From 229c8a41324f96dc832ee645df89837107875654 Mon Sep 17 00:00:00 2001
From: Felipe Noronha <felipenoris@gmail.com>
Date: Wed, 10 Apr 2019 09:50:34 -0300
Subject: [PATCH] handle 0-sized chunk case (#399)

---
 src/prelude.jl       | 2 +-
 test/GradientTest.jl | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/prelude.jl b/src/prelude.jl
index fb4fb13d..1eea8211 100644
--- a/src/prelude.jl
+++ b/src/prelude.jl
@@ -14,7 +14,7 @@ const CHUNKS = [Chunk{i}() for i in 1:DEFAULT_CHUNK_THRESHOLD]
 
 function Chunk(input_length::Integer, threshold::Integer = DEFAULT_CHUNK_THRESHOLD)
     N = pickchunksize(input_length, threshold)
-    N <= DEFAULT_CHUNK_THRESHOLD && return CHUNKS[N]
+    0 < N <= DEFAULT_CHUNK_THRESHOLD && return CHUNKS[N]
     return Chunk{N}()
 end
 
diff --git a/test/GradientTest.jl b/test/GradientTest.jl
index 6f30563b..5636e517 100644
--- a/test/GradientTest.jl
+++ b/test/GradientTest.jl
@@ -141,4 +141,13 @@ for T in (StaticArrays.SArray, StaticArrays.MArray)
     @test DiffResults.gradient(sresult3) == DiffResults.gradient(result)
 end
 
+# Issue 399
+
+let
+    f_const(x) = 1.0
+    g_grad_const = x -> ForwardDiff.gradient(f_const, x)
+    @test g_grad_const([1.0]) == [0.0]
+    @test isempty(g_grad_const(zeros(Float64, 0)))
+end
+
 end # module