Merging upstream version 126.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

4 files changed, 1032 insertions, 0 deletions

diff --git a/third_party/rust/smawk/src/brute_force.rs b/third_party/rust/smawk/src/brute_force.rs
new file mode 100644
index 0000000000..1ec0ca35a7
--- /dev/null
+++ b/third_party/rust/smawk/src/brute_force.rs
@@ -0,0 +1,150 @@
+//! Brute-force algorithm for finding column minima.
+//!
+//! The functions here are mostly meant to be used for testing
+//! correctness of the SMAWK implementation.
+//!
+//! **Note: this module is only available if you enable the `ndarray`
+//! Cargo feature.**
+
+use ndarray::{Array2, ArrayView1};
+
+/// Compute lane minimum by brute force.
+///
+/// This does a simple scan through the lane (row or column).
+#[inline]
+pub fn lane_minimum<T: Ord>(lane: ArrayView1<'_, T>) -> usize {
+    lane.iter()
+        .enumerate()
+        .min_by_key(|&(idx, elem)| (elem, idx))
+        .map(|(idx, _)| idx)
+        .expect("empty lane in matrix")
+}
+
+/// Compute row minima by brute force in O(*mn*) time.
+///
+/// This function implements a simple brute-force approach where each
+/// matrix row is scanned completely. This means that the function
+/// works on all matrices, not just Monge matrices.
+///
+/// # Examples
+///
+/// ```
+/// let matrix = ndarray::arr2(&[[4, 2, 4, 3],
+///                              [5, 3, 5, 3],
+///                              [5, 3, 3, 1]]);
+/// assert_eq!(smawk::brute_force::row_minima(&matrix),
+///            vec![1, 1, 3]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero columns.
+pub fn row_minima<T: Ord>(matrix: &Array2<T>) -> Vec<usize> {
+    matrix.rows().into_iter().map(lane_minimum).collect()
+}
+
+/// Compute column minima by brute force in O(*mn*) time.
+///
+/// This function implements a simple brute-force approach where each
+/// matrix column is scanned completely. This means that the function
+/// works on all matrices, not just Monge matrices.
+///
+/// # Examples
+///
+/// ```
+/// let matrix = ndarray::arr2(&[[4, 2, 4, 3],
+///                              [5, 3, 5, 3],
+///                              [5, 3, 3, 1]]);
+/// assert_eq!(smawk::brute_force::column_minima(&matrix),
+///            vec![0, 0, 2, 2]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero rows.
+pub fn column_minima<T: Ord>(matrix: &Array2<T>) -> Vec<usize> {
+    matrix.columns().into_iter().map(lane_minimum).collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ndarray::arr2;
+
+    #[test]
+    fn brute_force_1x1() {
+        let matrix = arr2(&[[2]]);
+        let minima = vec![0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_2x1() {
+        let matrix = arr2(&[
+            [3], //
+            [2],
+        ]);
+        let minima = vec![0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_1x2() {
+        let matrix = arr2(&[[2, 1]]);
+        let minima = vec![1];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_2x2() {
+        let matrix = arr2(&[
+            [3, 2], //
+            [2, 1],
+        ]);
+        let minima = vec![1, 1];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_3x3() {
+        let matrix = arr2(&[
+            [3, 4, 4], //
+            [3, 4, 4],
+            [2, 3, 3],
+        ]);
+        let minima = vec![0, 0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_4x4() {
+        let matrix = arr2(&[
+            [4, 5, 5, 5], //
+            [2, 3, 3, 3],
+            [2, 3, 3, 3],
+            [2, 2, 2, 2],
+        ]);
+        let minima = vec![0, 0, 0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn brute_force_5x5() {
+        let matrix = arr2(&[
+            [3, 2, 4, 5, 6],
+            [2, 1, 3, 3, 4],
+            [2, 1, 3, 3, 4],
+            [3, 2, 4, 3, 4],
+            [4, 3, 2, 1, 1],
+        ]);
+        let minima = vec![1, 1, 1, 1, 3];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+}
diff --git a/third_party/rust/smawk/src/lib.rs b/third_party/rust/smawk/src/lib.rs
new file mode 100644
index 0000000000..367d0337dc
--- /dev/null
+++ b/third_party/rust/smawk/src/lib.rs
@@ -0,0 +1,570 @@
+//! This crate implements various functions that help speed up dynamic
+//! programming, most importantly the SMAWK algorithm for finding row
+//! or column minima in a totally monotone matrix with *m* rows and
+//! *n* columns in time O(*m* + *n*). This is much better than the
+//! brute force solution which would take O(*mn*). When *m* and *n*
+//! are of the same order, this turns a quadratic function into a
+//! linear function.
+//!
+//! # Examples
+//!
+//! Computing the column minima of an *m* ✕ *n* Monge matrix can be
+//! done efficiently with `smawk::column_minima`:
+//!
+//! ```
+//! use smawk::Matrix;
+//!
+//! let matrix = vec![
+//!     vec![3, 2, 4, 5, 6],
+//!     vec![2, 1, 3, 3, 4],
+//!     vec![2, 1, 3, 3, 4],
+//!     vec![3, 2, 4, 3, 4],
+//!     vec![4, 3, 2, 1, 1],
+//! ];
+//! let minima = vec![1, 1, 4, 4, 4];
+//! assert_eq!(smawk::column_minima(&matrix), minima);
+//! ```
+//!
+//! The `minima` vector gives the index of the minimum value per
+//! column, so `minima[0] == 1` since the minimum value in the first
+//! column is 2 (row 1). Note that the smallest row index is returned.
+//!
+//! # Definitions
+//!
+//! Some of the functions in this crate only work on matrices that are
+//! *totally monotone*, which we will define below.
+//!
+//! ## Monotone Matrices
+//!
+//! We start with a helper definition. Given an *m* ✕ *n* matrix `M`,
+//! we say that `M` is *monotone* when the minimum value of row `i` is
+//! found to the left of the minimum value in row `i'` where `i < i'`.
+//!
+//! More formally, if we let `rm(i)` denote the column index of the
+//! left-most minimum value in row `i`, then we have
+//!
+//! ```text
+//! rm(0) ≤ rm(1) ≤ ... ≤ rm(m - 1)
+//! ```
+//!
+//! This means that as you go down the rows from top to bottom, the
+//! row-minima proceed from left to right.
+//!
+//! The algorithms in this crate deal with finding such row- and
+//! column-minima.
+//!
+//! ## Totally Monotone Matrices
+//!
+//! We say that a matrix `M` is *totally monotone* when every
+//! sub-matrix is monotone. A sub-matrix is formed by the intersection
+//! of any two rows `i < i'` and any two columns `j < j'`.
+//!
+//! This is often expressed as via this equivalent condition:
+//!
+//! ```text
+//! M[i, j] > M[i, j']  =>  M[i', j] > M[i', j']
+//! ```
+//!
+//! for all `i < i'` and `j < j'`.
+//!
+//! ## Monge Property for Matrices
+//!
+//! A matrix `M` is said to fulfill the *Monge property* if
+//!
+//! ```text
+//! M[i, j] + M[i', j'] ≤ M[i, j'] + M[i', j]
+//! ```
+//!
+//! for all `i < i'` and `j < j'`. This says that given any rectangle
+//! in the matrix, the sum of the top-left and bottom-right corners is
+//! less than or equal to the sum of the bottom-left and upper-right
+//! corners.
+//!
+//! All Monge matrices are totally monotone, so it is enough to
+//! establish that the Monge property holds in order to use a matrix
+//! with the functions in this crate. If your program is dealing with
+//! unknown inputs, it can use [`monge::is_monge`] to verify that a
+//! matrix is a Monge matrix.
+
+#![doc(html_root_url = "https://docs.rs/smawk/0.3.2")]
+// The s! macro from ndarray uses unsafe internally, so we can only
+// forbid unsafe code when building with the default features.
+#![cfg_attr(not(feature = "ndarray"), forbid(unsafe_code))]
+
+#[cfg(feature = "ndarray")]
+pub mod brute_force;
+pub mod monge;
+#[cfg(feature = "ndarray")]
+pub mod recursive;
+
+/// Minimal matrix trait for two-dimensional arrays.
+///
+/// This provides the functionality needed to represent a read-only
+/// numeric matrix. You can query the size of the matrix and access
+/// elements. Modeled after [`ndarray::Array2`] from the [ndarray
+/// crate](https://crates.io/crates/ndarray).
+///
+/// Enable the `ndarray` Cargo feature if you want to use it with
+/// `ndarray::Array2`.
+pub trait Matrix<T: Copy> {
+    /// Return the number of rows.
+    fn nrows(&self) -> usize;
+    /// Return the number of columns.
+    fn ncols(&self) -> usize;
+    /// Return a matrix element.
+    fn index(&self, row: usize, column: usize) -> T;
+}
+
+/// Simple and inefficient matrix representation used for doctest
+/// examples and simple unit tests.
+///
+/// You should prefer implementing it yourself, or you can enable the
+/// `ndarray` Cargo feature and use the provided implementation for
+/// [`ndarray::Array2`].
+impl<T: Copy> Matrix<T> for Vec<Vec<T>> {
+    fn nrows(&self) -> usize {
+        self.len()
+    }
+    fn ncols(&self) -> usize {
+        self[0].len()
+    }
+    fn index(&self, row: usize, column: usize) -> T {
+        self[row][column]
+    }
+}
+
+/// Adapting [`ndarray::Array2`] to the `Matrix` trait.
+///
+/// **Note: this implementation is only available if you enable the
+/// `ndarray` Cargo feature.**
+#[cfg(feature = "ndarray")]
+impl<T: Copy> Matrix<T> for ndarray::Array2<T> {
+    #[inline]
+    fn nrows(&self) -> usize {
+        self.nrows()
+    }
+    #[inline]
+    fn ncols(&self) -> usize {
+        self.ncols()
+    }
+    #[inline]
+    fn index(&self, row: usize, column: usize) -> T {
+        self[[row, column]]
+    }
+}
+
+/// Compute row minima in O(*m* + *n*) time.
+///
+/// This implements the [SMAWK algorithm] for efficiently finding row
+/// minima in a totally monotone matrix.
+///
+/// The SMAWK algorithm is from Agarwal, Klawe, Moran, Shor, and
+/// Wilbur, *Geometric applications of a matrix searching algorithm*,
+/// Algorithmica 2, pp. 195-208 (1987) and the code here is a
+/// translation [David Eppstein's Python code][pads].
+///
+/// Running time on an *m* ✕ *n* matrix: O(*m* + *n*).
+///
+/// # Examples
+///
+/// ```
+/// use smawk::Matrix;
+/// let matrix = vec![vec![4, 2, 4, 3],
+///                   vec![5, 3, 5, 3],
+///                   vec![5, 3, 3, 1]];
+/// assert_eq!(smawk::row_minima(&matrix),
+///            vec![1, 1, 3]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero columns.
+///
+/// [pads]: https://github.com/jfinkels/PADS/blob/master/pads/smawk.py
+/// [SMAWK algorithm]: https://en.wikipedia.org/wiki/SMAWK_algorithm
+pub fn row_minima<T: PartialOrd + Copy, M: Matrix<T>>(matrix: &M) -> Vec<usize> {
+    // Benchmarking shows that SMAWK performs roughly the same on row-
+    // and column-major matrices.
+    let mut minima = vec![0; matrix.nrows()];
+    smawk_inner(
+        &|j, i| matrix.index(i, j),
+        &(0..matrix.ncols()).collect::<Vec<_>>(),
+        &(0..matrix.nrows()).collect::<Vec<_>>(),
+        &mut minima,
+    );
+    minima
+}
+
+#[deprecated(since = "0.3.2", note = "Please use `row_minima` instead.")]
+pub fn smawk_row_minima<T: PartialOrd + Copy, M: Matrix<T>>(matrix: &M) -> Vec<usize> {
+    row_minima(matrix)
+}
+
+/// Compute column minima in O(*m* + *n*) time.
+///
+/// This implements the [SMAWK algorithm] for efficiently finding
+/// column minima in a totally monotone matrix.
+///
+/// The SMAWK algorithm is from Agarwal, Klawe, Moran, Shor, and
+/// Wilbur, *Geometric applications of a matrix searching algorithm*,
+/// Algorithmica 2, pp. 195-208 (1987) and the code here is a
+/// translation [David Eppstein's Python code][pads].
+///
+/// Running time on an *m* ✕ *n* matrix: O(*m* + *n*).
+///
+/// # Examples
+///
+/// ```
+/// use smawk::Matrix;
+/// let matrix = vec![vec![4, 2, 4, 3],
+///                   vec![5, 3, 5, 3],
+///                   vec![5, 3, 3, 1]];
+/// assert_eq!(smawk::column_minima(&matrix),
+///            vec![0, 0, 2, 2]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero rows.
+///
+/// [SMAWK algorithm]: https://en.wikipedia.org/wiki/SMAWK_algorithm
+/// [pads]: https://github.com/jfinkels/PADS/blob/master/pads/smawk.py
+pub fn column_minima<T: PartialOrd + Copy, M: Matrix<T>>(matrix: &M) -> Vec<usize> {
+    let mut minima = vec![0; matrix.ncols()];
+    smawk_inner(
+        &|i, j| matrix.index(i, j),
+        &(0..matrix.nrows()).collect::<Vec<_>>(),
+        &(0..matrix.ncols()).collect::<Vec<_>>(),
+        &mut minima,
+    );
+    minima
+}
+
+#[deprecated(since = "0.3.2", note = "Please use `column_minima` instead.")]
+pub fn smawk_column_minima<T: PartialOrd + Copy, M: Matrix<T>>(matrix: &M) -> Vec<usize> {
+    column_minima(matrix)
+}
+
+/// Compute column minima in the given area of the matrix. The
+/// `minima` slice is updated inplace.
+fn smawk_inner<T: PartialOrd + Copy, M: Fn(usize, usize) -> T>(
+    matrix: &M,
+    rows: &[usize],
+    cols: &[usize],
+    minima: &mut [usize],
+) {
+    if cols.is_empty() {
+        return;
+    }
+
+    let mut stack = Vec::with_capacity(cols.len());
+    for r in rows {
+        // TODO: use stack.last() instead of stack.is_empty() etc
+        while !stack.is_empty()
+            && matrix(stack[stack.len() - 1], cols[stack.len() - 1])
+                > matrix(*r, cols[stack.len() - 1])
+        {
+            stack.pop();
+        }
+        if stack.len() != cols.len() {
+            stack.push(*r);
+        }
+    }
+    let rows = &stack;
+
+    let mut odd_cols = Vec::with_capacity(1 + cols.len() / 2);
+    for (idx, c) in cols.iter().enumerate() {
+        if idx % 2 == 1 {
+            odd_cols.push(*c);
+        }
+    }
+
+    smawk_inner(matrix, rows, &odd_cols, minima);
+
+    let mut r = 0;
+    for (c, &col) in cols.iter().enumerate().filter(|(c, _)| c % 2 == 0) {
+        let mut row = rows[r];
+        let last_row = if c == cols.len() - 1 {
+            rows[rows.len() - 1]
+        } else {
+            minima[cols[c + 1]]
+        };
+        let mut pair = (matrix(row, col), row);
+        while row != last_row {
+            r += 1;
+            row = rows[r];
+            if (matrix(row, col), row) < pair {
+                pair = (matrix(row, col), row);
+            }
+        }
+        minima[col] = pair.1;
+    }
+}
+
+/// Compute upper-right column minima in O(*m* + *n*) time.
+///
+/// The input matrix must be totally monotone.
+///
+/// The function returns a vector of `(usize, T)`. The `usize` in the
+/// tuple at index `j` tells you the row of the minimum value in
+/// column `j` and the `T` value is minimum value itself.
+///
+/// The algorithm only considers values above the main diagonal, which
+/// means that it computes values `v(j)` where:
+///
+/// ```text
+/// v(0) = initial
+/// v(j) = min { M[i, j] | i < j } for j > 0
+/// ```
+///
+/// If we let `r(j)` denote the row index of the minimum value in
+/// column `j`, the tuples in the result vector become `(r(j), M[r(j),
+/// j])`.
+///
+/// The algorithm is an *online* algorithm, in the sense that `matrix`
+/// function can refer back to previously computed column minima when
+/// determining an entry in the matrix. The guarantee is that we only
+/// call `matrix(i, j)` after having computed `v(i)`. This is
+/// reflected in the `&[(usize, T)]` argument to `matrix`, which grows
+/// as more and more values are computed.
+pub fn online_column_minima<T: Copy + PartialOrd, M: Fn(&[(usize, T)], usize, usize) -> T>(
+    initial: T,
+    size: usize,
+    matrix: M,
+) -> Vec<(usize, T)> {
+    let mut result = vec![(0, initial)];
+
+    // State used by the algorithm.
+    let mut finished = 0;
+    let mut base = 0;
+    let mut tentative = 0;
+
+    // Shorthand for evaluating the matrix. We need a macro here since
+    // we don't want to borrow the result vector.
+    macro_rules! m {
+        ($i:expr, $j:expr) => {{
+            assert!($i < $j, "(i, j) not above diagonal: ({}, {})", $i, $j);
+            assert!(
+                $i < size && $j < size,
+                "(i, j) out of bounds: ({}, {}), size: {}",
+                $i,
+                $j,
+                size
+            );
+            matrix(&result[..finished + 1], $i, $j)
+        }};
+    }
+
+    // Keep going until we have finished all size columns. Since the
+    // columns are zero-indexed, we're done when finished == size - 1.
+    while finished < size - 1 {
+        // First case: we have already advanced past the previous
+        // tentative value. We make a new tentative value by applying
+        // smawk_inner to the largest square submatrix that fits under
+        // the base.
+        let i = finished + 1;
+        if i > tentative {
+            let rows = (base..finished + 1).collect::<Vec<_>>();
+            tentative = std::cmp::min(finished + rows.len(), size - 1);
+            let cols = (finished + 1..tentative + 1).collect::<Vec<_>>();
+            let mut minima = vec![0; tentative + 1];
+            smawk_inner(&|i, j| m![i, j], &rows, &cols, &mut minima);
+            for col in cols {
+                let row = minima[col];
+                let v = m![row, col];
+                if col >= result.len() {
+                    result.push((row, v));
+                } else if v < result[col].1 {
+                    result[col] = (row, v);
+                }
+            }
+            finished = i;
+            continue;
+        }
+
+        // Second case: the new column minimum is on the diagonal. All
+        // subsequent ones will be at least as low, so we can clear
+        // out all our work from higher rows. As in the fourth case,
+        // the loss of tentative is amortized against the increase in
+        // base.
+        let diag = m![i - 1, i];
+        if diag < result[i].1 {
+            result[i] = (i - 1, diag);
+            base = i - 1;
+            tentative = i;
+            finished = i;
+            continue;
+        }
+
+        // Third case: row i-1 does not supply a column minimum in any
+        // column up to tentative. We simply advance finished while
+        // maintaining the invariant.
+        if m![i - 1, tentative] >= result[tentative].1 {
+            finished = i;
+            continue;
+        }
+
+        // Fourth and final case: a new column minimum at tentative.
+        // This allows us to make progress by incorporating rows prior
+        // to finished into the base. The base invariant holds because
+        // these rows cannot supply any later column minima. The work
+        // done when we last advanced tentative (and undone by this
+        // step) can be amortized against the increase in base.
+        base = i - 1;
+        tentative = i;
+        finished = i;
+    }
+
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn smawk_1x1() {
+        let matrix = vec![vec![2]];
+        assert_eq!(row_minima(&matrix), vec![0]);
+        assert_eq!(column_minima(&matrix), vec![0]);
+    }
+
+    #[test]
+    fn smawk_2x1() {
+        let matrix = vec![
+            vec![3], //
+            vec![2],
+        ];
+        assert_eq!(row_minima(&matrix), vec![0, 0]);
+        assert_eq!(column_minima(&matrix), vec![1]);
+    }
+
+    #[test]
+    fn smawk_1x2() {
+        let matrix = vec![vec![2, 1]];
+        assert_eq!(row_minima(&matrix), vec![1]);
+        assert_eq!(column_minima(&matrix), vec![0, 0]);
+    }
+
+    #[test]
+    fn smawk_2x2() {
+        let matrix = vec![
+            vec![3, 2], //
+            vec![2, 1],
+        ];
+        assert_eq!(row_minima(&matrix), vec![1, 1]);
+        assert_eq!(column_minima(&matrix), vec![1, 1]);
+    }
+
+    #[test]
+    fn smawk_3x3() {
+        let matrix = vec![
+            vec![3, 4, 4], //
+            vec![3, 4, 4],
+            vec![2, 3, 3],
+        ];
+        assert_eq!(row_minima(&matrix), vec![0, 0, 0]);
+        assert_eq!(column_minima(&matrix), vec![2, 2, 2]);
+    }
+
+    #[test]
+    fn smawk_4x4() {
+        let matrix = vec![
+            vec![4, 5, 5, 5], //
+            vec![2, 3, 3, 3],
+            vec![2, 3, 3, 3],
+            vec![2, 2, 2, 2],
+        ];
+        assert_eq!(row_minima(&matrix), vec![0, 0, 0, 0]);
+        assert_eq!(column_minima(&matrix), vec![1, 3, 3, 3]);
+    }
+
+    #[test]
+    fn smawk_5x5() {
+        let matrix = vec![
+            vec![3, 2, 4, 5, 6],
+            vec![2, 1, 3, 3, 4],
+            vec![2, 1, 3, 3, 4],
+            vec![3, 2, 4, 3, 4],
+            vec![4, 3, 2, 1, 1],
+        ];
+        assert_eq!(row_minima(&matrix), vec![1, 1, 1, 1, 3]);
+        assert_eq!(column_minima(&matrix), vec![1, 1, 4, 4, 4]);
+    }
+
+    #[test]
+    fn online_1x1() {
+        let matrix = vec![vec![0]];
+        let minima = vec![(0, 0)];
+        assert_eq!(online_column_minima(0, 1, |_, i, j| matrix[i][j]), minima);
+    }
+
+    #[test]
+    fn online_2x2() {
+        let matrix = vec![
+            vec![0, 2], //
+            vec![0, 0],
+        ];
+        let minima = vec![(0, 0), (0, 2)];
+        assert_eq!(online_column_minima(0, 2, |_, i, j| matrix[i][j]), minima);
+    }
+
+    #[test]
+    fn online_3x3() {
+        let matrix = vec![
+            vec![0, 4, 4], //
+            vec![0, 0, 4],
+            vec![0, 0, 0],
+        ];
+        let minima = vec![(0, 0), (0, 4), (0, 4)];
+        assert_eq!(online_column_minima(0, 3, |_, i, j| matrix[i][j]), minima);
+    }
+
+    #[test]
+    fn online_4x4() {
+        let matrix = vec![
+            vec![0, 5, 5, 5], //
+            vec![0, 0, 3, 3],
+            vec![0, 0, 0, 3],
+            vec![0, 0, 0, 0],
+        ];
+        let minima = vec![(0, 0), (0, 5), (1, 3), (1, 3)];
+        assert_eq!(online_column_minima(0, 4, |_, i, j| matrix[i][j]), minima);
+    }
+
+    #[test]
+    fn online_5x5() {
+        let matrix = vec![
+            vec![0, 2, 4, 6, 7],
+            vec![0, 0, 3, 4, 5],
+            vec![0, 0, 0, 3, 4],
+            vec![0, 0, 0, 0, 4],
+            vec![0, 0, 0, 0, 0],
+        ];
+        let minima = vec![(0, 0), (0, 2), (1, 3), (2, 3), (2, 4)];
+        assert_eq!(online_column_minima(0, 5, |_, i, j| matrix[i][j]), minima);
+    }
+
+    #[test]
+    fn smawk_works_with_partial_ord() {
+        let matrix = vec![
+            vec![3.0, 2.0], //
+            vec![2.0, 1.0],
+        ];
+        assert_eq!(row_minima(&matrix), vec![1, 1]);
+        assert_eq!(column_minima(&matrix), vec![1, 1]);
+    }
+
+    #[test]
+    fn online_works_with_partial_ord() {
+        let matrix = vec![
+            vec![0.0, 2.0], //
+            vec![0.0, 0.0],
+        ];
+        let minima = vec![(0, 0.0), (0, 2.0)];
+        assert_eq!(
+            online_column_minima(0.0, 2, |_, i: usize, j: usize| matrix[i][j]),
+            minima
+        );
+    }
+}
diff --git a/third_party/rust/smawk/src/monge.rs b/third_party/rust/smawk/src/monge.rs
new file mode 100644
index 0000000000..dbc80e1517
--- /dev/null
+++ b/third_party/rust/smawk/src/monge.rs
@@ -0,0 +1,121 @@
+//! Functions for generating and checking Monge arrays.
+//!
+//! The functions here are mostly meant to be used for testing
+//! correctness of the SMAWK implementation.
+
+use crate::Matrix;
+use std::num::Wrapping;
+use std::ops::Add;
+
+/// Verify that a matrix is a Monge matrix.
+///
+/// A [Monge matrix] \(or array) is a matrix where the following
+/// inequality holds:
+///
+/// ```text
+/// M[i, j] + M[i', j'] <= M[i, j'] + M[i', j]  for all i < i', j < j'
+/// ```
+///
+/// The inequality says that the sum of the main diagonal is less than
+/// the sum of the antidiagonal. Checking this condition is done by
+/// checking *n* ✕ *m* submatrices, so the running time is O(*mn*).
+///
+/// [Monge matrix]: https://en.wikipedia.org/wiki/Monge_array
+pub fn is_monge<T: Ord + Copy, M: Matrix<T>>(matrix: &M) -> bool
+where
+    Wrapping<T>: Add<Output = Wrapping<T>>,
+{
+    /// Returns `Ok(a + b)` if the computation can be done without
+    /// overflow, otherwise `Err(a + b - T::MAX - 1)` is returned.
+    fn checked_add<T: Ord + Copy>(a: Wrapping<T>, b: Wrapping<T>) -> Result<T, T>
+    where
+        Wrapping<T>: Add<Output = Wrapping<T>>,
+    {
+        let sum = a + b;
+        if sum < a {
+            Err(sum.0)
+        } else {
+            Ok(sum.0)
+        }
+    }
+
+    (0..matrix.nrows() - 1)
+        .flat_map(|row| (0..matrix.ncols() - 1).map(move |col| (row, col)))
+        .all(|(row, col)| {
+            let top_left = Wrapping(matrix.index(row, col));
+            let top_right = Wrapping(matrix.index(row, col + 1));
+            let bot_left = Wrapping(matrix.index(row + 1, col));
+            let bot_right = Wrapping(matrix.index(row + 1, col + 1));
+
+            match (
+                checked_add(top_left, bot_right),
+                checked_add(bot_left, top_right),
+            ) {
+                (Ok(a), Ok(b)) => a <= b,   // No overflow.
+                (Err(a), Err(b)) => a <= b, // Double overflow.
+                (Ok(_), Err(_)) => true,    // Anti-diagonal overflow.
+                (Err(_), Ok(_)) => false,   // Main diagonal overflow.
+            }
+        })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_monge_handles_overflow() {
+        // The x + y <= z + w computations will overflow for an u8
+        // matrix unless is_monge is careful.
+        let matrix: Vec<Vec<u8>> = vec![
+            vec![200, 200, 200, 200],
+            vec![200, 200, 200, 200],
+            vec![200, 200, 200, 200],
+        ];
+        assert!(is_monge(&matrix));
+    }
+
+    #[test]
+    fn monge_constant_rows() {
+        let matrix = vec![
+            vec![42, 42, 42, 42],
+            vec![0, 0, 0, 0],
+            vec![100, 100, 100, 100],
+            vec![1000, 1000, 1000, 1000],
+        ];
+        assert!(is_monge(&matrix));
+    }
+
+    #[test]
+    fn monge_constant_cols() {
+        let matrix = vec![
+            vec![42, 0, 100, 1000],
+            vec![42, 0, 100, 1000],
+            vec![42, 0, 100, 1000],
+            vec![42, 0, 100, 1000],
+        ];
+        assert!(is_monge(&matrix));
+    }
+
+    #[test]
+    fn monge_upper_right() {
+        let matrix = vec![
+            vec![10, 10, 42, 42, 42],
+            vec![10, 10, 42, 42, 42],
+            vec![10, 10, 10, 10, 10],
+            vec![10, 10, 10, 10, 10],
+        ];
+        assert!(is_monge(&matrix));
+    }
+
+    #[test]
+    fn monge_lower_left() {
+        let matrix = vec![
+            vec![10, 10, 10, 10, 10],
+            vec![10, 10, 10, 10, 10],
+            vec![42, 42, 42, 10, 10],
+            vec![42, 42, 42, 10, 10],
+        ];
+        assert!(is_monge(&matrix));
+    }
+}
diff --git a/third_party/rust/smawk/src/recursive.rs b/third_party/rust/smawk/src/recursive.rs
new file mode 100644
index 0000000000..9df8b9c824
--- /dev/null
+++ b/third_party/rust/smawk/src/recursive.rs
@@ -0,0 +1,191 @@
+//! Recursive algorithm for finding column minima.
+//!
+//! The functions here are mostly meant to be used for testing
+//! correctness of the SMAWK implementation.
+//!
+//! **Note: this module is only available if you enable the `ndarray`
+//! Cargo feature.**
+
+use ndarray::{s, Array2, ArrayView2, Axis};
+
+/// Compute row minima in O(*m* + *n* log *m*) time.
+///
+/// This function computes row minima in a totally monotone matrix
+/// using a recursive algorithm.
+///
+/// Running time on an *m* ✕ *n* matrix: O(*m* + *n* log *m*).
+///
+/// # Examples
+///
+/// ```
+/// let matrix = ndarray::arr2(&[[4, 2, 4, 3],
+///                              [5, 3, 5, 3],
+///                              [5, 3, 3, 1]]);
+/// assert_eq!(smawk::recursive::row_minima(&matrix),
+///            vec![1, 1, 3]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero columns.
+pub fn row_minima<T: Ord>(matrix: &Array2<T>) -> Vec<usize> {
+    let mut minima = vec![0; matrix.nrows()];
+    recursive_inner(matrix.view(), &|| Direction::Row, 0, &mut minima);
+    minima
+}
+
+/// Compute column minima in O(*n* + *m* log *n*) time.
+///
+/// This function computes column minima in a totally monotone matrix
+/// using a recursive algorithm.
+///
+/// Running time on an *m* ✕ *n* matrix: O(*n* + *m* log *n*).
+///
+/// # Examples
+///
+/// ```
+/// let matrix = ndarray::arr2(&[[4, 2, 4, 3],
+///                              [5, 3, 5, 3],
+///                              [5, 3, 3, 1]]);
+/// assert_eq!(smawk::recursive::column_minima(&matrix),
+///            vec![0, 0, 2, 2]);
+/// ```
+///
+/// # Panics
+///
+/// It is an error to call this on a matrix with zero rows.
+pub fn column_minima<T: Ord>(matrix: &Array2<T>) -> Vec<usize> {
+    let mut minima = vec![0; matrix.ncols()];
+    recursive_inner(matrix.view(), &|| Direction::Column, 0, &mut minima);
+    minima
+}
+
+/// The type of minima (row or column) we compute.
+enum Direction {
+    Row,
+    Column,
+}
+
+/// Compute the minima along the given direction (`Direction::Row` for
+/// row minima and `Direction::Column` for column minima).
+///
+/// The direction is given as a generic function argument to allow
+/// monomorphization to kick in. The function calls will be inlined
+/// and optimized away and the result is that the compiler generates
+/// differnet code for finding row and column minima.
+fn recursive_inner<T: Ord, F: Fn() -> Direction>(
+    matrix: ArrayView2<'_, T>,
+    dir: &F,
+    offset: usize,
+    minima: &mut [usize],
+) {
+    if matrix.is_empty() {
+        return;
+    }
+
+    let axis = match dir() {
+        Direction::Row => Axis(0),
+        Direction::Column => Axis(1),
+    };
+    let mid = matrix.len_of(axis) / 2;
+    let min_idx = crate::brute_force::lane_minimum(matrix.index_axis(axis, mid));
+    minima[mid] = offset + min_idx;
+
+    if mid == 0 {
+        return; // Matrix has a single row or column, so we're done.
+    }
+
+    let top_left = match dir() {
+        Direction::Row => matrix.slice(s![..mid, ..(min_idx + 1)]),
+        Direction::Column => matrix.slice(s![..(min_idx + 1), ..mid]),
+    };
+    let bot_right = match dir() {
+        Direction::Row => matrix.slice(s![(mid + 1).., min_idx..]),
+        Direction::Column => matrix.slice(s![min_idx.., (mid + 1)..]),
+    };
+    recursive_inner(top_left, dir, offset, &mut minima[..mid]);
+    recursive_inner(bot_right, dir, offset + min_idx, &mut minima[mid + 1..]);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ndarray::arr2;
+
+    #[test]
+    fn recursive_1x1() {
+        let matrix = arr2(&[[2]]);
+        let minima = vec![0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_2x1() {
+        let matrix = arr2(&[
+            [3], //
+            [2],
+        ]);
+        let minima = vec![0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_1x2() {
+        let matrix = arr2(&[[2, 1]]);
+        let minima = vec![1];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_2x2() {
+        let matrix = arr2(&[
+            [3, 2], //
+            [2, 1],
+        ]);
+        let minima = vec![1, 1];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_3x3() {
+        let matrix = arr2(&[
+            [3, 4, 4], //
+            [3, 4, 4],
+            [2, 3, 3],
+        ]);
+        let minima = vec![0, 0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_4x4() {
+        let matrix = arr2(&[
+            [4, 5, 5, 5], //
+            [2, 3, 3, 3],
+            [2, 3, 3, 3],
+            [2, 2, 2, 2],
+        ]);
+        let minima = vec![0, 0, 0, 0];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+
+    #[test]
+    fn recursive_5x5() {
+        let matrix = arr2(&[
+            [3, 2, 4, 5, 6],
+            [2, 1, 3, 3, 4],
+            [2, 1, 3, 3, 4],
+            [3, 2, 4, 3, 4],
+            [4, 3, 2, 1, 1],
+        ]);
+        let minima = vec![1, 1, 1, 1, 3];
+        assert_eq!(row_minima(&matrix), minima);
+        assert_eq!(column_minima(&matrix.reversed_axes()), minima);
+    }
+}