Fix coloring with empty matrix as input

src/interface.jl

@@ -305,7 +305,13 @@ function _coloring(
         vertices_in_order = vertices(ag, order)
         return acyclic_coloring(ag, vertices_in_order, algo.postprocessing)
     end
-    color, tree_set = argmin(maximum ∘ first, color_and_tree_set_by_order)
+    # if `color` is empty, `maximum` will fail but `color_and_tree_set_by_order`
+    # is also one so we can just add a special case for this
+    if length(color_and_tree_set_by_order) == 1
+        color, tree_set = only(color_and_tree_set_by_order)
+    else
+        color, tree_set = argmin(maximum ∘ first, color_and_tree_set_by_order)
+    end
     if speed_setting isa WithResult
         return TreeSetColoringResult(A, ag, color, tree_set, R)
     else

src/result.jl

@@ -82,6 +82,9 @@ Create a color-indexed vector `group` such that `i ∈ group[c]` iff `color[i] =
 Assumes the colors are contiguously numbered from `0` to some `cmax`.
 """
 function group_by_color(::Type{T}, color::AbstractVector) where {T<:Integer}
+    if isempty(color)
+        return typeof(view(T[], 1:0))[]
+    end
     cmin, cmax = extrema(color)
     @assert cmin >= 0
     # Compute group sizes and offsets for a joint storage

test/check.jl

@@ -1,4 +1,5 @@
 using LinearAlgebra
+using SparseArrays
 using SparseMatrixColorings:
     structurally_orthogonal_columns,
     symmetrically_orthogonal_columns,
@@ -308,3 +309,12 @@ end
     log = (:warn, "4 colors provided for 3 rows.")
     @test_logs log !substitutable_bidirectional(A, B, [1, 0, 0, 1], [0, 1, 1]; verbose=true)
 end
+
+# See https://github.com/gdalle/SparseMatrixColorings.jl/pull/300
+@testset "Empty matrix" begin
+    problem = ColoringProblem(; structure=:symmetric, partition=:column)
+    algo = GreedyColoringAlgorithm(; decompression=:substitution)
+    S = spzeros(Int, 0, 0)
+    result = coloring(S, problem, algo)
+    @test isempty(result.color)
+end