SCNGeometrySource+vertices.swift

/**
 Returns an array of vertex positions.
 
 - precondition: the geoemetry must have vertices which are 3-vectors with floating point component values
 
 */
fileprivate func vertices(source:SCNGeometrySource) -> [SCNVector3]
{
    precondition(source.usesFloatComponents == true, "I can only handle three-vectors whose components are floating-point values, i.e., floats or doubles")
    precondition(source.componentsPerVector == 3, "I can only be used for three vectors")
    
    let shouldUseFloatNotDouble:Bool
    if source.bytesPerComponent == 4 {
        shouldUseFloatNotDouble = true
    }
    else if source.bytesPerComponent == 8 {
        shouldUseFloatNotDouble = false
    }
    else {
        assert(false, "The SCNGeometrySource has reported an unexpected byte size for its vector components, not 4 bytes (float) or 8 bytes (double) but \(source.bytesPerComponent). I am not equipped for this so I am going to use floats and hope for the best. This will probably not work. Sorry.")
        shouldUseFloatNotDouble = true
    }
    
    let vectors = source.data.withUnsafeBytes {
        (p:UnsafePointer<UInt8>) -> [SCNVector3] in
        let rawPtr = UnsafeRawPointer(p)
        var v:[SCNVector3] = []
        v.reserveCapacity(source.vectorCount)
        for byteOffset in stride(from: source.dataOffset,
                                 to: source.dataOffset + source.dataStride * source.vectorCount,
                                 by: source.dataStride)
        {
            if shouldUseFloatNotDouble {
                let ff = rawPtr.load(fromByteOffset: byteOffset, as: float3.self)
                let val = SCNVector3(ff)
                v.append(val)
            }
            else {
                let ff = rawPtr.load(fromByteOffset: byteOffset, as: double3.self)
                let val = SCNVector3(ff)
                v.append(val)
            }
        }
        return v
    }
    return vectors
}