src/kernels/base.rs


//! Things for constructing new kernels from component kernels and traits for analysing them
use serde::Serialize;
use numeric_literals::replace_float_literals;

use alg_tools::types::*;
use alg_tools::norms::*;
use alg_tools::loc::Loc;
use alg_tools::sets::Cube;
use alg_tools::bisection_tree::{
    Support,
    Bounds,
    LocalAnalysis,
    GlobalAnalysis,
    Bounded,
};
use alg_tools::mapping::Apply;
use alg_tools::maputil::{array_init, map2};
use alg_tools::sets::SetOrd;

use crate::fourier::Fourier;

/// Representation of the product of two kernels.
///
/// The kernels typically implement [`Support`] and [`Mapping`][alg_tools::mapping::Mapping].
///
/// The implementation [`Support`] only uses the [`Support::support_hint`] of the first parameter!
#[derive(Copy,Clone,Serialize,Debug)]
pub struct SupportProductFirst<A, B>(
    /// First kernel
    pub A,
    /// Second kernel
    pub B
);

impl<A, B, F : Float, const N : usize> Apply<Loc<F, N>>
for SupportProductFirst<A, B>
where A : for<'a> Apply<&'a Loc<F, N>, Output=F>,
      B : for<'a> Apply<&'a Loc<F, N>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : Loc<F, N>) -> Self::Output {
        self.0.apply(&x) * self.1.apply(&x)
    }
}

impl<'a, A, B, F : Float, const N : usize> Apply<&'a Loc<F, N>>
for SupportProductFirst<A, B>
where A : Apply<&'a Loc<F, N>, Output=F>,
      B : Apply<&'a Loc<F, N>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : &'a Loc<F, N>) -> Self::Output {
        self.0.apply(x) * self.1.apply(x)
    }
}

impl<'a, A, B, F : Float, const N : usize> Support<F, N>
for SupportProductFirst<A, B>
where A : Support<F, N>,
      B : Support<F, N> {
    #[inline]
    fn support_hint(&self) -> Cube<F, N> {
        self.0.support_hint()
    }

    #[inline]
    fn in_support(&self, x : &Loc<F, N>) -> bool {
        self.0.in_support(x)
    }

    #[inline]
    fn bisection_hint(&self, cube : &Cube<F, N>) -> [Option<F>; N] {
        self.0.bisection_hint(cube)
    }
}

impl<'a, A, B, F : Float> GlobalAnalysis<F, Bounds<F>>
for SupportProductFirst<A, B>
where A : GlobalAnalysis<F, Bounds<F>>,
      B : GlobalAnalysis<F, Bounds<F>> {
    #[inline]
    fn global_analysis(&self) -> Bounds<F> {
        self.0.global_analysis() * self.1.global_analysis()
    }
}

impl<'a, A, B, F : Float, const N : usize> LocalAnalysis<F, Bounds<F>, N>
for SupportProductFirst<A, B>
where A : LocalAnalysis<F, Bounds<F>, N>,
      B : LocalAnalysis<F, Bounds<F>, N> {
    #[inline]
    fn local_analysis(&self, cube : &Cube<F, N>) -> Bounds<F> {
        self.0.local_analysis(cube) * self.1.local_analysis(cube)
    }
}

/// Representation of the sum of two kernels
///
/// The kernels typically implement [`Support`] and [`Mapping`][alg_tools::mapping::Mapping].
///
/// The implementation [`Support`] only uses the [`Support::support_hint`] of the first parameter!
#[derive(Copy,Clone,Serialize,Debug)]
pub struct SupportSum<A, B>(
    /// First kernel
    pub A,
    /// Second kernel
    pub B
);

impl<'a, A, B, F : Float, const N : usize> Apply<&'a Loc<F, N>>
for SupportSum<A, B>
where A : Apply<&'a Loc<F, N>, Output=F>,
      B : Apply<&'a Loc<F, N>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : &'a Loc<F, N>) -> Self::Output {
        self.0.apply(x) + self.1.apply(x)
    }
}

impl<A, B, F : Float, const N : usize> Apply<Loc<F, N>>
for SupportSum<A, B>
where A : for<'a> Apply<&'a Loc<F, N>, Output=F>,
      B : for<'a> Apply<&'a Loc<F, N>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : Loc<F, N>) -> Self::Output {
        self.0.apply(&x) + self.1.apply(&x)
    }
}

impl<'a, A, B, F : Float, const N : usize> Support<F, N>
for SupportSum<A, B>
where A : Support<F, N>,
      B : Support<F, N>,
      Cube<F, N> : SetOrd {
    #[inline]
    fn support_hint(&self) -> Cube<F, N> {
        self.0.support_hint().common(&self.1.support_hint())
    }

    #[inline]
    fn in_support(&self, x : &Loc<F, N>) -> bool {
        self.0.in_support(x) || self.1.in_support(x)
    }

    #[inline]
    fn bisection_hint(&self, cube : &Cube<F, N>) -> [Option<F>; N] {
        map2(self.0.bisection_hint(cube),
             self.1.bisection_hint(cube),
             |a, b| a.or(b))
    }
}

impl<'a, A, B, F : Float> GlobalAnalysis<F, Bounds<F>>
for SupportSum<A, B>
where A : GlobalAnalysis<F, Bounds<F>>,
      B : GlobalAnalysis<F, Bounds<F>> {
    #[inline]
    fn global_analysis(&self) -> Bounds<F> {
        self.0.global_analysis() + self.1.global_analysis()
    }
}

impl<'a, A, B, F : Float, const N : usize> LocalAnalysis<F, Bounds<F>, N>
for SupportSum<A, B>
where A : LocalAnalysis<F, Bounds<F>, N>,
      B : LocalAnalysis<F, Bounds<F>, N>,
      Cube<F, N> : SetOrd {
    #[inline]
    fn local_analysis(&self, cube : &Cube<F, N>) -> Bounds<F> {
        self.0.local_analysis(cube) + self.1.local_analysis(cube)
    }
}

/// Representation of the convolution of two kernels.
///
/// The kernels typically implement [`Support`]s and [`Mapping`][alg_tools::mapping::Mapping].
//
/// Trait implementations have to be on a case-by-case basis.
#[derive(Copy,Clone,Serialize,Debug,Eq,PartialEq)]
pub struct Convolution<A, B>(
    /// First kernel
    pub A,
    /// Second kernel
    pub B
);

/// Representation of the autoconvolution of a kernel.
///
/// The kernel typically implements [`Support`] and [`Mapping`][alg_tools::mapping::Mapping].
///
/// Trait implementations have to be on a case-by-case basis.
#[derive(Copy,Clone,Serialize,Debug,Eq,PartialEq)]
pub struct AutoConvolution<A>(
    /// The kernel to be autoconvolved
    pub A
);

/// Representation a multi-dimensional product of a one-dimensional kernel.
///
/// For $G: ℝ → ℝ$, this is the function $F(x\_1, …, x\_n) := \prod_{i=1}^n G(x\_i)$.
/// The kernel $G$ typically implements [`Support`] and [`Mapping`][alg_tools::mapping::Mapping]
/// on [`Loc<F, 1>`]. Then the product implements them on [`Loc<F, N>`].
#[derive(Copy,Clone,Serialize,Debug,Eq,PartialEq)]
struct UniformProduct<G, const N : usize>(
    /// The one-dimensional kernel
    G
);

impl<'a, G, F : Float, const N : usize> Apply<&'a Loc<F, N>>
for UniformProduct<G, N>
where G : Apply<Loc<F, 1>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : &'a Loc<F, N>) -> F {
        x.iter().map(|&y| self.0.apply(Loc([y]))).product()
    }
}

impl<G, F : Float, const N : usize> Apply<Loc<F, N>>
for UniformProduct<G, N>
where G : Apply<Loc<F, 1>, Output=F> {
    type Output = F;
    #[inline]
    fn apply(&self, x : Loc<F, N>) -> F {
        x.into_iter().map(|y| self.0.apply(Loc([y]))).product()
    }
}

impl<G, F : Float, const N : usize> Support<F, N>
for UniformProduct<G, N>
where G : Support<F, 1> {
    #[inline]
    fn support_hint(&self) -> Cube<F, N> {
        let [a] : [[F; 2]; 1] = self.0.support_hint().into();
        array_init(|| a.clone()).into()
    }

    #[inline]
    fn in_support(&self, x : &Loc<F, N>) -> bool {
        x.iter().all(|&y| self.0.in_support(&Loc([y])))
    }

    #[inline]
    fn bisection_hint(&self, cube : &Cube<F, N>) -> [Option<F>; N] {
        cube.map(|a, b| {
            let [h] = self.0.bisection_hint(&[[a, b]].into());
            h
        })
    }
}

impl<G, F : Float, const N : usize> GlobalAnalysis<F, Bounds<F>>
for UniformProduct<G, N>
where G : GlobalAnalysis<F, Bounds<F>> {
    #[inline]
    fn global_analysis(&self) -> Bounds<F> {
        let g = self.0.global_analysis();
        (0..N).map(|_| g).product()
    }
}

impl<G, F : Float, const N : usize> LocalAnalysis<F, Bounds<F>, N>
for UniformProduct<G, N>
where G : LocalAnalysis<F, Bounds<F>, 1> {
    #[inline]
    fn local_analysis(&self, cube : &Cube<F, N>) -> Bounds<F> {
        cube.iter_coords().map(
            |&[a, b]| self.0.local_analysis(&([[a, b]].into()))
        ).product()
    }
}

macro_rules! product_lpnorm {
    ($lp:ident) => {
        impl<G, F : Float, const N : usize> Norm<F, $lp>
        for UniformProduct<G, N>
        where G : Norm<F, $lp> {
            #[inline]
            fn norm(&self, lp : $lp) -> F {
                self.0.norm(lp).powi(N as i32)
            }
        }
    }
}

product_lpnorm!(L1);
product_lpnorm!(L2);
product_lpnorm!(Linfinity);


/// Trait for bounding one kernel with respect to another.
///
/// The type `F` is the scalar field, and `T` another kernel to which `Self` is compared.
pub trait BoundedBy<F : Num, T> {
    /// Calclate a bounding factor $c$ such that the Fourier transforms $ℱ\[v\] ≤ c ℱ\[u\]$ for
    /// $v$ `self` and $u$ `other`.
    ///
    /// If no such factors exits, `None` is returned.
    fn bounding_factor(&self, other : &T) -> Option<F>;
}

/// This [`BoundedBy`] implementation bounds $(uv) * (uv)$ by $(ψ * ψ) u$.
#[replace_float_literals(F::cast_from(literal))]
impl<F, C, BaseP>
BoundedBy<F, SupportProductFirst<AutoConvolution<C>, BaseP>>
for AutoConvolution<SupportProductFirst<C, BaseP>>
where F : Float,
      C : Clone + PartialEq,
      BaseP :  Fourier<F> + PartialOrd, // TODO: replace by BoundedBy,
      <BaseP as Fourier<F>>::Transformed : Bounded<F> + Norm<F, L1> {

    fn bounding_factor(&self, kernel : &SupportProductFirst<AutoConvolution<C>, BaseP>) -> Option<F> {
        let SupportProductFirst(AutoConvolution(ref cutoff2), base_spread2) = kernel;
        let AutoConvolution(SupportProductFirst(ref cutoff, ref base_spread)) = self;
        let v̂ = base_spread.fourier();

        // Verify that the cut-off and ideal physical model (base spread) are the same.
        if cutoff == cutoff2
           && base_spread <= base_spread2
           && v̂.bounds().lower() >= 0.0 {
            // Calculate the factor between the convolution approximation
            // `AutoConvolution<SupportProductFirst<C, BaseP>>` of $A_*A$ and the
            // kernel of the seminorm. This depends on the physical model P being
            // `SupportProductFirst<C, BaseP>` with the kernel `K` being
            // a `SupportSum` involving `SupportProductFirst<AutoConvolution<C>, BaseP>`.
            Some(v̂.norm(L1))
        } else {
            // We cannot compare
            None
        }
    }
}

impl<F : Float, A, B, C> BoundedBy<F, SupportSum<B, C>> for A
where A : BoundedBy<F, B>,
      C : Bounded<F> {

    #[replace_float_literals(F::cast_from(literal))]
    fn bounding_factor(&self, SupportSum(ref kernel1, kernel2) : &SupportSum<B, C>) -> Option<F> {
        if kernel2.bounds().lower() >= 0.0 {
            self.bounding_factor(kernel1)
        } else {
            None
        }
    }
}

/// Generates on $[a, b]$ [`Support::support_hint`] for a symmetric interval $[-r, r]$.
///
/// It will attempt to place the subdivision point at $-r$ or $r$.
/// If neither of these points lies within $[a, b]$, `None` is returned.
#[inline]
pub(super) fn symmetric_interval_hint<F : Float>(r : F, a : F, b : F) -> Option<F> {
    if a < -r && -r < b {
        Some(-r)
    } else if a < r && r < b {
        Some(r)
    } else {
        None
    }
}

/// Generates on $[a, b]$ [`Support::support_hint`] for a function with monotone derivative,
/// support on $[-r, r]$ and peak at $0.
///
/// It will attempt to place the subdivision point at $-r$, $r$, or $0$, depending on which
/// gives the longer length for the shorter of the two subintervals. If none of these points
/// lies within $[a, b]$, or the resulting interval would be shorter than $0.3r$, `None` is
/// returned.
#[replace_float_literals(F::cast_from(literal))]
#[inline]
pub(super) fn symmetric_peak_hint<F : Float>(r : F, a : F, b : F) -> Option<F> {
    let stage1 = if a < -r {
        if b <= -r {
            None
        } else if a + r < -b {
            Some(-r)
        } else {
            Some(0.0)
        }
    } else if a < 0.0 {
        if b <= 0.0 {
            None
        } else if a < r - b {
            Some(0.0)
        } else {
            Some(r)
        }
    } else if a < r && b > r {
        Some(r)
    } else {
        None
    };

    // Ignore stage1 hint if either side of subdivision would be just a small fraction of the
    // interval
    match stage1 {
        Some(h) if (h - a).min(b-h) >= 0.3 * r => Some(h),
        _ => None
    }
}