From c0a6df55b17b767340e2bd65e98df136b356ff2c Mon Sep 17 00:00:00 2001
From: Fredrik Jansson <fredrik.jansson.se@gmail.com>
Date: Wed, 1 Apr 2020 22:36:05 +0200
Subject: [PATCH] Addition of matrix exponent for static size matrices.

---
 src/linalg/exp.rs   | 445 ++++++++++++++++++++++++++++++++++++++++++++
 src/linalg/mod.rs   |   1 +
 tests/linalg/exp.rs |  97 ++++++++++
 tests/linalg/mod.rs |   2 +
 4 files changed, 545 insertions(+)
 create mode 100644 src/linalg/exp.rs
 create mode 100644 tests/linalg/exp.rs

diff --git a/src/linalg/exp.rs b/src/linalg/exp.rs
new file mode 100644
index 00000000..8e8227d5
--- /dev/null
+++ b/src/linalg/exp.rs
@@ -0,0 +1,445 @@
+use crate::{
+    base::{
+        allocator::Allocator,
+        dimension::{DimMin, DimMinimum, DimName},
+        DefaultAllocator,
+    },
+    try_convert, ComplexField, MatrixN, RealField,
+};
+
+// https://github.com/scipy/scipy/blob/c1372d8aa90a73d8a52f135529293ff4edb98fc8/scipy/sparse/linalg/matfuncs.py
+struct ExpmPadeHelper<N, R>
+where
+    N: RealField,
+    R: DimName + DimMin<R>,
+    DefaultAllocator: Allocator<N, R, R> + Allocator<(usize, usize), DimMinimum<R, R>>,
+{
+    use_exact_norm: bool,
+    ident: MatrixN<N, R>,
+
+    a: MatrixN<N, R>,
+    a2: Option<MatrixN<N, R>>,
+    a4: Option<MatrixN<N, R>>,
+    a6: Option<MatrixN<N, R>>,
+    a8: Option<MatrixN<N, R>>,
+    a10: Option<MatrixN<N, R>>,
+
+    d4_exact: Option<N>,
+    d6_exact: Option<N>,
+    d8_exact: Option<N>,
+    d10_exact: Option<N>,
+
+    d4_approx: Option<N>,
+    d6_approx: Option<N>,
+    d8_approx: Option<N>,
+    d10_approx: Option<N>,
+}
+
+impl<N, R> ExpmPadeHelper<N, R>
+where
+    N: RealField,
+    R: DimName + DimMin<R>,
+    DefaultAllocator: Allocator<N, R, R> + Allocator<(usize, usize), DimMinimum<R, R>>,
+{
+    fn new(a: MatrixN<N, R>, use_exact_norm: bool) -> Self {
+        ExpmPadeHelper {
+            use_exact_norm,
+            ident: MatrixN::<N, R>::identity(),
+            a,
+            a2: None,
+            a4: None,
+            a6: None,
+            a8: None,
+            a10: None,
+            d4_exact: None,
+            d6_exact: None,
+            d8_exact: None,
+            d10_exact: None,
+            d4_approx: None,
+            d6_approx: None,
+            d8_approx: None,
+            d10_approx: None,
+        }
+    }
+
+    fn a2(&self) -> &MatrixN<N, R> {
+        if self.a2.is_none() {
+            let ap = &self.a2 as *const Option<MatrixN<N, R>> as *mut Option<MatrixN<N, R>>;
+            unsafe {
+                *ap = Some(&self.a * &self.a);
+            }
+        }
+        self.a2.as_ref().unwrap()
+    }
+
+    fn a4(&self) -> &MatrixN<N, R> {
+        if self.a4.is_none() {
+            let ap = &self.a4 as *const Option<MatrixN<N, R>> as *mut Option<MatrixN<N, R>>;
+            let a2 = self.a2();
+            unsafe {
+                *ap = Some(a2 * a2);
+            }
+        }
+        self.a4.as_ref().unwrap()
+    }
+
+    fn a6(&self) -> &MatrixN<N, R> {
+        if self.a6.is_none() {
+            let a2 = self.a2();
+            let a4 = self.a4();
+            let ap = &self.a6 as *const Option<MatrixN<N, R>> as *mut Option<MatrixN<N, R>>;
+            unsafe {
+                *ap = Some(a4 * a2);
+            }
+        }
+        self.a6.as_ref().unwrap()
+    }
+
+    fn a8(&self) -> &MatrixN<N, R> {
+        if self.a8.is_none() {
+            let a2 = self.a2();
+            let a6 = self.a6();
+            let ap = &self.a8 as *const Option<MatrixN<N, R>> as *mut Option<MatrixN<N, R>>;
+            unsafe {
+                *ap = Some(a6 * a2);
+            }
+        }
+        self.a8.as_ref().unwrap()
+    }
+
+    fn a10(&mut self) -> &MatrixN<N, R> {
+        if self.a10.is_none() {
+            let a4 = self.a4();
+            let a6 = self.a6();
+            let ap = &self.a10 as *const Option<MatrixN<N, R>> as *mut Option<MatrixN<N, R>>;
+            unsafe {
+                *ap = Some(a6 * a4);
+            }
+        }
+        self.a10.as_ref().unwrap()
+    }
+
+    fn d4_tight(&mut self) -> N {
+        if self.d4_exact.is_none() {
+            self.d4_exact = Some(self.a4().amax().powf(N::from_f64(0.25).unwrap()));
+        }
+        self.d4_exact.unwrap()
+    }
+
+    fn d6_tight(&mut self) -> N {
+        if self.d6_exact.is_none() {
+            self.d6_exact = Some(self.a6().amax().powf(N::from_f64(1.0 / 6.0).unwrap()));
+        }
+        self.d6_exact.unwrap()
+    }
+
+    fn d8_tight(&mut self) -> N {
+        if self.d8_exact.is_none() {
+            self.d8_exact = Some(self.a8().amax().powf(N::from_f64(1.0 / 8.0).unwrap()));
+        }
+        self.d8_exact.unwrap()
+    }
+
+    fn d10_tight(&mut self) -> N {
+        if self.d10_exact.is_none() {
+            self.d10_exact = Some(self.a10().amax().powf(N::from_f64(1.0 / 10.0).unwrap()));
+        }
+        self.d10_exact.unwrap()
+    }
+
+    fn d4_loose(&mut self) -> N {
+        if self.use_exact_norm {
+            return self.d4_tight();
+        }
+
+        if self.d4_exact.is_some() {
+            return self.d4_exact.unwrap();
+        }
+
+        if self.d4_approx.is_none() {
+            self.d4_approx = Some(self.a4().amax().powf(N::from_f64(0.25).unwrap()));
+        }
+
+        self.d4_approx.unwrap()
+    }
+
+    fn d6_loose(&mut self) -> N {
+        if self.use_exact_norm {
+            return self.d6_tight();
+        }
+
+        if self.d6_exact.is_some() {
+            return self.d6_exact.unwrap();
+        }
+
+        if self.d6_approx.is_none() {
+            self.d6_approx = Some(self.a6().amax().powf(N::from_f64(1.0 / 6.0).unwrap()));
+        }
+
+        self.d6_approx.unwrap()
+    }
+
+    fn d8_loose(&mut self) -> N {
+        if self.use_exact_norm {
+            return self.d8_tight();
+        }
+
+        if self.d8_exact.is_some() {
+            return self.d8_exact.unwrap();
+        }
+
+        if self.d8_approx.is_none() {
+            self.d8_approx = Some(self.a8().amax().powf(N::from_f64(1.0 / 8.0).unwrap()));
+        }
+
+        self.d8_approx.unwrap()
+    }
+
+    fn d10_loose(&mut self) -> N {
+        if self.use_exact_norm {
+            return self.d10_tight();
+        }
+
+        if self.d10_exact.is_some() {
+            return self.d10_exact.unwrap();
+        }
+
+        if self.d10_approx.is_none() {
+            self.d10_approx = Some(self.a10().amax().powf(N::from_f64(1.0 / 10.0).unwrap()));
+        }
+
+        self.d10_approx.unwrap()
+    }
+
+    fn pade3(&mut self) -> (MatrixN<N, R>, MatrixN<N, R>) {
+        let b = [
+            N::from_f64(120.0).unwrap(),
+            N::from_f64(60.0).unwrap(),
+            N::from_f64(12.0).unwrap(),
+            N::from_f64(1.0).unwrap(),
+        ];
+        let u = &self.a * (self.a2() * b[3] + &self.ident * b[1]);
+        let v = self.a2() * b[2] + &self.ident * b[0];
+        (u, v)
+    }
+
+    fn pade5(&mut self) -> (MatrixN<N, R>, MatrixN<N, R>) {
+        let b = [
+            N::from_f64(30240.0).unwrap(),
+            N::from_f64(15120.0).unwrap(),
+            N::from_f64(3360.0).unwrap(),
+            N::from_f64(420.0).unwrap(),
+            N::from_f64(30.0).unwrap(),
+            N::from_f64(1.0).unwrap(),
+        ];
+        let u = &self.a * (self.a4() * b[5] + self.a2() * b[3] + &self.ident * b[1]);
+        let v = self.a4() * b[4] + self.a2() * b[2] + &self.ident * b[0];
+        (u, v)
+    }
+
+    fn pade7(&mut self) -> (MatrixN<N, R>, MatrixN<N, R>) {
+        let b = [
+            N::from_f64(17297280.0).unwrap(),
+            N::from_f64(8648640.0).unwrap(),
+            N::from_f64(1995840.0).unwrap(),
+            N::from_f64(277200.0).unwrap(),
+            N::from_f64(25200.0).unwrap(),
+            N::from_f64(1512.0).unwrap(),
+            N::from_f64(56.0).unwrap(),
+            N::from_f64(1.0).unwrap(),
+        ];
+        let u =
+            &self.a * (self.a6() * b[7] + self.a4() * b[5] + self.a2() * b[3] + &self.ident * b[1]);
+        let v = self.a6() * b[6] + self.a4() * b[4] + self.a2() * b[2] + &self.ident * b[0];
+        (u, v)
+    }
+
+    fn pade9(&mut self) -> (MatrixN<N, R>, MatrixN<N, R>) {
+        let b = [
+            N::from_f64(17643225600.0).unwrap(),
+            N::from_f64(8821612800.0).unwrap(),
+            N::from_f64(2075673600.0).unwrap(),
+            N::from_f64(302702400.0).unwrap(),
+            N::from_f64(30270240.0).unwrap(),
+            N::from_f64(2162160.0).unwrap(),
+            N::from_f64(110880.0).unwrap(),
+            N::from_f64(3960.0).unwrap(),
+            N::from_f64(90.0).unwrap(),
+            N::from_f64(1.0).unwrap(),
+        ];
+        let u = &self.a
+            * (self.a8() * b[9]
+                + self.a6() * b[7]
+                + self.a4() * b[5]
+                + self.a2() * b[3]
+                + &self.ident * b[1]);
+        let v = self.a8() * b[8]
+            + self.a6() * b[6]
+            + self.a4() * b[4]
+            + self.a2() * b[2]
+            + &self.ident * b[0];
+        (u, v)
+    }
+
+    fn pade13_scaled(&mut self, s: u64) -> (MatrixN<N, R>, MatrixN<N, R>) {
+        let b = [
+            N::from_f64(64764752532480000.0).unwrap(),
+            N::from_f64(32382376266240000.0).unwrap(),
+            N::from_f64(7771770303897600.0).unwrap(),
+            N::from_f64(1187353796428800.0).unwrap(),
+            N::from_f64(129060195264000.0).unwrap(),
+            N::from_f64(10559470521600.0).unwrap(),
+            N::from_f64(670442572800.0).unwrap(),
+            N::from_f64(33522128640.0).unwrap(),
+            N::from_f64(1323241920.0).unwrap(),
+            N::from_f64(40840800.0).unwrap(),
+            N::from_f64(960960.0).unwrap(),
+            N::from_f64(16380.0).unwrap(),
+            N::from_f64(182.0).unwrap(),
+            N::from_f64(1.0).unwrap(),
+        ];
+        let s = s as f64;
+
+        let mb = &self.a * N::from_f64(2.0.powf(-s)).unwrap();
+        let mb2 = self.a2() * N::from_f64(2.0.powf(-2.0 * s)).unwrap();
+        let mb4 = self.a4() * N::from_f64(2.0.powf(-4.0 * s)).unwrap();
+        let mb6 = self.a6() * N::from_f64(2.0.powf(-6.0 * s)).unwrap();
+
+        let u2 = &mb6 * (&mb6 * b[13] + &mb4 * b[11] + &mb2 * b[9]);
+        let u = &mb * (&u2 + &mb6 * b[7] + &mb4 * b[5] + &mb2 * b[3] + &self.ident * b[1]);
+        let v2 = &mb6 * (&mb6 * b[12] + &mb4 * b[10] + &mb2 * b[8]);
+        let v = v2 + &mb6 * b[6] + &mb4 * b[4] + &mb2 * b[2] + &self.ident * b[0];
+        (u, v)
+    }
+}
+
+fn factorial(n: u128) -> u128 {
+    if n == 1 {
+        return 1;
+    }
+    n * factorial(n - 1)
+}
+
+fn onenorm_matrix_power_nnm<N, R>(a: &MatrixN<N, R>, p: u64) -> N
+where
+    N: RealField,
+    R: DimName,
+    DefaultAllocator: Allocator<N, R, R>,
+{
+    let mut v = MatrixN::<N, R>::repeat(N::from_f64(1.0).unwrap());
+    let m = a.transpose();
+
+    for _ in 0..p {
+        v = &m * v;
+    }
+
+    v.amax()
+}
+
+fn ell<N, R>(a: &MatrixN<N, R>, m: u64) -> u64
+where
+    N: RealField,
+    R: DimName,
+    DefaultAllocator: Allocator<N, R, R>,
+{
+    // 2m choose m = (2m)!/(m! * (2m-m)!)
+
+    let a_abs_onenorm = onenorm_matrix_power_nnm(&a.abs(), 2 * m + 1);
+
+    if a_abs_onenorm == N::zero() {
+        return 0;
+    }
+
+    let choose_2m_m =
+        factorial(2 * m as u128) / (factorial(m as u128) * factorial(2 * m as u128 - m as u128));
+    let abs_c_recip = choose_2m_m * factorial(2 * m as u128 + 1);
+    let alpha = a_abs_onenorm / a.amax();
+    let alpha = alpha / N::from_u128(abs_c_recip).unwrap();
+
+    let u = N::from_f64(2_f64.powf(-53.0)).unwrap();
+    let log2_alpha_div_u = try_convert((alpha / u).log2()).unwrap();
+    let value = (log2_alpha_div_u / (2.0 * m as f64)).ceil();
+    if value > 0.0 {
+        value as u64
+    } else {
+        0
+    }
+}
+
+fn solve_p_q<N, R>(u: MatrixN<N, R>, v: MatrixN<N, R>) -> MatrixN<N, R>
+where
+    N: ComplexField,
+    R: DimMin<R, Output = R> + DimName,
+    DefaultAllocator: Allocator<N, R, R> + Allocator<(usize, usize), DimMinimum<R, R>>,
+{
+    let p = &u + &v;
+    let q = &v - &u;
+
+    q.lu().solve(&p).unwrap()
+}
+
+impl<N: RealField, R: DimMin<R, Output = R> + DimName> MatrixN<N, R>
+where
+    DefaultAllocator: Allocator<N, R, R> + Allocator<(usize, usize), DimMinimum<R, R>>,
+{
+    /// Computes exp of this matrix
+    pub fn exp(&self) -> Self {
+        // Simple case
+        if self.nrows() == 1 {
+            return self.clone().map(|v| v.exp());
+        }
+
+        let mut h = ExpmPadeHelper::new(self.clone(), true);
+
+        let eta_1 = N::max(h.d4_loose(), h.d6_loose());
+        if eta_1 < N::from_f64(1.495585217958292e-002).unwrap() && ell(&h.a, 3) == 0 {
+            let (u, v) = h.pade3();
+            return solve_p_q(u, v);
+        }
+
+        let eta_2 = N::max(h.d4_tight(), h.d6_loose());
+        if eta_2 < N::from_f64(2.539398330063230e-001).unwrap() && ell(&h.a, 5) == 0 {
+            let (u, v) = h.pade5();
+            return solve_p_q(u, v);
+        }
+
+        let eta_3 = N::max(h.d6_tight(), h.d8_loose());
+        if eta_3 < N::from_f64(9.504178996162932e-001).unwrap() && ell(&h.a, 7) == 0 {
+            let (u, v) = h.pade7();
+            return solve_p_q(u, v);
+        }
+        if eta_3 < N::from_f64(2.097847961257068e+000).unwrap() && ell(&h.a, 9) == 0 {
+            let (u, v) = h.pade9();
+            return solve_p_q(u, v);
+        }
+
+        let eta_4 = N::max(h.d8_loose(), h.d10_loose());
+        let eta_5 = N::min(eta_3, eta_4);
+        let theta_13 = N::from_f64(4.25).unwrap();
+
+        let mut s = if eta_5 == N::zero() {
+            0
+        } else {
+            let l2 = try_convert((eta_5 / theta_13).log2().ceil()).unwrap();
+
+            if l2 < 0.0 {
+                0
+            } else {
+                l2 as u64
+            }
+        };
+
+        s += ell(
+            &(&h.a * N::from_f64(2.0_f64.powf(-(s as f64))).unwrap()),
+            13,
+        );
+
+        let (u, v) = h.pade13_scaled(s);
+        let mut x = solve_p_q(u, v);
+
+        for _ in 0..s {
+            x = &x * &x;
+        }
+        x
+    }
+}
diff --git a/src/linalg/mod.rs b/src/linalg/mod.rs
index de1108f7..42bfde63 100644
--- a/src/linalg/mod.rs
+++ b/src/linalg/mod.rs
@@ -5,6 +5,7 @@ mod bidiagonal;
 mod cholesky;
 mod convolution;
 mod determinant;
+mod exp;
 mod full_piv_lu;
 pub mod givens;
 mod hessenberg;
diff --git a/tests/linalg/exp.rs b/tests/linalg/exp.rs
new file mode 100644
index 00000000..ac3f8dd7
--- /dev/null
+++ b/tests/linalg/exp.rs
@@ -0,0 +1,97 @@
+#[cfg(test)]
+mod tests {
+    //https://github.com/scipy/scipy/blob/c1372d8aa90a73d8a52f135529293ff4edb98fc8/scipy/sparse/linalg/tests/test_matfuncs.py
+    #[test]
+    fn exp() {
+        use nalgebra::{Matrix1, Matrix2, Matrix3};
+
+        {
+            let m = Matrix1::new(1.0);
+
+            let f = m.exp();
+
+            assert_eq!(f, Matrix1::new(1_f64.exp()));
+        }
+
+        {
+            let m = Matrix2::new(0.0, 1.0, 0.0, 0.0);
+
+            assert_eq!(m.exp(), Matrix2::new(1.0, 1.0, 0.0, 1.0));
+        }
+
+        {
+            let a: f64 = 1.0;
+            let b: f64 = 2.0;
+            let c: f64 = 3.0;
+            let d: f64 = 4.0;
+            let m = Matrix2::new(a, b, c, d);
+
+            let delta = ((a - d).powf(2.0) + 4.0 * b * c).sqrt();
+            let delta_2 = delta / 2.0;
+            let ad_2 = (a + d) / 2.0;
+            let m11 = ad_2.exp() * (delta * delta_2.cosh() + (a - d) * delta_2.sinh());
+            let m12 = 2.0 * b * ad_2.exp() * delta_2.sinh();
+            let m21 = 2.0 * c * ad_2.exp() * delta_2.sinh();
+            let m22 = ad_2.exp() * (delta * delta_2.cosh() + (d - a) * delta_2.sinh());
+
+            let f = Matrix2::new(m11, m12, m21, m22) / delta;
+            assert!((f - m.exp()).iter().all(|v| v.abs() <= 0.00005));
+        }
+
+        {
+            // https://mathworld.wolfram.com/MatrixExponential.html
+            use rand::{
+                distributions::{Distribution, Uniform},
+                thread_rng,
+            };
+            let mut rng = thread_rng();
+            let dist = Uniform::new(-10.0, 10.0);
+            loop {
+                let a: f64 = dist.sample(&mut rng);
+                let b: f64 = dist.sample(&mut rng);
+                let c: f64 = dist.sample(&mut rng);
+                let d: f64 = dist.sample(&mut rng);
+                let m = Matrix2::new(a, b, c, d);
+
+                let delta_sq = (a - d).powf(2.0) + 4.0 * b * c;
+                if delta_sq < 0.0 {
+                    continue;
+                }
+
+                let delta = delta_sq.sqrt();
+                let delta_2 = delta / 2.0;
+                let ad_2 = (a + d) / 2.0;
+                let m11 = ad_2.exp() * (delta * delta_2.cosh() + (a - d) * delta_2.sinh());
+                let m12 = 2.0 * b * ad_2.exp() * delta_2.sinh();
+                let m21 = 2.0 * c * ad_2.exp() * delta_2.sinh();
+                let m22 = ad_2.exp() * (delta * delta_2.cosh() + (d - a) * delta_2.sinh());
+
+                let f = Matrix2::new(m11, m12, m21, m22) / delta;
+                println!("a: {}", m);
+                assert!((f - m.exp()).iter().all(|v| v.abs() <= 0.00005));
+                break;
+            }
+        }
+
+        {
+            let m = Matrix3::new(1.0, 3.0, 0.0, 0.0, 1.0, 5.0, 0.0, 0.0, 2.0);
+
+            let e1 = 1.0_f64.exp();
+            let e2 = 2.0_f64.exp();
+
+            let f = Matrix3::new(
+                e1,
+                3.0 * e1,
+                15.0 * (e2 - 2.0 * e1),
+                0.0,
+                e1,
+                5.0 * (e2 - e1),
+                0.0,
+                0.0,
+                e2,
+            );
+
+            assert!((f - m.exp()).iter().all(|v| v.abs() <= 0.00005));
+        }
+    }
+}
diff --git a/tests/linalg/mod.rs b/tests/linalg/mod.rs
index 234cac39..c810df1f 100644
--- a/tests/linalg/mod.rs
+++ b/tests/linalg/mod.rs
@@ -1,7 +1,9 @@
 mod balancing;
 mod bidiagonal;
 mod cholesky;
+mod convolution;
 mod eigen;
+mod exp;
 mod full_piv_lu;
 mod hessenberg;
 mod inverse;