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I managed to find a way to rotate a slice in-place (previous question on SO). The slice is linear, but represents a 2d square of elements.

Is this approach efficient?

Please check the previous question linked above if anything is unclear about the algorithm.

Code

enum Rotation {
    Clockwise,
    Counterclockwise,
}

fn rotate_square_slice<T>(slice: &mut [T], s: usize, rotation: Rotation) {
    // iterate ringwise, from outer to inner
    // skip center when size % 2 == 1
    for r in 0..s / 2 { 
        // for all unique indices under rotational symmetry ...
        for i in 0..s - (2 * r) - 1{
            // ... get their 4 corresponding positions ...
            let a = s * (   r   ) +   r+i   ;
            let b = s * (  r+i  ) +  s-r-1  ;
            let c = s * ( s-r-1 ) + s-r-i-1 ;
            let d = s * (s-r-i-1) +    r    ;

            //... and swap them in the correct direction.
            match rotation {
                Rotation::Clockwise => {
                    slice.swap(a, b);
                    slice.swap(a, c);
                    slice.swap(a, d);
                },
                Rotation::Counterclockwise => {
                    slice.swap(a, b);
                    slice.swap(c, d);
                    slice.swap(b, d);
                }
            }
        }
    }
}
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