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is it possible to construct simd_float4x4 from a string, eg: I had a string which stored simd_float4x4.debugdescription's value ?

aznelite89
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    Are you looking for an efficient SIMD implementation of `atof()`? There's an x86 SIMD `atoi()` at [How to implement atoi using SIMD?](https://stackoverflow.com/q/35127060), but NEON has different horizontal-add instructions. If you want to parse commas, [Fastest way to get IPv4 address from string](https://stackoverflow.com/q/31679341) might be interesting, but again that's very x86-specific. ARM doesn't have `pmovmskb`. (And arbitrary-length FP values as strings wouldn't work well, unlike dotted-quads). – Peter Cordes Jul 29 '18 at 15:19

1 Answers1

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Here is an extension for simd_float4x4 that adds a failable init that takes a debug description and creates the simd_float4x4. It is a failable init because the string might be ill formed.

import simd

extension simd_float4x4 {
    init?(_ string: String) {
        let prefix = "simd_float4x4"
        guard string.hasPrefix(prefix) else { return nil }

        let csv = string.dropFirst(prefix.count).components(separatedBy: ",")
        let filtered = csv.map { $0.filter { Array("-01234567890.").contains($0) } }
        let floats = filtered.compactMap(Float.init)
        guard floats.count == 16 else { return nil }

        let f0 = float4(Array(floats[0...3]))
        let f1 = float4(Array(floats[4...7]))
        let f2 = float4(Array(floats[8...11]))
        let f3 = float4(Array(floats[12...15]))

        self = simd_float4x4(f0, f1, f2, f3)
    }
}

Test

let col0 = float4(0.1, 0.2, 0.3, 0.4)
let col1 = float4(1.1, 1.2, 1.3, 1.4)
let col2 = float4(2.1, 2.2, 2.3, 2.4)
let col3 = float4(-3.1, -3.2, -3.3, -3.4)

var x = simd_float4x4(col0, col1, col2, col3)
print(x)

let xDesc = x.debugDescription

if let y = simd_float4x4(xDesc) {
    print(y)
}

Output

simd_float4x4([[0.1, 0.2, 0.3, 0.4)], [1.1, 1.2, 1.3, 1.4)], [2.1, 2.2, 2.3, 2.4)], [-3.1, -3.2, -3.3, -3.4)]])
simd_float4x4([[0.1, 0.2, 0.3, 0.4)], [1.1, 1.2, 1.3, 1.4)], [2.1, 2.2, 2.3, 2.4)], [-3.1, -3.2, -3.3, -3.4)]])
vacawama
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