[swift-users] appending packed bytes to [UInt8]

[Nevin Brackett-Rozinsky]

Conversely, since I’m pretty sure the memory layout of a *tuple* is
guaranteed to be as it appears, I would probably start with something like
this:

typealias Vec3rgba = (x: Float, y: Float, z: Float, r: Int8, g: Int8, b:
Int8, a: Int8)

Nevin


On Wed, Jan 10, 2018 at 4:27 PM, Jens Persson via swift-users <
swift-users at swift.org> wrote:

> I'm not sure what you mean by "I need the buffer to be a vector /.../" but
> perhaps this may be of some help:
>
> struct S {
>     var x: Float
>     var y: Float
>     var z: Float
>     var r: UInt8
>     var g: UInt8
>     var b: UInt8
>     var a: UInt8
> }
> var buf = S(x: 0.1, y: 1.2, z: 2.3, r: 11, g: 22, b: 33, a: 44)
> print(MemoryLayout<S>.stride) // 16
> withUnsafeMutableBytes(of: &buf) { ptr in
>     print("x:", ptr.load(fromByteOffset: 0, as: Float.self)) // 0.1
>     print("y:", ptr.load(fromByteOffset: 4, as: Float.self)) // 1.2
>     print("z:", ptr.load(fromByteOffset: 8, as: Float.self)) // 2.3
>     print("r:", ptr.load(fromByteOffset: 12, as: UInt8.self)) // 11
>     print("g:", ptr.load(fromByteOffset: 13, as: UInt8.self)) // 22
>     print("b:", ptr.load(fromByteOffset: 14, as: UInt8.self)) // 33
>     print("a:", ptr.load(fromByteOffset: 15, as: UInt8.self)) // 44
> }
>
> NOTE however that the memory layout of Swift-structs is not guaranteed to
> remain like this and is thus not future-proof, although I think that
> if/when things changes, there will be some way to tell the compiler that
> you want the memory to be this "expected" "C-like" layout.
>
> /Jens
>
>
> On Wed, Jan 10, 2018 at 10:03 PM, Kelvin Ma via swift-users <
> swift-users at swift.org> wrote:
>
>> I want to create a buffer with the layout
>>
>> 0           4           8           12      13      14      15      16
>> [   x:Float   |   y:Float   |   z:Float   | r:UInt8 | g:UInt8 | b:UInt8
>> | _:UInt8 ]
>>
>> Normally, I’d use UnsafeRawBufferPointer for this, but I need the buffer
>> to be a vector (i.e. with append(_:)), and for it to return a
>> Swift-managed Array<UInt8>. How should I do this? The project does not
>> use Foundation.
>>
>> _______________________________________________
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>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
>
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