From b558c57ad8b865b7f9f51f4b1e757b664b45eb74 Mon Sep 17 00:00:00 2001
From: Oliver Schulz <oschulz@mpp.mpg.de>
Date: Wed, 24 Jul 2024 12:38:24 +0200
Subject: [PATCH] Support views on empty UnsafeArrays

---
 src/unsafe_array.jl  | 12 +++++++++++-
 test/unsafe_array.jl | 12 ++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/unsafe_array.jl b/src/unsafe_array.jl
index da3e51d..337a411 100644
--- a/src/unsafe_array.jl
+++ b/src/unsafe_array.jl
@@ -87,12 +87,22 @@ Base.@propagate_inbounds _unsafe_view_impl(IFwd::NTuple{M,Base.ViewIndex}, A::Un
     _unsafe_view_impl((IFwd..., i), A, I...)
 
 @inline function _unsafe_view_impl(IFwd::NTuple{M,Base.Slice}, A::UnsafeArray{T,N}, i::DenseIdx, I::Integer...) where {T,M,N}
+    #@info "DEBUG" _unsafe_view_impl IFwd i I T M N
+    #global g_state = (;IFwd, i, I, T, M, N)
+    ax_A = axes(A)
     @assert IFwd == ntuple(i -> axes(A)[i], Val{M}())
     I_all = (IFwd..., i, I...)
     @boundscheck checkbounds(A, I_all...)
     startidxs = map(first, (IFwd..., i, I...))
+    firstidxs = map(first, ax_A)
     sub_s = _sub_size(I_all...)
-    p = pointer(A, LinearIndices(size(A))[startidxs...])
+    A_isempty = length(A) == 0
+    view_isempty = prod(sub_s) == 0
+    # getindex on LinearIndices fails with startidxs if A is empty, so use pseudo-size in that case:
+    sz = size(A)
+    pseudo_sz = ntuple(_ -> 1, Val(N))
+    mod_size, mod_startidxs = ifelse(A_isempty && view_isempty, (pseudo_sz, firstidxs), (sz, startidxs))
+    p = pointer(A, LinearIndices(mod_size)[mod_startidxs...])
     UnsafeArray(p, sub_s)
 end
 
diff --git a/test/unsafe_array.jl b/test/unsafe_array.jl
index 4c32b10..14f180a 100644
--- a/test/unsafe_array.jl
+++ b/test/unsafe_array.jl
@@ -34,6 +34,13 @@ using Random
     end
 
 
+    function test_empty_A_UA(test_code::Function, ::Type{T}, Val_N::Val{N}) where {T, N}
+        A = rand(T, 0, fill(4, N-1)...)
+        UA = UnsafeArray(pointer(A), size(A))
+        UnsafeArrays.@gc_preserve A test_code(A, UA)
+    end
+
+
     @testset "ctors" begin
         test_A(Float64, Val(0)) do A
             ptr = pointer(A)
@@ -146,6 +153,11 @@ using Random
             @test isbits(view(UA, :, :, :)) == true
             @test isbits(view(UA, :, 3, :)) == true
         end
+
+        test_empty_A_UA(Float32, Val(2)) do A, UA
+            @test @inferred(view(UA, :, 2:3)) isa UnsafeArray
+            @test size(@inferred(view(UA, :, 2:3))) == (0, 2)
+        end
     end