src/kernels/hat_convolution.rs

//! Implementation of the convolution of two hat functions,
//! and its convolution with a [`CubeIndicator`].

use alg_tools::bisection_tree::{Constant, Support};
use alg_tools::bounds::{Bounded, Bounds, GlobalAnalysis, LocalAnalysis};
use alg_tools::error::DynResult;
use alg_tools::loc::Loc;
use alg_tools::mapping::{DifferentiableImpl, Instance, LipschitzDifferentiableImpl, Mapping};
use alg_tools::maputil::array_init;
use alg_tools::norms::*;
use alg_tools::sets::Cube;
use alg_tools::types::*;
use anyhow::anyhow;
use numeric_literals::replace_float_literals;
use serde::Serialize;

use super::ball_indicator::CubeIndicator;
use super::base::*;
use crate::types::Lipschitz;

/// Hat convolution kernel.
///
/// This struct represents the function
/// $$
///     f(x\_1, …, x\_n) = \prod\_{i=1}^n \frac{4}{σ} (h\*h)(x\_i/σ)
/// $$
/// where the “hat function” $h(y)= \max(0, 1 - |2y|)$.
/// The factor $4/σ$ normalises $∫ f d x = 1$.
/// We have
/// $$
///     (h*h)(y) =
///     \begin{cases}
///         \frac{2}{3} (y+1)^3 & -1<y\leq -\frac{1}{2}, \\\\
///         -2 y^3-2 y^2+\frac{1}{3} & -\frac{1}{2}<y\leq 0, \\\\
///         2 y^3-2 y^2+\frac{1}{3} & 0<y<\frac{1}{2}, \\\\
///         -\frac{2}{3} (y-1)^3 & \frac{1}{2}\leq y<1. \\\\
///     \end{cases}
/// $$
// Hence
// $$
//     (h\*h)'(y) =
//     \begin{cases}
//         2 (y+1)^2 & -1<y\leq -\frac{1}{2}, \\\\
//         -6 y^2-4 y & -\frac{1}{2}<y\leq 0, \\\\
//         6 y^2-4 y & 0<y<\frac{1}{2}, \\\\
//         -2 (y-1)^2 & \frac{1}{2}\leq y<1. \\\\
//     \end{cases}
// $$
// as well as
// $$
//     (h\*h)''(y) =
//     \begin{cases}
//         4 (y+1) & -1<y\leq -\frac{1}{2}, \\\\
//         -12 y-4 & -\frac{1}{2}<y\leq 0, \\\\
//         12 y-4 & 0<y<\frac{1}{2}, \\\\
//         -4 (y-1) & \frac{1}{2}\leq y<1. \\\\
//     \end{cases}
// $$
// This is maximised at y=±1/2 with value 2, and minimised at y=0 with value -4.
// Now observe that
// $$
//     [∇f(x\_1, …, x\_n)]_j = \frac{4}{σ} (h\*h)'(x\_j/σ) \prod\_{j ≠ i} \frac{4}{σ} (h\*h)(x\_i/σ)
// $$
#[derive(Copy, Clone, Debug, Serialize, Eq)]
pub struct HatConv<S: Constant, const N: usize> {
    /// The parameter $σ$ of the kernel.
    pub radius: S,
}

impl<S1, S2, const N: usize> PartialEq<HatConv<S2, N>> for HatConv<S1, N>
where
    S1: Constant,
    S2: Constant<Type = S1::Type>,
{
    fn eq(&self, other: &HatConv<S2, N>) -> bool {
        self.radius.value() == other.radius.value()
    }
}

impl<'a, S, const N: usize> HatConv<S, N>
where
    S: Constant,
{
    /// Returns the $σ$ parameter of the kernel.
    #[inline]
    pub fn radius(&self) -> S::Type {
        self.radius.value()
    }
}

impl<'a, S, const N: usize> Mapping<Loc<N, S::Type>> for HatConv<S, N>
where
    S: Constant,
{
    type Codomain = S::Type;

    #[inline]
    fn apply<I: Instance<Loc<N, S::Type>>>(&self, y: I) -> Self::Codomain {
        let σ = self.radius();
        y.decompose().product_map(|x| self.value_1d_σ1(x / σ)) / σ.powi(N as i32)
    }
}

#[replace_float_literals(S::Type::cast_from(literal))]
impl<S, const N: usize> Lipschitz<L2> for HatConv<S, N>
where
    S: Constant,
{
    type FloatType = S::Type;
    #[inline]
    fn lipschitz_factor(&self, _L2: L2) -> DynResult<Self::FloatType> {
        // For arbitrary ψ(x) = ∏_{i=1}^n ψ_i(x_i), we have
        // ψ(x) - ψ(y) = ∑_i [ψ_i(x_i)-ψ_i(y_i)] ∏_{j < i} ψ_j(y_j) ∏_{j > i} ψ_j(x_j)
        // by a simple recursive argument. In particular, if ψ_i=g for all i, j, we have
        // |ψ(x) - ψ(y)| ≤  ∑_i L_g|x_i-y_i| M_g^{n-1} ≤  √n L_g M_g^{n-1} |x-y|₂
        // where L_g is the Lipschitz factor of g, and M_g a bound on it.
        let σ = self.radius();
        let l1d = self.lipschitz_1d_σ1();
        let m1d = self.value_1d_σ1(0.0);
        Ok(
            S::Type::cast_from(N).sqrt() * l1d * m1d.powi((N - 1) as i32)
                / σ.powi((N - 1 + 2) as i32),
        )
    }
}

impl<'a, S, const N: usize> DifferentiableImpl<Loc<N, S::Type>> for HatConv<S, N>
where
    S: Constant,
{
    type Derivative = Loc<N, S::Type>;

    #[inline]
    fn differential_impl<I: Instance<Loc<N, S::Type>>>(&self, y0: I) -> Self::Derivative {
        let y = y0.decompose();
        let σ = self.radius();
        let tmp = y.map(|x| x / σ);
        let vs = tmp.map(|x| self.value_1d_σ1(x));
        product_differential(&tmp, &vs, |x| self.diff_1d_σ1(x)) / σ.powi((N + 1) as i32)
    }
}

#[replace_float_literals(S::Type::cast_from(literal))]
impl<'a, F: Float, S, const N: usize> LipschitzDifferentiableImpl<Loc<N, S::Type>, L2>
    for HatConv<S, N>
where
    S: Constant<Type = F>,
{
    type FloatType = F;

    #[inline]
    fn diff_lipschitz_factor(&self, _l2: L2) -> DynResult<F> {
        let σ = self.radius();
        product_differential_lipschitz_factor::<F, N>(
            self.value_1d_σ1(0.0),
            self.lipschitz_1d_σ1(),
            self.maxabsdiff_1d_σ1(),
            self.lipschitz_diff_1d_σ1(),
        )
        .map(|l| l / σ.powi((N + 2) as i32))
        // We have f(x)=f₀(x/σ)/σ^N, where f₀(z)=∏_{i=1}^N f₁(z_i).
        // Thus |f'(x)-f'(y)| = |f₀'(x/σ)-f₀'(y/σ)|/σ^{N+1} ≤ L_{f₀'}/σ^{N+2}.
    }
}

#[replace_float_literals(S::Type::cast_from(literal))]
impl<'a, F: Float, S, const N: usize> HatConv<S, N>
where
    S: Constant<Type = F>,
{
    /// Computes the value of the kernel for $n=1$ with $σ=1$.
    #[inline]
    fn value_1d_σ1(&self, x: F) -> F {
        let y = x.abs();
        if y >= 1.0 {
            0.0
        } else if y > 0.5 {
            -(8.0 / 3.0) * (y - 1.0).powi(3)
        } else
        /* 0 ≤ y ≤ 0.5 */
        {
            (4.0 / 3.0) + 8.0 * y * y * (y - 1.0)
        }
    }

    /// Computes the differential of the kernel for $n=1$ with $σ=1$.
    #[inline]
    fn diff_1d_σ1(&self, x: F) -> F {
        if x >= 1.0 || x <= -1.0 {
            0.0
        } else if x > 0.5 {
            -8.0 * (x - 1.0).powi(2)
        } else if x < -0.5 {
            8.0 * (x + 1.0).powi(2)
        } else {
            /* 0 ≤ y ≤ 0.5 */
            (24.0 * x.abs() - 16.0) * x
        }
    }

    /// Computes the Lipschitz factor of the kernel for $n=1$ with $σ=1$.
    #[inline]
    fn lipschitz_1d_σ1(&self) -> F {
        // Maximal absolute differential achieved at ±0.5 by diff_1d_σ1 analysis
        2.0
    }

    /// Computes the maximum absolute differential of the kernel for $n=1$ with $σ=1$.
    #[inline]
    fn maxabsdiff_1d_σ1(&self) -> F {
        // Maximal absolute differential achieved at ±0.5 by diff_1d_σ1 analysis
        2.0
    }

    /// Computes the differential of the kernel for $n=1$ with $σ=1$.
    #[inline]
    fn lipschitz_diff_1d_σ1(&self) -> F {
        // Maximal absolute second differential achieved at 0 by diff2_1d_σ1 analysis
        16.0
    }
}

impl<'a, S, const N: usize> Support<N, S::Type> for HatConv<S, N>
where
    S: Constant,
{
    #[inline]
    fn support_hint(&self) -> Cube<N, S::Type> {
        let σ = self.radius();
        array_init(|| [-σ, σ]).into()
    }

    #[inline]
    fn in_support(&self, y: &Loc<N, S::Type>) -> bool {
        let σ = self.radius();
        y.iter().all(|x| x.abs() <= σ)
    }

    #[inline]
    fn bisection_hint(&self, cube: &Cube<N, S::Type>) -> [Option<S::Type>; N] {
        let σ = self.radius();
        cube.map(|c, d| symmetric_peak_hint(σ, c, d))
    }
}

#[replace_float_literals(S::Type::cast_from(literal))]
impl<S, const N: usize> GlobalAnalysis<S::Type, Bounds<S::Type>> for HatConv<S, N>
where
    S: Constant,
{
    #[inline]
    fn global_analysis(&self) -> Bounds<S::Type> {
        Bounds(0.0, self.apply(Loc::ORIGIN))
    }
}

impl<S, const N: usize> LocalAnalysis<S::Type, Bounds<S::Type>, N> for HatConv<S, N>
where
    S: Constant,
{
    #[inline]
    fn local_analysis(&self, cube: &Cube<N, S::Type>) -> Bounds<S::Type> {
        // The function is maximised/minimised where the 2-norm is minimised/maximised.
        let lower = self.apply(cube.maxnorm_point());
        let upper = self.apply(cube.minnorm_point());
        Bounds(lower, upper)
    }
}

#[replace_float_literals(C::Type::cast_from(literal))]
impl<'a, C: Constant, const N: usize> Norm<L1, C::Type> for HatConv<C, N> {
    #[inline]
    fn norm(&self, _: L1) -> C::Type {
        1.0
    }
}

#[replace_float_literals(C::Type::cast_from(literal))]
impl<'a, C: Constant, const N: usize> Norm<Linfinity, C::Type> for HatConv<C, N> {
    #[inline]
    fn norm(&self, _: Linfinity) -> C::Type {
        self.bounds().upper()
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F: Float, R, C, const N: usize> Mapping<Loc<N, F>>
    for Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    type Codomain = F;

    #[inline]
    fn apply<I: Instance<Loc<N, F>>>(&self, y: I) -> F {
        let Convolution(ref ind, ref hatconv) = self;
        let β = ind.r.value();
        let σ = hatconv.radius();

        // This is just a product of one-dimensional versions
        y.decompose().product_map(|x| {
            // With $u_σ(x) = u_1(x/σ)/σ$ the normalised hat convolution
            // we have
            // $$
            //      [χ_{-β,β} * u_σ](x)
            //      = ∫_{x-β}^{x+β} u_σ(z) d z
            //      = (1/σ)∫_{x-β}^{x+β} u_1(z/σ) d z
            //      = ∫_{(x-β)/σ}^{(x+β)/σ} u_1(z) d z
            //      = (χ_{[-β/σ, β/σ]} * u_1)(x/σ)
            // $$
            self.value_1d_σ1(x / σ, β / σ)
        })
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F: Float, R, C, const N: usize> DifferentiableImpl<Loc<N, F>>
    for Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    type Derivative = Loc<N, F>;

    #[inline]
    fn differential_impl<I: Instance<Loc<N, F>>>(&self, y0: I) -> Loc<N, F> {
        let y = y0.decompose();
        let Convolution(ref ind, ref hatconv) = self;
        let β = ind.r.value();
        let σ = hatconv.radius();

        let vs = y.map(|x| self.value_1d_σ1(x / σ, β / σ));
        product_differential(&*y, &vs, |x| self.diff_1d_σ1(x / σ, β / σ)) / σ
    }
}

/// Integrate $f'$, whose support is $[c, d]$, on $[a, b]$.
/// The value $f$ is given, not the derivative.
/// If $b > d$, add $g()$ to the result.
#[inline]
#[replace_float_literals(F::cast_from(literal))]
fn i<F: Float>(a: F, b: F, c: F, d: F, f: impl Fn(F) -> F, g: impl Fn() -> F) -> F {
    if b < c {
        0.0
    } else if b <= d {
        if a <= c {
            f(b) - f(c)
        } else {
            f(b) - f(a)
        }
    } else
    /* b > d */
    {
        g() + if a <= c {
            f(d) - f(c)
        } else if a < d {
            f(d) - f(a)
        } else {
            0.0
        }
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<F: Float, C, R, const N: usize> Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    /// Calculates the value of the 1D hat convolution further convolved by a interval indicator.
    /// As both functions are piecewise polynomials, this is implemented by explicit integral over
    /// all subintervals of polynomiality of the cube indicator, using easily formed
    /// antiderivatives.
    #[inline]
    pub fn value_1d_σ1(&self, x: F, β: F) -> F {
        // The integration interval
        let a = x - β;
        let b = x + β;

        // Observe the factor 1/6 at the front from the antiderivatives below.
        // The factor 4 is from normalisation of the original function.
        (4.0 / 6.0)
            * i(
                a,
                b,
                -1.0,
                -0.5,
                // (2/3) (y+1)^3  on  -1 < y ≤ -1/2
                // The antiderivative is  (2/12)(y+1)^4 = (1/6)(y+1)^4
                |y| (y + 1.0).powi(4),
                || {
                    i(
                        a,
                        b,
                        -0.5,
                        0.5,
                        // -2 y^3 - 2 y^2 + 1/3  on  -1/2 < y ≤ 0
                        // The antiderivative is -1/2 y^4 - 2/3 y^3 + 1/3 y
                        // 2 y^3 - 2 y^2 + 1/3 on 0 < y < 1/2
                        // The antiderivative is 1/2 y^4 - 2/3 y^3 + 1/3 y
                        |y| y * (y * y * (y.abs() * 3.0 - 4.0) + 2.0),
                        || {
                            i(
                                a,
                                b,
                                0.5,
                                1.0,
                                // -(2/3) (y-1)^3  on  1/2 < y ≤ 1
                                // The antiderivative is  -(2/12)(y-1)^4 = -(1/6)(y-1)^4
                                |y| -(y - 1.0).powi(4),
                                || 0.0,
                            )
                        },
                    )
                },
            )
    }

    /// Calculates the derivative of the 1D hat convolution further convolved by a interval
    /// indicator. The implementation is similar to [`Self::value_1d_σ1`], using the fact that
    /// $(θ * ψ)' = θ * ψ'$.
    #[inline]
    pub fn diff_1d_σ1(&self, x: F, β: F) -> F {
        // The integration interval
        let a = x - β;
        let b = x + β;

        i(
            a,
            b,
            -1.0,
            -0.5,
            // (2/3) (y+1)^3  on  -1 < y ≤ -1/2
            |y| (8.0 / 3.0) * (y + 1.0).powi(3),
            || {
                i(
                    a,
                    b,
                    -0.5,
                    0.5,
                    // 2 y^3 - 2 y^2 + 1/3 on 0 < y < 1/2
                    |y| 8.0 * (y.abs() - 1.0) * y * y + (4.0 / 3.0),
                    || {
                        i(
                            a,
                            b,
                            0.5,
                            1.0,
                            // -(2/3) (y-1)^3  on  1/2 < y ≤ 1
                            |y| -(8.0 / 3.0) * (y - 1.0).powi(3),
                            || 0.0,
                        )
                    },
                )
            },
        )
    }
}

/*
impl<'a, F : Float, R, C, const N : usize> Lipschitz<L2>
for Differential<Loc<N, F>, Convolution<CubeIndicator<R, N>, HatConv<C, N>>>
where R : Constant<Type=F>,
      C : Constant<Type=F> {

    type FloatType = F;

    #[inline]
    fn lipschitz_factor(&self, _l2 : L2) -> Option<F> {
        dbg!("unimplemented");
        None
    }
}
*/

#[replace_float_literals(F::cast_from(literal))]
impl<F: Float, R, C, const N: usize> Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    #[inline]
    fn get_r(&self) -> F {
        let Convolution(ref ind, ref hatconv) = self;
        ind.r.value() + hatconv.radius()
    }
}

impl<F: Float, R, C, const N: usize> Support<N, F>
    for Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    #[inline]
    fn support_hint(&self) -> Cube<N, F> {
        let r = self.get_r();
        array_init(|| [-r, r]).into()
    }

    #[inline]
    fn in_support(&self, y: &Loc<N, F>) -> bool {
        let r = self.get_r();
        y.iter().all(|x| x.abs() <= r)
    }

    #[inline]
    fn bisection_hint(&self, cube: &Cube<N, F>) -> [Option<F>; N] {
        // It is not difficult to verify that [`HatConv`] is C^2.
        // Therefore, so is [`Convolution<CubeIndicator<R, N>, HatConv<C, N>>`] so that a finer
        // subdivision for the hint than this is not particularly useful.
        let r = self.get_r();
        cube.map(|c, d| symmetric_peak_hint(r, c, d))
    }
}

impl<F: Float, R, C, const N: usize> GlobalAnalysis<F, Bounds<F>>
    for Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    #[inline]
    fn global_analysis(&self) -> Bounds<F> {
        Bounds(F::ZERO, self.apply(Loc::ORIGIN))
    }
}

impl<F: Float, R, C, const N: usize> LocalAnalysis<F, Bounds<F>, N>
    for Convolution<CubeIndicator<R, N>, HatConv<C, N>>
where
    R: Constant<Type = F>,
    C: Constant<Type = F>,
{
    #[inline]
    fn local_analysis(&self, cube: &Cube<N, F>) -> Bounds<F> {
        // The function is maximised/minimised where the absolute value is minimised/maximised.
        let lower = self.apply(cube.maxnorm_point());
        let upper = self.apply(cube.minnorm_point());
        //assert!(upper >= lower);
        if upper < lower {
            let Convolution(ref ind, ref hatconv) = self;
            let β = ind.r.value();
            let σ = hatconv.radius();
            eprintln!("WARNING: Hat convolution {β} {σ} upper bound {upper} < lower bound {lower} on {cube:?} with min-norm point {:?} and max-norm point {:?}", cube.minnorm_point(), cube.maxnorm_point());
            Bounds(upper, lower)
        } else {
            Bounds(lower, upper)
        }
    }
}

/// This [`BoundedBy`] implementation bounds $u * u$ by $(ψ * ψ) u$ for $u$ a hat convolution and
/// $ψ = χ_{[-a,a]^N}$ for some $a>0$.
///
/// This is based on the general formula for bounding $(uχ) * (uχ)$ by $(ψ * ψ) u$,
/// where we take $ψ = χ_{[-a,a]^N}$ and $χ = χ_{[-σ,σ]^N}$ for $σ$ the width of the hat
/// convolution.
#[replace_float_literals(F::cast_from(literal))]
impl<F, C, S, const N: usize>
    BoundedBy<F, SupportProductFirst<AutoConvolution<CubeIndicator<S, N>>, HatConv<C, N>>>
    for AutoConvolution<HatConv<C, N>>
where
    F: Float,
    C: Constant<Type = F>,
    S: Constant<Type = F>,
{
    fn bounding_factor(
        &self,
        kernel: &SupportProductFirst<AutoConvolution<CubeIndicator<S, N>>, HatConv<C, N>>,
    ) -> DynResult<F> {
        // We use the comparison $ℱ[𝒜(ψ v)] ≤ L_1 ℱ[𝒜(ψ)u] ⟺ I_{v̂} v̂ ≤ L_1 û$ with
        // $ψ = χ_{[-w, w]}$ satisfying $supp v ⊂ [-w, w]$, i.e. $w ≥ σ$. Here $v̂ = ℱ[v]$ and
        // $I_{v̂} = ∫ v̂ d ξ. For this relationship to be valid, we need $v̂ ≥ 0$, which is guaranteed
        // by $v̂ = u_σ$ being an autoconvolution. With $u = v$, therefore $L_1 = I_v̂ = ∫ u_σ(ξ) d ξ$.
        let SupportProductFirst(AutoConvolution(ref ind), hatconv2) = kernel;
        let σ = self.0.radius();
        let a = ind.r.value();
        let bounding_1d = 4.0 / (3.0 * σ);

        // Check that the cutting indicator of the comparison
        // `SupportProductFirst<AutoConvolution<CubeIndicator<S, N>>, HatConv<C, N>>`
        // is wide enough, and that the hat convolution has the same radius as ours.
        if σ <= a && hatconv2 == &self.0 {
            Ok(bounding_1d.powi(N as i32))
        } else {
            // We cannot compare
            Err(anyhow!("Incomparable factors"))
        }
    }
}

/// This [`BoundedBy`] implementation bounds $u * u$ by $u$ for $u$ a hat convolution.
///
/// This is based on Example 3.3 in the manuscript.
#[replace_float_literals(F::cast_from(literal))]
impl<F, C, const N: usize> BoundedBy<F, HatConv<C, N>> for AutoConvolution<HatConv<C, N>>
where
    F: Float,
    C: Constant<Type = F>,
{
    /// Returns an estimate of the factor $L_1$.
    ///
    /// Returns  `None` if `kernel` does not have the same width as hat convolution that `self`
    /// is based on.
    fn bounding_factor(&self, kernel: &HatConv<C, N>) -> DynResult<F> {
        if kernel == &self.0 {
            Ok(1.0)
        } else {
            // We cannot compare
            Err(anyhow!("Incomparable kernels"))
        }
    }
}

#[cfg(test)]
mod tests {
    use super::HatConv;
    use crate::kernels::{BallIndicator, Convolution, CubeIndicator};
    use alg_tools::lingrid::linspace;
    use alg_tools::loc::Loc;
    use alg_tools::mapping::Mapping;
    use alg_tools::norms::Linfinity;

    /// Tests numerically that [`HatConv<f64, 1>`] is monotone.
    #[test]
    fn hatconv_monotonicity() {
        let grid = linspace(0.0, 1.0, 100000);
        let hatconv: HatConv<f64, 1> = HatConv { radius: 1.0 };
        let mut vals = grid.into_iter().map(|t| hatconv.apply(Loc::from(t)));
        let first = vals.next().unwrap();
        let monotone = vals
            .fold((first, true), |(prev, ok), t| (prev, ok && prev >= t))
            .1;
        assert!(monotone);
    }

    /// Tests numerically that [`Convolution<CubeIndicator<f64, 1>, HatConv<f64, 1>>`] is monotone.
    #[test]
    fn convolution_cubeind_hatconv_monotonicity() {
        let grid = linspace(-2.0, 0.0, 100000);
        let hatconv: Convolution<CubeIndicator<f64, 1>, HatConv<f64, 1>> =
            Convolution(BallIndicator { r: 0.5, exponent: Linfinity }, HatConv {
                radius: 1.0,
            });
        let mut vals = grid.into_iter().map(|t| hatconv.apply(Loc::from(t)));
        let first = vals.next().unwrap();
        let monotone = vals
            .fold((first, true), |(prev, ok), t| (prev, ok && prev <= t))
            .1;
        assert!(monotone);

        let grid = linspace(0.0, 2.0, 100000);
        let hatconv: Convolution<CubeIndicator<f64, 1>, HatConv<f64, 1>> =
            Convolution(BallIndicator { r: 0.5, exponent: Linfinity }, HatConv {
                radius: 1.0,
            });
        let mut vals = grid.into_iter().map(|t| hatconv.apply(Loc::from(t)));
        let first = vals.next().unwrap();
        let monotone = vals
            .fold((first, true), |(prev, ok), t| (prev, ok && prev >= t))
            .1;
        assert!(monotone);
    }
}