self.values {
            *val *= scalar;
        }
    }

    /// Add a scalar to the diagonal
    pub fn add_diagonal(&mut self, scalar: T) {
        let n = self.num_rows.min(self.num_cols);

        for i in 0..n {
            for idx in self.row_range(i) {
                if self.col_indices[idx] == i {
                    self.values[idx] += scalar;
                    break;
                }
            }
        }
    }

    /// Create identity matrix in CSR format
    pub fn identity(n: usize) -> Self {
        Self {
            num_rows: n,
            num_cols: n,
            values: vec![T::one(); n],
            col_indices: (0..n).collect(),
            row_ptrs: (0..=n).collect(),
        }
    }

    /// Create diagonal matrix from vector
    pub fn from_diagonal(diag: &Array1<T>) -> Self {
        let n = diag.len();
        Self {
            num_rows: n,
            num_cols: n,
            values: diag.to_vec(),
            col_indices: (0..n).collect(),
            row_ptrs: (0..=n).collect(),
        }
    }

    /// Convert to dense matrix (for debugging/small matrices)
    pub fn to_dense(&self) -> Array2<T> {
        let mut dense = Array2::from_elem((self.num_rows, self.num_cols), T::zero());

        for i in 0..self.num_rows {
            for idx in self.row_range(i) {
                let j = self.col_indices[idx];
                dense[[i, j]] = self.values[idx];
            }
        }

        dense
    }
}

impl<T: ComplexField> LinearOperator<T> for CsrMatrix<T> {
    fn num_rows(&self) -> usize {
        self.num_rows
    }

    fn num_cols(&self) -> usize {
        self.num_cols
    }

    fn apply(&self, x: &Array1<T>) -> Array1<T> {
        self.matvec(x)
    }

    fn apply_transpose(&self, x: &Array1<T>) -> Array1<T> {
        self.matvec_transpose(x)
    }

    fn apply_hermitian(&self, x: &Array1<T>) -> Array1<T> {
        self.matvec_hermitian(x)
    }
}

/// Builder for constructing CSR matrices row by row
pub struct CsrBuilder<T: ComplexField> {
    num_rows: usize,
    num_cols: usize,
    values: Vec<T>,
    col_indices: Vec<usize>,
    row_ptrs: Vec<usize>,
    current_row: usize,
}

impl<T: ComplexField> CsrBuilder<T> {
    /// Create a new CSR builder
    pub fn new(num_rows: usize, num_cols: usize) -> Self {
        Self {
            num_rows,
            num_cols,
            values: Vec::new(),
            col_indices: Vec::new(),
            row_ptrs: vec![0],
            current_row: 0,
        }
    }

    /// Create a new CSR builder with estimated non-zeros
    pub fn with_capacity(num_rows: usize, num_cols: usize, nnz_estimate: usize) -> Self {
        Self {
            num_rows,
            num_cols,
            values: Vec::with_capacity(nnz_estimate),
            col_indices: Vec::with_capacity(nnz_estimate),
            row_ptrs: Vec::with_capacity(num_rows + 1),
            current_row: 0,
        }
    }

    /// Add entries for the current row (must be added in column order)
    pub fn add_row_entries(&mut self, entries: impl Iterator<Item = (usize, T)>) {
        for (col, val) in entries {
            if val.norm() > T::Real::zero() {
                self.values.push(val);
                self.col_indices.push(col);
            }
        }
        self.row_ptrs.push(self.values.len());
        self.current_row += 1;
    }

    /// Finish building and return the CSR matrix
    pub fn finish(mut self) -> CsrMatrix<T> {
        // Fill remaining rows if not all rows were added
        while self.current_row < self.num_rows {
            self.row_ptrs.push(self.values.len());
            self.current_row += 1;
        }

        CsrMatrix {
            num_rows: self.num_rows,
            num_cols: self.num_cols,
            values: self.values,
            col_indices: self.col_indices,
            row_ptrs: self.row_ptrs,
        }
    }
}

/// Blocked CSR format for hierarchical matrices
///
/// Stores the matrix as a collection of dense blocks at the leaf level
/// of a hierarchical decomposition.
#[derive(Debug, Clone)]
pub struct BlockedCsr<T: ComplexField> {
    /// Number of rows
    pub num_rows: usize,
    /// Number of columns
    pub num_cols: usize,
    /// Block size (rows and columns per block)
    pub block_size: usize,
    /// Number of block rows
    pub num_block_rows: usize,
    /// Number of block columns
    pub num_block_cols: usize,
    /// Dense blocks stored in CSR-like format
    pub blocks: Vec<Array2<T>>,
    /// Block column indices
    pub block_col_indices: Vec<usize>,
    /// Block row pointers
    pub block_row_ptrs: Vec<usize>,
}

impl<T: ComplexField> BlockedCsr<T> {
    /// Create a new blocked CSR matrix
    pub fn new(num_rows: usize, num_cols: usize, block_size: usize) -> Self {
        let num_block_rows = num_rows.div_ceil(block_size);
        let num_block_cols = num_cols.div_ceil(block_size);

        Self {
            num_rows,
            num_cols,
            block_size,
            num_block_rows,
            num_block_cols,
            blocks: Vec::new(),
            block_col_indices: Vec::new(),
            block_row_ptrs: vec![0; num_block_rows + 1],
        }
    }

    /// Matrix-vector product using blocked structure
    pub fn matvec(&self, x: &Array1<T>) -> Array1<T> {
        assert_eq!(x.len(), self.num_cols, "Input vector size mismatch");

        let mut y = Array1::from_elem(self.num_rows, T::zero());

        for block_i in 0..self.num_block_rows {
            let row_start = block_i * self.block_size;
            let row_end = (row_start + self.block_size).min(self.num_rows);
            let local_rows = row_end - row_start;

            for idx in self.block_row_ptrs[block_i]..self.block_row_ptrs[block_i + 1] {
                let block_j = self.block_col_indices[idx];
                let block = &self.blocks[idx];

                let col_start = block_j * self.block_size;
                let col_end = (col_start + self.block_size).min(self.num_cols);
                let local_cols = col_end - col_start;

                // Extract local x
                let x_local: Array1<T> = Array1::from_iter((col_start..col_end).map(|j| x[j]));

                // Apply block
                for i in 0..local_rows {
                    let mut sum = T::zero();
                    for j in 0..local_cols {
                        sum += block[[i, j]] * x_local[j];
                    }
                    y[row_start + i] += sum;
                }
            }
        }

        y
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use approx::assert_relative_eq;
    use ndarray::array;
    use num_complex::Complex64;

    #[test]
    fn test_csr_from_dense() {
        let dense = array![
            [
                Complex64::new(1.0, 0.0),
                Complex64::new(0.0, 0.0),
                Complex64::new(2.0, 0.0)
            ],
            [
                Complex64::new(0.0, 0.0),
                Complex64::new(3.0, 0.0),
                Complex64::new(0.0, 0.0)
            ],
            [
                Complex64::new(4.0, 0.0),
                Complex64::new(0.0, 0.0),
                Complex64::new(5.0, 0.0)
            ],
        ];

        let csr = CsrMatrix::from_dense(&dense, 1e-15);

        assert_eq!(csr.num_rows, 3);
        assert_eq!(csr.num_cols, 3);
        assert_eq!(csr.nnz(), 5);

        assert_relative_eq!(csr.get(0, 0).re, 1.0);
        assert_relative_eq!(csr.get(0, 2).re, 2.0);
        assert_relative_eq!(csr.get(1, 1).re, 3.0);
        assert_relative_eq!(csr.get(2, 0).re, 4.0);
        assert_relative_eq!(csr.get(2, 2).re, 5.0);
    }

    #[test]
    fn test_csr_matvec() {
        let dense = array![
            [Complex64::new(1.0, 0.0), Complex64::new(2.0, 0.0)],
            [Complex64::new(3.0, 0.0), Complex64::new(4.0, 0.0)],
        ];

        let csr = CsrMatrix::from_dense(&dense, 1e-15);
        let x = array![Complex64::new(1.0, 0.0), Complex64::new(2.0, 0.0)];

        let y = csr.matvec(&x);

        // [1 2] * [1]   [5]
        // [3 4]   [2] = [11]
        assert_relative_eq!(y[0].re, 5.0, epsilon = 1e-10);
        assert_relative_eq!(y[1].re, 11.0, epsilon = 1e-10);
    }

    #[test]
    fn test_csr_from_triplets() {
        let triplets = vec![
            (0, 0, Complex64::new(1.0, 0.0)),
            (0, 2, Complex64::new(2.0, 0.0)),
            (1, 1, Complex64::new(3.0, 0.0)),
            (2, 0, Complex64::new(4.0, 0.0)),
            (2, 2, Complex64::new(5.0, 0.0)),
        ];

        let csr = CsrMatrix::from_triplets(3, 3, triplets);

        assert_eq!(csr.nnz(), 5);
        assert_relative_eq!(csr.get(0, 0).re, 1.0);
        assert_relative_eq!(csr.get(1, 1).re, 3.0);
    }

    #[test]
    fn test_csr_triplets_duplicate() {
        let triplets = vec![
            (0, 0, Complex64::new(1.0, 0.0)),
            (0, 0, Complex64::new(2.0, 0.0)), // Duplicate!
            (1, 1, Complex64::new(3.0, 0.0)),
        ];

        let csr = CsrMatrix::from_triplets(2, 2, triplets);

        assert_relative_eq!(csr.get(0, 0).re, 3.0); // 1 + 2 = 3
    }

    #[test]
    fn test_csr_identity() {
        let id: CsrMatrix<Complex64> = CsrMatrix::identity(3);

        assert_eq!(id.nnz(), 3);
        assert_relative_eq!(id.get(0, 0).re, 1.0);
        assert_relative_eq!(id.get(1, 1).re, 1.0);
        assert_relative_eq!(id.get(2, 2).re, 1.0);
        assert_relative_eq!(id.get(0, 1).norm(), 0.0);
    }

    #[test]
    fn test_csr_builder() {
        let mut builder: CsrBuilder<Complex64> = CsrBuilder::new(3, 3);

        builder.add_row_entries(
            [(0, Complex64::new(1.0, 0.0)), (2, Complex64::new(2.0, 0.0))].into_iter(),
        );
        builder.add_row_entries([(1, Complex64::new(3.0, 0.0))].into_iter());
        builder.add_row_entries(
            [(0, Complex64::new(4.0, 0.0)), (2, Complex64::new(5.0, 0.0))].into_iter(),
        );

        let csr = builder.finish();

        assert_eq!(csr.nnz(), 5);
        assert_relative_eq!(csr.get(0, 0).re, 1.0);
        assert_relative_eq!(csr.get(1, 1).re, 3.0);
    }

    #[test]
    fn test_csr_to_dense_roundtrip() {
        let original = array![
            [Complex64::new(1.0, 0.5), Complex64::new(0.0, 0.0)],
            [Complex64::new(2.0, -1.0), Complex64::new(3.0, 0.0)],
        ];

        let csr = CsrMatrix::from_dense(&original, 1e-15);
        let recovered = csr.to_dense();

        for i in 0..2 {
            for j in 0..2 {
                assert_relative_eq!(
                    (original[[i, j]] - recovered[[i, j]]).norm(),
                    0.0,
                    epsilon = 1e-10
                );
            }
        }
    }

    #[test]
    fn test_linear_operator_impl() {
        let dense = array![
            [Complex64::new(1.0, 0.0), Complex64::new(2.0, 0.0)],
            [Complex64::new(3.0, 0.0), Complex64::new(4.0, 0.0)],
        ];

        let csr = CsrMatrix::from_dense(&dense, 1e-15);
        let x = array![Complex64::new(1.0, 0.0), Complex64::new(2.0, 0.0)];

        // Test via LinearOperator trait
        let y = csr.apply(&x);
        assert_relative_eq!(y[0].re, 5.0, epsilon = 1e-10);
        assert_relative_eq!(y[1].re, 11.0, epsilon = 1e-10);

        assert!(csr.is_square());
        assert_eq!(csr.num_rows(), 2);
        assert_eq!(csr.num_cols(), 2);
    }

    #[test]
    fn test_f64_csr() {
        let dense = array![[1.0_f64, 2.0], [3.0, 4.0],];

        let csr = CsrMatrix::from_dense(&dense, 1e-15);
        let x = array![1.0_f64, 2.0];

        let y = csr.matvec(&x);
        assert_relative_eq!(y[0], 5.0, epsilon = 1e-10);
        assert_relative_eq!(y[1], 11.0, epsilon = 1e-10);
    }
}