From 81b4b4c1c9572c08d1b83a1ba597602aed308a76 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=BCrgen=20Fuhrmann?= <juergen-fuhrmann@web.de>
Date: Fri, 28 Nov 2025 20:37:32 -0800
Subject: [PATCH] Attempt the implementation of reinit! for dual cache

---
 ext/LinearSolveForwardDiffExt.jl | 38 ++++++++++++++++++++++++++++++--
 test/forwarddiff_overloads.jl    |  8 ++++++-
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/ext/LinearSolveForwardDiffExt.jl b/ext/LinearSolveForwardDiffExt.jl
index cacba0112..467c2b852 100644
--- a/ext/LinearSolveForwardDiffExt.jl
+++ b/ext/LinearSolveForwardDiffExt.jl
@@ -2,7 +2,7 @@ module LinearSolveForwardDiffExt
 
 using LinearSolve
 using LinearSolve: SciMLLinearSolveAlgorithm, __init, LinearVerbosity, DefaultLinearSolver,
-                   DefaultAlgorithmChoice, defaultalg
+                   DefaultAlgorithmChoice, defaultalg, reinit!
 using LinearAlgebra
 using ForwardDiff
 using ForwardDiff: Dual, Partials
@@ -342,6 +342,38 @@ function setu!(dc::DualLinearCache, u)
     partial_vals!(getfield(dc, :partials_u), u) # Update in-place
 end
 
+function SciMLBase.reinit!(cache::DualLinearCache;
+        A = nothing,
+        b = nothing,
+        u = nothing,
+        p = nothing,
+        reuse_precs = false)
+    if !isnothing(A)
+        setA!(cache, A)
+    end
+
+    if !isnothing(b)
+        setb!(cache, b)
+    end
+
+    if !isnothing(u)
+        setu!(cache, u)
+    end
+
+    if !isnothing(p)
+        cache.linear_cache.p=p
+    end
+
+    isfresh = !isnothing(A)
+    precsisfresh = !reuse_precs && (isfresh || !isnothing(p))
+    isfresh |= cache.isfresh
+    precsisfresh |= cache.linear_cache.precsisfresh
+    cache.linear_cache.isfresh = true
+    cache.linear_cache.precsisfresh = precsisfresh
+
+    nothing
+end
+
 function Base.setproperty!(dc::DualLinearCache, sym::Symbol, val)
     # If the property is A or b, also update it in the LinearCache
     if sym === :A
@@ -390,7 +422,9 @@ partial_vals!(out, x) = map!(partial_vals, out, x) # Update in-place
 nodual_value(x) = x
 nodual_value(x::Dual{T, V, P}) where {T, V <: AbstractFloat, P} = ForwardDiff.value(x)
 nodual_value(x::Dual{T, V, P}) where {T, V <: Dual, P} = x.value  # Keep the inner dual intact
-nodual_value(x::AbstractArray{<:Dual}) = nodual_value!(similar(x, typeof(nodual_value(first(x)))), x)
+function nodual_value(x::AbstractArray{<:Dual})
+    nodual_value!(similar(x, typeof(nodual_value(first(x)))), x)
+end
 nodual_value!(out, x) = map!(nodual_value, out, x) # Update in-place
 
 function update_partials_list!(partial_matrix::AbstractVector{T}, list_cache) where {T}
diff --git a/test/forwarddiff_overloads.jl b/test/forwarddiff_overloads.jl
index 46116e7a3..d70e86fb6 100644
--- a/test/forwarddiff_overloads.jl
+++ b/test/forwarddiff_overloads.jl
@@ -188,6 +188,12 @@ backslash_x_p = A \ b
 
 @test ≈(overload_x_p, backslash_x_p, rtol = 1e-9)
 
+A[1, 1]+=2
+cache = overload_x_p.cache
+reinit!(cache; A = sparse(A))
+overload_x_p = solve!(cache, UMFPACKFactorization())
+backslash_x_p = A \ b
+@test ≈ (overload_x_p, backslash_x_p, rtol = 1e-9)
 
 # Test that GenericLU doesn't create a DualLinearCache
 A, b = h([ForwardDiff.Dual(5.0, 1.0, 0.0), ForwardDiff.Dual(5.0, 0.0, 1.0)])
@@ -234,4 +240,4 @@ grad = ForwardDiff.gradient(component_linsolve, p_test)
 @test grad isa Vector
 @test length(grad) == 2
 @test !any(isnan, grad)
-@test !any(isinf, grad)
\ No newline at end of file
+@test !any(isinf, grad)