sukker 2.1.2

Linear Algebra and Matrices made easy!
Documentation 
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use std::{error::Error, ops::RangeInclusive, str::FromStr};

use rayon::prelude::*;

use crate::{at, Matrix, MatrixElement};

pub fn swap(lhs: &mut usize, rhs: &mut usize) {
    let temp = *lhs;
    *lhs = *rhs;
    *rhs = temp;
}

// simd
impl<'a, T> Matrix<'a, T>
where
    T: MatrixElement + 'a,
    <T as FromStr>::Err: Error + 'static,
    Vec<T>: IntoParallelIterator,
    Vec<&'a T>: IntoParallelRefIterator<'a>,
{
    pub fn determinant_helper(&self) -> T {
        match self.nrows {
            1 => self.at(0, 0),
            2 => Self::det_2x2(self),
            3 => Self::det_3x3(self),
            n => Self::det_nxn(self.data.clone(), n),
        }
    }

    // General helper function calling out to other matmuls based on target architecture
    pub fn matmul_helper(&self, other: &Self) -> Self {
        match (self.shape(), other.shape()) {
            ((1, 2), (2, 1)) => return self.onetwo_by_twoone(other),
            ((2, 2), (2, 1)) => return self.twotwo_by_twoone(other),
            ((1, 2), (2, 2)) => return self.onetwo_by_twotwo(other),
            ((2, 2), (2, 2)) => return self.twotwo_by_twotwo(other),
            _ => {}
        };

        // Target Detection

        // if let Some(result) = optim::get_optimized_matmul(self, other) {
        //     return result;
        // }

        let blck_size = Self::get_block_size(self, other);

        // println!("BS: {}", blck_size);

        if self.shape() == other.shape() {
            // Calculated from lowest possible size where
            // nrows & blck_size == 0.
            // Block size will never be more than 50
            return Self::blocked_matmul(self, other, blck_size);
        }

        Self::optimized_blocked_matmul(self, other, blck_size)
    }

    // Calculate efficient blocksize
    #[inline(always)]
    pub fn get_block_size(&self, other: &Self) -> usize {
        let range = Self::get_range_for_block_size(self, other);

        range
            .collect::<Vec<_>>()
            .into_par_iter()
            .find_last(|b| self.ncols % b == 0 || self.nrows % b == 0 || other.ncols % b == 0)
            .unwrap()
    }

    #[inline(always)]
    pub fn get_range_for_block_size(&self, other: &Self) -> RangeInclusive<usize> {
        if self.nrows < 30 && self.ncols < 30 || other.nrows < 30 && other.ncols < 30 {
            2..=10
        } else if self.nrows < 100 && self.ncols < 100 || other.nrows < 100 && other.ncols < 100 {
            10..=30
        } else {
            30..=50
        }
    }

    // ===================================================
    //           Determinant
    // ===================================================

    #[inline(always)]
    fn det_2x2(&self) -> T {
        self.at(0, 0) * self.at(1, 1) - self.at(0, 1) * self.at(1, 0)
    }

    #[inline(always)]
    fn det_3x3(&self) -> T {
        let a = self.at(0, 0);
        let b = self.at(0, 1);
        let c = self.at(0, 2);
        let d = self.at(1, 0);
        let e = self.at(1, 1);
        let f = self.at(1, 2);
        let g = self.at(2, 0);
        let h = self.at(2, 1);
        let i = self.at(2, 2);

        a * (e * i - f * h) - b * (d * i - f * g) + c * (d * h - e * g)
    }

    fn det_nxn(matrix: Vec<T>, n: usize) -> T {
        if n == 1 {
            return matrix[0];
        }

        let mut det = T::zero();
        let mut sign = T::one();

        for col in 0..n {
            let sub_det = Self::det_nxn(Self::submatrix(matrix.clone(), n, 0, col), n - 1);

            det += sign * matrix[col] * sub_det;

            sign *= -T::one();
        }

        det
    }

    fn submatrix(matrix: Vec<T>, n: usize, row_to_remove: usize, col_to_remove: usize) -> Vec<T> {
        matrix
            .par_iter()
            .enumerate()
            .filter_map(|(i, &value)| {
                let row = i / n;
                let col = i % n;
                if row != row_to_remove && col != col_to_remove {
                    Some(value)
                } else {
                    None
                }
            })
            .collect()
    }

    // ===================================================
    //           Extremely specific optimizations
    // ===================================================

    // 1x2 @ 2x1 matrix
    #[inline(always)]
    fn onetwo_by_twoone(&self, other: &Self) -> Self {
        let a = self.at(0, 0) * other.at(0, 0) + self.at(0, 1) * other.at(1, 0);

        Self::new(vec![a], (1, 1)).unwrap()
    }

    // 2x2 @ 2x1 matrix
    #[inline(always)]
    fn twotwo_by_twoone(&self, other: &Self) -> Self {
        let a = self.at(0, 0) * other.at(0, 0) + self.at(0, 1) * other.at(1, 0);
        let b = self.at(1, 0) * other.at(0, 0) + self.at(1, 1) * other.at(1, 0);

        Self::new(vec![a, b], (2, 1)).unwrap()
    }

    //
    // 1x2 @ 2x2 matrix
    #[inline(always)]
    fn onetwo_by_twotwo(&self, other: &Self) -> Self {
        let a = self.at(0, 0) * other.at(0, 0) + self.at(0, 1) * other.at(1, 0);
        let b = self.at(0, 0) * other.at(1, 0) + self.at(0, 1) * other.at(1, 1);

        Self::new(vec![a, b], (1, 2)).unwrap()
    }

    // 2x2 @ 2x2 matrix
    #[inline(always)]
    fn twotwo_by_twotwo(&self, other: &Self) -> Self {
        let a = self.at(0, 0) * other.at(0, 0) + self.at(1, 0) * other.at(1, 0);
        let b = self.at(0, 0) * other.at(0, 1) + self.at(0, 1) * other.at(1, 1);
        let c = self.at(1, 0) * other.at(0, 0) + self.at(1, 1) * other.at(1, 0);
        let d = self.at(1, 0) * other.at(1, 0) + self.at(1, 1) * other.at(1, 1);

        Self::new(vec![a, b, c, d], (2, 2)).unwrap()
    }

    // ========================================================================
    //
    //    General solutions for matrix multiplication
    //
    // ========================================================================

    /// Naive matmul if you don't have any SIMD intrinsincts
    ///
    /// Also blocked, but doing different than just N
    fn optimized_blocked_matmul(&self, other: &Self, block_size: usize) -> Self {
        let M = self.nrows;
        let N = self.ncols;
        let P = other.ncols;

        let mut data = vec![T::zero(); M * P];

        //let t_other = other.transpose_copy();

        for kk in (0..N).step_by(block_size) {
            for jj in (0..P).step_by(block_size) {
                for ii in (0..M).step_by(block_size) {
                    let block_end_i = (ii + block_size).min(M);
                    let block_end_j = (jj + block_size).min(P);
                    let block_end_k = (kk + block_size).min(N);

                    // Blocking for L0 memory
                    for i in ii..block_end_i {
                        for j in jj..block_end_j {
                            // for k in kk..block_end_k {
                            //     data[at!(i, j, P)] += self.at(i, k) * other.at(k, j);
                            // }
                            data[at!(i, j, P)] = (kk..block_end_k)
                                .into_par_iter()
                                .map(|k| self.at(i, k) * other.at(k, j))
                                .sum();
                        }
                    }
                }
            }
        }
        Self::new(data, (M, P)).unwrap()
    }

    // SUMMA Algorithm
    // https://www.netlib.org/lapack/lawnspdf/lawn96.pdf
    fn summa(&self, other: &Self, block_size: usize) -> Self {
        todo!()
    }

    // The magnum opus of matrix multiply, also known as naive matmul
    // Only optimization is a parallelized innermost summation
    fn naive(&self, other: &Self) -> Self {
        let M = self.nrows;
        let N = self.ncols;
        let P = other.ncols;

        let mut data = vec![T::zero(); M * P];

        for i in 0..M {
            for j in 0..P {
                data[at!(i, j, P)] = (0..N)
                    .into_par_iter()
                    .map(|k| self.at(i, k) * other.at(k, j))
                    .sum();
            }
        }

        Self::new(data, (M, P)).unwrap()
    }

    // Blocked matmul if you don't have any SIMD intrinsincts
    // https://csapp.cs.cmu.edu/public/waside/waside-blocking.pdf
    //
    // Modification involves transposing the B matrix, at the cost
    // of increased space complexity, but better cache hit rate
    //
    // NOTE: Only works for M N @ N M matrices for now
    fn blocked_matmul(&self, other: &Self, block_size: usize) -> Self {
        let n = self.nrows;

        let en = block_size * (n / block_size);

        let mut data = vec![T::zero(); n * n];

        let t_other = other.transpose_copy();

        for kk in (0..n).step_by(en) {
            for jj in (0..n).step_by(en) {
                for i in 0..n {
                    for j in jj..jj + block_size {
                        data[at!(i, j, n)] = (kk..kk + block_size)
                            .into_par_iter()
                            .map(|k| self.at(i, k) * t_other.at(j, k))
                            .sum();
                    }
                }
            }
        }
        Self::new(data, (n, n)).unwrap()
    }
}