Add function unsafe_from_parts (#25)

Author: jakobnissen

CHANGELOG.md

@@ -4,6 +4,10 @@ Any new features, or breaking changes, will be written in this file.
 Bugfixes, internal refactors, documentation improvements and style changes will
 not be mentioned here, because they do not impact how the package is to be used.
 
+## 0.3.4
+* Add new function `unsafe_from_parts` to construct a memory view from a
+  `MemoryRef`.
+
 ## 0.3.3
 ### New features
 * Add a new `split_each` function, which iterates over memory views delimited

Project.toml

@@ -1,6 +1,6 @@
 name = "MemoryViews"
 uuid = "a791c907-b98b-4e44-8f4d-e4c2362c6b2f"
-version = "0.3.3"
+version = "0.3.4"
 authors = ["Jakob Nybo Nissen <jakobnybonissen@gmail.com>"]
 
 [weakdeps]

src/MemoryViews.jl

@@ -7,7 +7,8 @@ export MemoryView,
     IsMemory,
     NotMemory,
     inner,
-    split_each
+    split_each,
+    unsafe_from_parts
 
 public Mutable, Immutable, DelimitedIterator
 
@@ -96,6 +97,35 @@ end
 const MutableMemoryView{T} = MemoryView{T, Mutable}
 const ImmutableMemoryView{T} = MemoryView{T, Immutable}
 
+"""
+    unsafe_from_parts(ref::MemoryRef{T}, len::Int)::MutableMemoryView{T}
+
+Create a mutable memory view from its parts.
+
+**Safety:** Callers are responsible to ensure that:
+* `len` is not negative
+* All indices `i in 1:len` are valid for `ref` (i.e. `memoryref(ref, i)` would
+  not throw)
+* If `ref` is derived from immutable memory, the returned memory view must not
+  be mutated. The caller should immediately convert it to an immutable view.
+
+# Examples
+```jldoctest
+julia> v = [1,2,3,4];
+
+julia> ref = Base.cconvert(Ptr, v);
+
+julia> view = unsafe_from_parts(ref, 3)
+3-element MutableMemoryView{Int64}:
+ 1
+ 2
+ 3
+```
+"""
+function unsafe_from_parts(ref::MemoryRef{T}, len::Int) where {T}
+    return MemoryView{T, Mutable}(unsafe, ref, len)
+end
+
 _get_mutability(::MemoryView{T, M}) where {T, M} = M
 
 # Mutable mem views can turn into immutable ones, but not vice versa

test/runtests.jl

@@ -693,6 +693,16 @@ end
     end
 end
 
+@testset "From parts" begin
+    v = [1 2; 3 4]
+    ref = Base.cconvert(Ptr, v)
+    mem = unsafe_from_parts(ref, 3)
+    @test mem == [1, 3, 2]
+    @test unsafe_from_parts(ref, 2) == [1, 3]
+    @test isempty(unsafe_from_parts(ref, 0))
+    @test mem isa MutableMemoryView{Int}
+end
+
 @testset "MemoryKind" begin
     @test MemoryKind(Vector{Int16}) == IsMemory(MutableMemoryView{Int16})
     @test MemoryKind(typeof(codeunits(view("abc", 2:3)))) ==