# HG changeset patch
# User Tuomo Valkonen <tuomov@iki.fi>
# Date 1745931318 18000
# Node ID 103aa137fcb24ffed2e1d6f21d480ac64f70bb23
# Parent  e7f1cb4bec7887cb69da592d5a52eb05de3831b8
sketch

diff -r e7f1cb4bec78 -r 103aa137fcb2 src/convex.rs
--- a/src/convex.rs	Tue Apr 29 00:03:12 2025 -0500
+++ b/src/convex.rs	Tue Apr 29 07:55:18 2025 -0500
@@ -319,15 +319,15 @@
 }
 
 /// The squared Euclidean norm divided by two
-pub struct Norm222<Domain: Space, F: Float>(PhantomData<(Domain, F)>);
+pub struct Norm222<F: Float>(PhantomData<F>);
 
-impl<Domain: Euclidean<F>, F: Float> Norm222<Domain, F> {
+impl</*Domain: Euclidean<F>,*/ F: Float> Norm222<F> {
     pub fn new() -> Self {
         Norm222(PhantomData)
     }
 }
 
-impl<Domain: Euclidean<F>, F: Float> Mapping<Domain> for Norm222<Domain, F> {
+impl<Domain: Euclidean<F>, F: Float> Mapping<Domain> for Norm222<F> {
     type Codomain = F;
 
     /// Compute the value of `self` at `x`.
@@ -336,9 +336,9 @@
     }
 }
 
-impl<Domain: Euclidean<F>, F: Float> ConvexMapping<Domain, F> for Norm222<Domain, F> {}
+impl<Domain: Euclidean<F>, F: Float> ConvexMapping<Domain, F> for Norm222<F> {}
 
-impl<Domain: Euclidean<F>, F: Float> Conjugable<Domain, F> for Norm222<Domain, F> {
+impl<Domain: Euclidean<F>, F: Float> Conjugable<Domain, F> for Norm222<F> {
     type Conjugate<'a>
         = Self
     where
@@ -350,7 +350,7 @@
     }
 }
 
-impl<Domain: Euclidean<F>, F: Float> Preconjugable<Domain, Domain, F> for Norm222<Domain, F> {
+impl<Domain: Euclidean<F>, F: Float> Preconjugable<Domain, Domain, F> for Norm222<F> {
     type Preconjugate<'a>
         = Self
     where
@@ -362,7 +362,7 @@
     }
 }
 
-impl<Domain, F> Prox<Domain> for Norm222<Domain, F>
+impl<Domain, F> Prox<Domain> for Norm222<F>
 where
     F: Float,
     Domain: Euclidean<F, Output = Domain>,
diff -r e7f1cb4bec78 -r 103aa137fcb2 src/mapping.rs
--- a/src/mapping.rs	Tue Apr 29 00:03:12 2025 -0500
+++ b/src/mapping.rs	Tue Apr 29 07:55:18 2025 -0500
@@ -9,6 +9,7 @@
 use crate::types::{ClosedMul, Float, Num};
 use std::borrow::Cow;
 use std::marker::PhantomData;
+use std::ops::Mul;
 
 /// A mapping from `Domain` to `Self::Codomain`.
 pub trait Mapping<Domain: Space> {
@@ -283,8 +284,39 @@
     }
 }
 
+/// Helper trait for implementing [`DifferentiableMapping`]
+impl<S, T, X, E, Y> DifferentiableImpl<X> for Composition<S, T, E>
+where
+    X: Space,
+    T: DifferentiableImpl<X> + Mapping<X>,
+    S: DifferentiableImpl<T::Codomain>,
+    E: Copy,
+    //Composition<S::Derivative, T::Derivative, E>: Space,
+    S::Derivative: Mul<T::Derivative, Output = Y>,
+    Y: Space,
+{
+    //type Derivative = Composition<S::Derivative, T::Derivative, E>;
+    type Derivative = Y;
+
+    /// Compute the differential of `self` at `x`, consuming the input.
+    fn differential_impl<I: Instance<X>>(&self, x: I) -> Self::Derivative {
+        // Composition {
+        //     outer: self
+        //         .outer
+        //         .differential_impl(self.inner.apply(x.ref_instance())),
+        //     inner: self.inner.differential_impl(x),
+        //     intermediate_norm_exponent: self.intermediate_norm_exponent,
+        // }
+        self.outer
+            .differential_impl(self.inner.apply(x.ref_instance()))
+            * self.inner.differential_impl(x)
+    }
+}
+
 mod quadratic;
 pub use quadratic::Quadratic;
+mod dataterm;
+pub use dataterm::DataTerm;
 
 /// Trait for indicating that `Self` is Lipschitz with respect to the (semi)norm `D`.
 pub trait Lipschitz<M> {
diff -r e7f1cb4bec78 -r 103aa137fcb2 src/mapping/dataterm.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/mapping/dataterm.rs	Tue Apr 29 07:55:18 2025 -0500
@@ -0,0 +1,124 @@
+/*!
+General deata terms of the form $g(Ax-b)$ for an operator $A$
+to a [`Euclidean`] space, and a function g on that space.
+*/
+
+#![allow(non_snake_case)]
+
+use super::{DifferentiableImpl, DifferentiableMapping, /*LipschitzDifferentiableImpl,*/ Mapping,};
+use crate::convex::ConvexMapping;
+use crate::instance::{Instance, Space};
+use crate::linops::{/*BoundedLinear,*/ Linear, Preadjointable};
+//use crate::norms::{Norm, NormExponent, L2};
+use crate::types::Float;
+use std::ops::Sub;
+//use serde::{Deserialize, Serialize};
+
+/// Functions of the form $\frac{1}{2}\|Ax-b\|_2^2$ for an operator $A$
+/// to a [`Euclidean`] space.
+pub struct DataTerm<
+    F: Float,
+    Domain: Space,
+    A: Mapping<Domain>,
+    G: Mapping<A::Codomain, Codomain = F>,
+> {
+    opA: A,
+    b: <A as Mapping<Domain>>::Codomain,
+    g: G,
+}
+
+#[allow(non_snake_case)]
+impl<F: Float, Domain: Space, A: Mapping<Domain>, G: Mapping<A::Codomain, Codomain = F>>
+    DataTerm<F, Domain, A, G>
+{
+    pub fn new(opA: A, b: A::Codomain, g: G) -> Self {
+        DataTerm { opA, b, g }
+    }
+
+    pub fn operator(&self) -> &'_ A {
+        &self.opA
+    }
+
+    pub fn data(&self) -> &'_ <A as Mapping<Domain>>::Codomain {
+        &self.b
+    }
+
+    pub fn fidelity(&self) -> &'_ G {
+        &self.g
+    }
+}
+
+//+ AdjointProductBoundedBy<RNDM<F, N>, P, FloatType = F>,
+
+impl<F, X, A, G> Mapping<X> for DataTerm<F, X, A, G>
+where
+    F: Float,
+    X: Space,
+    A: Mapping<X>,
+    G: Mapping<A::Codomain, Codomain = F>,
+    A::Codomain: for<'a> Sub<&'a A::Codomain, Output = A::Codomain>,
+{
+    type Codomain = F;
+
+    fn apply<I: Instance<X>>(&self, x: I) -> F {
+        // TODO: possibly (if at all more effcient) use GEMV once generalised
+        // to not require preallocation. However, Rust should be pretty efficient
+        // at not doing preallocations or anything here, as the result of self.opA.apply()
+        // can be consumed, so maybe GEMV is no use.
+        self.g.apply(self.opA.apply(x) - &self.b)
+    }
+}
+
+impl<F, X, A, G> ConvexMapping<X, F> for DataTerm<F, X, A, G>
+where
+    F: Float,
+    X: Space,
+    A: Linear<X>,
+    G: ConvexMapping<A::Codomain, F>,
+    A::Codomain: for<'a> Sub<&'a A::Codomain, Output = A::Codomain>,
+{
+}
+
+impl<F, X, Y, A, G> DifferentiableImpl<X> for DataTerm<F, X, A, G>
+where
+    F: Float,
+    X: Space,
+    Y: Space + for<'a> Sub<&'a Y, Output = Y>,
+    //<A as Mapping<X>>::Codomain: Euclidean<F>,
+    A: Linear<X, Codomain = Y> + Preadjointable<X, G::DerivativeDomain>,
+    //<<A as Mapping<X>>::Codomain as Euclidean<F>>::Output: Instance<<A as Mapping<X>>::Codomain>,
+    G: DifferentiableMapping<Y, Codomain = F>,
+{
+    type Derivative = A::PreadjointCodomain;
+
+    fn differential_impl<I: Instance<X>>(&self, x: I) -> Self::Derivative {
+        // TODO: possibly (if at all more effcient) use GEMV once generalised
+        // to not require preallocation. However, Rust should be pretty efficient
+        // at not doing preallocations or anything here, as the result of self.opA.apply()
+        // can be consumed, so maybe GEMV is no use.
+        //self.opA.preadjoint().apply(self.opA.apply(x) - self.b)
+        self.opA
+            .preadjoint()
+            .apply(self.g.diff_ref().apply(self.opA.apply(x) - &self.b))
+    }
+}
+
+/*
+impl<'a, F, X, ExpX, Y, ExpY, A, G> LipschitzDifferentiableImpl<X, ExpX> for DataTerm<F, X, A, G>
+where
+    F: Float,
+    X: Space + Clone + Norm<F, ExpX>,
+    Y: Space + Norm<F, ExpY>,
+    ExpX: NormExponent,
+    ExpY: NormExponent,
+    A: Clone + BoundedLinear<X, ExpX, L2, F, Codomain = Y>,
+    G: Mapping<Y, Codomain = F> + LipschitzDifferentiableImpl<Y, ExpY>,
+    Self: DifferentiableImpl<X>,
+{
+    type FloatType = F;
+
+    fn diff_lipschitz_factor(&self, seminorm: ExpX) -> Option<F> {
+        Some(self.opA.opnorm_bound(seminorm, L2).powi(2))
+    }
+}
+*/