src/kernels/ball_indicator.rs


//! Implementation of the indicator function of a ball with respect to various norms.
use float_extras::f64::{tgamma as gamma};
use numeric_literals::replace_float_literals;
use serde::Serialize;
use alg_tools::types::*;
use alg_tools::norms::*;
use alg_tools::loc::Loc;
use alg_tools::sets::Cube;
use alg_tools::bisection_tree::{
    Support,
    Constant,
    Bounds,
    LocalAnalysis,
    GlobalAnalysis,
};
use alg_tools::mapping::Apply;
use alg_tools::maputil::array_init;
use alg_tools::coefficients::factorial;

use super::base::*;

/// Representation of the indicator of the ball $𝔹_q = \\{ x ∈ ℝ^N \mid \\|x\\|\_q ≤ r \\}$,
/// where $q$ is the `Exponent`, and $r$ is the radius [`Constant`] `C`.
#[derive(Copy,Clone,Serialize,Debug,Eq,PartialEq)]
pub struct BallIndicator<C : Constant, Exponent : NormExponent, const N : usize> {
    /// The radius of the ball.
    pub r : C,
    /// The exponent $q$ of the norm creating the ball
    pub exponent : Exponent,
}

/// Alias for the representation of the indicator of the $∞$-norm-ball
/// $𝔹_∞ = \\{ x ∈ ℝ^N \mid \\|x\\|\_∞ ≤ c \\}$.
pub type CubeIndicator<C, const N : usize> = BallIndicator<C, Linfinity, N>;

#[replace_float_literals(C::Type::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, Exponent : NormExponent, const N : usize>
Apply<&'a Loc<C::Type, N>>
for BallIndicator<C, Exponent, N>
where Loc<F, N> : Norm<F, Exponent> {
    type Output = C::Type;
    #[inline]
    fn apply(&self, x : &'a Loc<C::Type, N>) -> Self::Output {
        let r = self.r.value();
        let n = x.norm(self.exponent);
        if n <= r {
            1.0
        } else {
            0.0
        }
    }
}

impl<F : Float, C : Constant<Type=F>, Exponent : NormExponent, const N : usize>
Apply<Loc<C::Type, N>>
for BallIndicator<C, Exponent, N>
where Loc<F, N> : Norm<F, Exponent> {
    type Output = C::Type;
    #[inline]
    fn apply(&self, x : Loc<C::Type, N>) -> Self::Output {
        self.apply(&x)
    }
}


impl<'a, F : Float, C : Constant<Type=F>, Exponent : NormExponent, const N : usize>
Support<C::Type, N>
for BallIndicator<C, Exponent, N>
where Loc<F, N> : Norm<F, Exponent>,
      Linfinity : Dominated<F, Exponent, Loc<F, N>> {

    #[inline]
    fn support_hint(&self) -> Cube<F,N> {
        let r = Linfinity.from_norm(self.r.value(), self.exponent);
        array_init(|| [-r, r]).into()
    }

    #[inline]
    fn in_support(&self, x : &Loc<F,N>) -> bool {
        let r = Linfinity.from_norm(self.r.value(), self.exponent);
        x.norm(self.exponent) <= r
    }

    /// This can only really work in a reasonable fashion for N=1.
    #[inline]
    fn bisection_hint(&self, cube : &Cube<F, N>) -> [Option<F>; N] {
        let r = Linfinity.from_norm(self.r.value(), self.exponent);
        cube.map(|a, b| symmetric_interval_hint(r, a, b))
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, Exponent : NormExponent, const N : usize>
GlobalAnalysis<F, Bounds<F>>
for BallIndicator<C, Exponent, N>
where Loc<F, N> : Norm<F, Exponent> {
    #[inline]
    fn global_analysis(&self) -> Bounds<F> {
        Bounds(0.0, 1.0)
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, Exponent : NormExponent, const N : usize>
Norm<F, Linfinity>
for BallIndicator<C, Exponent, N>
where Loc<F, N> : Norm<F, Exponent> {
    #[inline]
    fn norm(&self, _ : Linfinity) -> F {
        1.0
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, const N : usize>
Norm<F, L1>
for BallIndicator<C, L1, N> {
    #[inline]
    fn norm(&self, _ : L1) -> F {
        // Using https://en.wikipedia.org/wiki/Volume_of_an_n-ball#Balls_in_Lp_norms,
        // we have V_N^1(r) = (2r)^N / N!
        let r = self.r.value();
        if N==1 {
            2.0 * r
        } else if N==2 {
            r*r
        } else {
            (2.0 * r).powi(N as i32) * F::cast_from(factorial(N))
        }
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, const N : usize>
Norm<F, L1>
for BallIndicator<C, L2, N> {
    #[inline]
    fn norm(&self, _ : L1) -> F {
        // See https://en.wikipedia.org/wiki/Volume_of_an_n-ball#The_volume.
        let r = self.r.value();
        let π = F::PI;
        if N==1 {
            2.0 * r
        } else if N==2 {
            π * (r * r)
        } else {
            let ndiv2 = F::cast_from(N) / 2.0;
            let γ = F::cast_from(gamma((ndiv2 + 1.0).as_()));
            π.powf(ndiv2) / γ * r.powi(N as i32)
        }
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, C : Constant<Type=F>, const N : usize>
Norm<F, L1>
for BallIndicator<C, Linfinity, N> {
    #[inline]
    fn norm(&self, _ : L1) -> F {
        let two_r = 2.0 * self.r.value();
        two_r.powi(N as i32)
    }
}


macro_rules! indicator_local_analysis {
    ($exponent:ident) => {
        impl<'a, F : Float, C : Constant<Type=F>, const N : usize>
        LocalAnalysis<F, Bounds<F>, N>
        for BallIndicator<C, $exponent, N>
        where Loc<F, N> : Norm<F, $exponent>,
            Linfinity : Dominated<F, $exponent, Loc<F, N>> {
            #[inline]
            fn local_analysis(&self, cube : &Cube<F, N>) -> Bounds<F> {
                // 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)
            }
        }
    }
}

indicator_local_analysis!(L1);
indicator_local_analysis!(L2);
indicator_local_analysis!(Linfinity);


#[replace_float_literals(F::cast_from(literal))]
impl<'a, F : Float, R, const N : usize> Apply<&'a Loc<F, N>>
for AutoConvolution<CubeIndicator<R, N>>
where R : Constant<Type=F> {
    type Output = F;

    #[inline]
    fn apply(&self, y : &'a Loc<F, N>) -> F {
        let two_r = 2.0 * self.0.r.value();
        // This is just a product of one-dimensional versions
        y.iter().map(|&x| {
            0.0.max(two_r - x.abs())
        }).product()
    }
}

impl<F : Float, R, const N : usize> Apply<Loc<F, N>>
for AutoConvolution<CubeIndicator<R, N>>
where R : Constant<Type=F> {
    type Output = F;

    #[inline]
    fn apply(&self, y : Loc<F, N>) -> F {
        self.apply(&y)
    }
}

#[replace_float_literals(F::cast_from(literal))]
impl<F : Float, R, const N : usize> Support<F, N>
for AutoConvolution<CubeIndicator<R, N>>
where R : Constant<Type=F> {
    #[inline]
    fn support_hint(&self) -> Cube<F, N> {
        let two_r = 2.0 * self.0.r.value();
        array_init(|| [-two_r, two_r]).into()
    }

    #[inline]
    fn in_support(&self, y : &Loc<F, N>) -> bool {
        let two_r = 2.0 * self.0.r.value();
        y.iter().all(|x| x.abs() <= two_r)
    }

    #[inline]
    fn bisection_hint(&self, cube : &Cube<F, N>) -> [Option<F>; N] {
        let two_r = 2.0 * self.0.r.value();
        cube.map(|c, d| symmetric_interval_hint(two_r, c, d))
    }
}

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

impl<F : Float, R, const N : usize> LocalAnalysis<F, Bounds<F>, N>
for AutoConvolution<CubeIndicator<R, N>>
where R : Constant<Type=F> {
    #[inline]
    fn local_analysis(&self, cube : &Cube<F, N>) -> 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());
        Bounds(lower, upper)
    }
}