From 30b85ba7d676f44ee8b5bb967b4976485fbf729e Mon Sep 17 00:00:00 2001
From: Jukka Aho <ahojukka5@gmail.com>
Date: Sat, 9 May 2026 18:38:50 +0300
Subject: [PATCH] test(elements): add interpolate_field_value regression

---
 test/elements/test_interpolate_field_value.jl | 71 +++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 test/elements/test_interpolate_field_value.jl

diff --git a/test/elements/test_interpolate_field_value.jl b/test/elements/test_interpolate_field_value.jl
new file mode 100644
index 0000000..06980a1
--- /dev/null
+++ b/test/elements/test_interpolate_field_value.jl
@@ -0,0 +1,71 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using Test
+using JuliaFEM
+using Tensors
+
+uint_tuple(n::Int) = tuple([UInt(i) for i in 1:n]...)
+
+# Regression coverage for the interpolate_field_value vector branch.
+# Previously the @generated body referenced the function `quantity_type`
+# instead of the local variable `Q`, so the elseif silently failed and
+# the vector branch was unreachable. These tests would have errored out
+# before the fix.
+@testset "interpolate_field_value" begin
+    @testset "Scalar (Temperature) field" begin
+        S = @DOFSet{T::DOF{Temperature, Vertex}}
+        elem = Element{Tetrahedron{4}, Lagrange{1}, S, 4}(UInt(1), uint_tuple(4))
+
+        # Use a constant field; any interpolation must reproduce that value.
+        u_global = fill(2.5, 4)
+        ξ = Vec((0.25, 0.25, 0.25))
+
+        T_val = interpolate_field_value(elem, u_global, :T, ξ)
+        @test T_val isa Float64
+        @test T_val ≈ 2.5
+
+        @inferred interpolate_field_value(elem, u_global, :T, ξ)
+    end
+
+    @testset "Vector (Displacement{3}) field" begin
+        S = @DOFSet{u::DOF{Displacement{3}, Vertex}}
+        elem = Element{Tetrahedron{4}, Lagrange{1}, S, 12}(UInt(1), uint_tuple(12))
+
+        # Constant per-component values; vector interpolation must reproduce
+        # the per-component constant.
+        u_global = Float64[
+            1.0, 2.0, 3.0,  # Node 1
+            1.0, 2.0, 3.0,  # Node 2
+            1.0, 2.0, 3.0,  # Node 3
+            1.0, 2.0, 3.0,  # Node 4
+        ]
+        ξ = Vec((0.25, 0.25, 0.25))
+
+        u_val = interpolate_field_value(elem, u_global, :u, ξ)
+        @test u_val isa Vec{3, Float64}
+        @test u_val ≈ Vec{3}((1.0, 2.0, 3.0))
+
+        @inferred interpolate_field_value(elem, u_global, :u, ξ)
+    end
+
+    @testset "Multi-field (T + u)" begin
+        S = @DOFSet{T::DOF{Temperature, Vertex}, u::DOF{Displacement{3}, Vertex}}
+        elem = Element{Tetrahedron{4}, Lagrange{1}, S, 16}(UInt(1), uint_tuple(16))
+
+        # First 4 entries are temperature, next 12 are displacement.
+        u_global = Float64[
+            5.0, 5.0, 5.0, 5.0,
+            0.5, 0.0, 0.0,
+            0.5, 0.0, 0.0,
+            0.5, 0.0, 0.0,
+            0.5, 0.0, 0.0,
+        ]
+        ξ = Vec((0.25, 0.25, 0.25))
+
+        @test interpolate_field_value(elem, u_global, :T, ξ) ≈ 5.0
+        @test interpolate_field_value(elem, u_global, :u, ξ) ≈ Vec{3}((0.5, 0.0, 0.0))
+
+        @test_throws ErrorException interpolate_field_value(elem, u_global, :nope, ξ)
+    end
+end