State keeps the vector field of an empty state variable

CHANGELOG.md

@@ -23,6 +23,14 @@ makes it worth keeping.
   `src/public.jl`. The file tests the public names of `src/methods.jl`, but `test/methods.jl`
   already exists, so it moved to `test/integration/`.
 
+### Fixed
+
+- **`State` keeps the vector field of an empty state variable.** A field was dropped where
+  `all(ismissing, x)` held, which is true of every empty array, so a state variable with a zero
+  extent lost its field and a `6 × 0` particle matrix had no `q̇` to fill — `integrate` raised
+  `FieldError` on any problem whose state is empty. A field is now dropped only where
+  `_vectorfield` returns `missing`, so a vector field mirrors the shape of its state variable.
+
 ## [0.15.0] — 2026-10-01
 
 ### Breaking Changes

KNOWN_ISSUES.md

@@ -47,3 +47,18 @@ Defects found in review and not yet fixed.
   1.14.3–2.2.2 by Julia.
 - **kind:** not verified
 - **found:** PR #32
+
+### K6 · `L2norm(x, y)` raises on an empty argument, so a nonlinear solve of an empty state stops in its status.
+
+- **location:** `src/utils/norms.jl:2-5`
+- **evidence:** `L2norm(x, y)` is `mapfoldl(z -> (z[1] - z[2])^2, +, zip(x, y))`, and `mapfoldl` with
+  `+` refuses an empty collection: `L2norm(Float64[], Float64[])` gives
+  `ArgumentError: reducing over an empty collection is not allowed; consider supplying init to the
+  reducer`. The one-argument method at `:14-17` guards with `isempty(x) && return zero(T)`, so a
+  norm of an empty array is `0`; the two-argument method has no such guard. Its callers include
+  `SimpleSolvers` `src/nonlinear/nonlinear_solver_status.jl:143,145`, which `residuals(state)`
+  computes at every iteration of every nonlinear solve — the step an implicit integrator takes —
+  and GeometricOptimizers' line searches. So a solve over an empty state fails in the status, with
+  a message that names no solver and no integrator.
+- **kind:** defect
+- **found:** 2026-10-11

Project.toml

@@ -1,6 +1,6 @@
 name = "GeometricBase"
 uuid = "9a0b12b7-583b-4f04-aa1f-d8551b6addc9"
-version = "0.15.0"
+version = "0.15.1"
 authors = ["Michael Kraus"]
 
 [deps]

src/data/state.jl

@@ -76,9 +76,11 @@ function State(initialtime::Real, ics::NamedTuple; initialize = true)
     vectorfield = map(x -> _vectorfield(x), ics)
 
     # remove all fields that are missing, i.e., that correspond to a variable without vectorfield.
-    # `_vectorfield` returns the scalar `missing` for those, so that case is tested on its own:
-    # `all(ismissing, missing)` cannot iterate a scalar, and a broadcast here would allocate.
-    _novectorfield(x) = ismissing(x) || all(ismissing, x)
+    # A field is dropped only where `_vectorfield` returns `missing`, which it does for a time,
+    # algebraic or vector-field variable and nowhere else. A vector field mirrors the shape of its
+    # state variable, so an empty state variable keeps its field: `all(ismissing, x)` is true of
+    # every empty array, which would drop the vector field of a `6 × 0` state.
+    _novectorfield(x) = ismissing(x)
     vectorfield_filtered = NamedTuple{filter(k -> !_novectorfield(vectorfield[k]), keys(vectorfield))}(vectorfield)
 
     # create vector field symbols with dotted solution symbols

test/data/state.jl

@@ -171,3 +171,21 @@ end
 
     @test isnan(st)
 end
+
+@testset "$(rpad("State of an Empty or Scalar State Variable",80))" begin
+    # A vector field mirrors the shape of its state variable, so a state variable with a zero
+    # extent keeps its field. `all(ismissing, x)` is true of every empty array, and dropping the
+    # `q̇` of a `6 × 0` state leaves it without a vector field.
+    for q in (zeros(6, 0), zeros(0), zeros(3, 0, 2), fill(1.0))
+        st = State(0.0, (q = StateVariable(q),))
+        @test :q̇ ∈ keys(st)
+        @test size(st.q̇) == size(st.q)
+    end
+
+    # a field is still dropped where `_vectorfield` returns `missing`
+    st = State(0.0, (t = TimeVariable(1.0), q = StateVariable(zeros(6, 0)),
+        λ = AlgebraicVariable(zeros(0))))
+    @test :q̇ ∈ keys(st)
+    @test :q ∈ keys(vectorfield(st))
+    @test length(vectorfield(st)) == 1
+end