Check if a buffer for the decompression of acyclic coloring is needed

src/decompression.jl

@@ -523,7 +523,7 @@ function decompress!(
     R = eltype(A)
     fill!(A, zero(R))
 
-    if eltype(buffer) == R
+    if eltype(buffer) == R || isempty(buffer)
         buffer_right_type = buffer
     else
         buffer_right_type = similar(buffer, R)
@@ -599,7 +599,7 @@ function decompress!(
     nzA = nonzeros(A)
     uplo == :F && check_same_pattern(A, S)
 
-    if eltype(buffer) == R
+    if eltype(buffer) == R || isempty(buffer)
         buffer_right_type = buffer
     else
         buffer_right_type = similar(buffer, R)

src/result.jl

@@ -362,7 +362,8 @@ function TreeSetColoringResult(
 
     # buffer holds the sum of edge values for subtrees in a tree.
     # For each vertex i, buffer[i] is the sum of edge values in the subtree rooted at i.
-    buffer = Vector{R}(undef, nvertices)
+    # Note that we don't need a buffer is all trees are stars.
+    buffer = all(is_star) ? R[] : Vector{R}(undef, nvertices)
 
     return TreeSetColoringResult(
         A,

test/allocations.jl

@@ -132,3 +132,21 @@ end;
         test_noallocs_structured_decompression(1000; structure, partition, decompression)
     end
 end;
+
+@testset "Multi-precision acyclic decompression" begin
+    @testset "$format" for format in ("dense", "sparse")
+        A = [0 0 1; 0 1 0; 1 0 0]
+        if format == "sparse"
+            A = sparse(A)
+        end
+        problem = ColoringProblem(; structure=:symmetric, partition=:column)
+        result = coloring(A, problem, GreedyColoringAlgorithm{:substitution}())
+        @test isempty(result.buffer)
+        for T in (Float32, Float64)
+            C = rand(T) * T.(A)
+            B = compress(C, result)
+            bench_multiprecision = @be decompress!(C, B, result)
+            @test minimum(bench_multiprecision).allocs == 0
+        end
+    end
+end