Update MTLBuffer from Array with offset

Question

I'm trying some test coding with swift, Metal and iOS. I have an MTLBuffer and an Array:

    var uint16Array:[UInt16] = Array(repeating: 0, count: 16)
    var mtlBuffer:MTLBuffer = device.makeBuffer(bytes: uint16Array, length: 32)!

If I update the array

    uint16Array[0] = 22
    uint16Array[7] = 22
    uint16Array[15] = 22

then I can update the mtlbuffer:

    mtlBuffer.contents().copyMemory(from: uint16Array, byteCount:32)

But... If I want to copy from array index != 0.... How to do?.

"pseudocode":

     mtlBuffer.contents().copyMemory(from: uint16Array[4], byteCount:22) 

(not works, of course)

I have tried some code with "withUnsafeBufferPointer", "UnsafeRawPointer(uint16Array + 4)", etc but without success.

Answer 1

Access the non-zero index of uint16Array via withUnsafePointer

import Metal

var uint16Array:[UInt16] = Array(repeating: 0, count: 16)
let device = MTLCreateSystemDefaultDevice()!
var mtlBuffer: MTLBuffer = device.makeBuffer(
    bytes: uint16Array, 
    length: uint16Array.count * MemoryLayout<Int16>.stride
)!

// Print the original values
print("Original mtlBuffer:")
printBufferValues(buffer: mtlBuffer) 
// [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

uint16Array = [UInt16](0..<16)

let firstIndex = 3
let lastIndex = 10
let range = lastIndex - firstIndex

withUnsafePointer(to: &uint16Array[firstIndex]) {
    mtlBuffer
        .contents()
        .advanced(by: firstIndex * MemoryLayout<Int16>.stride)
        .copyMemory(from: $0, byteCount: range * MemoryLayout<Int16>.stride)
}


// Print the modified values
print("\nModified mtlBuffer:")
printBufferValues(buffer: mtlBuffer) 
// [0, 0, 0, 3, 4, 5, 6, 7, 8, 9, 0, 0, 0, 0, 0, 0]

// Function to print buffer values
func printBufferValues(buffer: MTLBuffer) {
    let pointer = buffer.contents().assumingMemoryBound(to: Int16.self)
    let bufferValues = UnsafeBufferPointer(start: pointer, count: buffer.length / MemoryLayout<Int16>.stride)
    print(Array(bufferValues))
}