Rust 2D sub-window iterator - matrix

Question

I am using a Vec to store a 2D (row major) matrix of values. I would like to iterate over this matrix with a sliding 2D sub-window to apply a filter (which is unfortunately non-separable).
I have seen in the slice documentation that a windows function exists, which is what I want but in 2 dimensions.
I thought about implementing this as:
fn main() {
// 4 rows 3 columns
let dim: (usize, usize) = (4, 3);
// Place-holder matrix
#[rustfmt::skip]
let mat = vec![0, 1, 2,
3, 4, 5,
6, 7, 8,
9, 10, 11];
// 2D index to 1D index
let linearize = |r, c| r * dim.1 + c;
// The dimensions of my sub-window
let win_size: usize = 2;
// Calculate the bounds for which the top left corner of each window may exist
let bounds: (usize, usize) = (dim.0 - win_size + 1, dim.1 - win_size + 1);
// Convert window 1D index into a 2D index
let split = |i| (i / win_size, i % win_size);
// Iterate over all the top left corners
let window_2d = (0..bounds.0 * bounds.1).map(|i| {
// Get the 2D index of the top left corner
let (r, c) = (i / bounds.1, i % bounds.1);
// Borrow the matrix, so our closure may own the reference
let bmat = &mat;
// Return an iterator for this window
return (0..win_size * win_size).map(move |x| {
let (wr, wc) = split(x);
return bmat[linearize(wr + r, wc + c)];
});
});
// Print the windows out
window_2d.for_each(|it| {
print!("[ ");
it.for_each(|x| print!("{} ", x));
println!("]");
});
}
Essentially creating an iterator over a range of indices and then mapping to the square bracket operator of the matrix.
As far as I know, this is going to have the overhead of a bounds check for each deref of the iterator.
I'm wondering if there's an alternative which would elide the bounds checks? Maybe using a combination of chunks, windows and zip, to chunk the matrix into rows, each with a sliding window, then zip the row's windows and flatten the result?
Thanks!
Edit:
I'm not looking to simply iterate over a 2D array, I want to slide a 2D window over the array, similar to how the std::slice::windows function works.

Answer 1

The best I've got for now is wrapping the matrix access in an unsafe block to elide the bounds check.
With some other misc changes, this is the full example now:
fn split_factory(cols: usize) -> impl Fn(usize) -> (usize, usize) {
// Declaring that cols must be positive allows more aggressive optimisation of div and mod.
if cols < 1 {
unreachable!()
}
move |i| (i / cols, i % cols)
}
fn main() {
// 4 rows 3 columns
let dim: (usize, usize) = (4, 3);
// Place-holder matrix
#[rustfmt::skip]
let mat = vec![0, 1, 2,
3, 4, 5,
6, 7, 8,
9, 10, 11];
// The dimensions of my sub-window
let win_dim = (3usize, 2usize);
// Calculate the bounds for which the top left corner of each window may exist
let bounds = (dim.0 - win_dim.0 + 1, dim.1 - win_dim.1 + 1);
// Iterate over all the top left corners
let convolution_iter = (0..bounds.0 * bounds.1)
.map(split_factory(bounds.1))
.map(|(r, c)| {
// Borrow the matrix, so our closure may own the reference
let bmat = &mat;
// Return an iterator for this window
return (0..win_dim.0 * win_dim.1)
.map(split_factory(win_dim.1))
.map(move |(wr, wc)| {
let px = (wr + r) * dim.1 + (wc + c);
(px, unsafe { *bmat.get_unchecked(px) })
});
});
// Print the windows out (badly...)
convolution_iter.for_each(|it| println!("{:?}", it.collect::<Vec<(usize, i32)>>()));
}
Would still be nicer to avoid unsafe and the indirection of looking up the matrix with indices.