A cheat sheet for byte conversion of number types in Swift 3.0

endian-conversion-cheat-sheet.swift

func _convertToBytes<T>(_ value: T, withCapacity capacity: Int) -> [UInt8] {
    
    var mutableValue = value
    return withUnsafePointer(to: &mutableValue) {
        
        return $0.withMemoryRebound(to: UInt8.self, capacity: capacity) {
            
            return Array(UnsafeBufferPointer(start: $0, count: capacity))
        }
    }
}

extension Integer {
    
    var bytes: [UInt8] {
        
        return _convertToBytes(self, withCapacity: MemoryLayout<Self>.size)
    }
    
    init?(_ bytes: [UInt8]) {
        
        guard bytes.count == MemoryLayout<Self>.size else { return nil }
        
        self = bytes.withUnsafeBytes {
            
            return $0.load(as: Self.self)
        }
    }
}

extension FloatingPoint {
    
    var bytes: [UInt8] {
        
        return _convertToBytes(self, withCapacity: MemoryLayout<Self>.size)
    }
    
    init?(_ bytes: [UInt8]) {
        
        guard bytes.count == MemoryLayout<Self>.size else { return nil }
        
        self = bytes.withUnsafeBytes {
            
            return $0.load(as: Self.self)
        }
    }
}

let u_int_1: UInt64 = 0x08_07_06_05_04_03_02_01  // 578437695752307201
u_int_1.littleEndian.bytes                       // [1, 2, 3, 4, 5, 6, 7, 8]
u_int_1.bigEndian.bytes                          // [8, 7, 6, 5, 4, 3, 2, 1]
                                                 //
let u_int_2: UInt32 = 0x04_03_02_01              // 67305985
u_int_2.littleEndian.bytes                       // [1, 2, 3, 4]
u_int_2.bigEndian.bytes                          // [4, 3, 2, 1]
                                                 //
let u_int_3: UInt16 = 0x02_01                    // 513
u_int_3.littleEndian.bytes                       // [1, 2]
u_int_3.bigEndian.bytes                          // [2, 1]
                                                 //
let int_1: Int64 = 0x08_07_06_05_04_03_02_01     // 578437695752307201
int_1.littleEndian.bytes                         // [1, 2, 3, 4, 5, 6, 7, 8]
int_1.bigEndian.bytes                            // [8, 7, 6, 5, 4, 3, 2, 1]
                                                 //
let int_2: Int32 = 0x04_03_02_01                 // 67305985
int_2.littleEndian.bytes                         // [1, 2, 3, 4]
int_2.bigEndian.bytes                            // [4, 3, 2, 1]
                                                 //
let int_3: Int16 = 0x02_01                       // 513
int_3.littleEndian.bytes                         // [1, 2]
int_3.bigEndian.bytes                            // [2, 1]


let flp_1: Double = 4.0                             // (on macOS)
flp_1.bytes                                         // [0, 0, 0, 0, 0, 0, 16, 64] - in little endian
flp_1.bytes.reversed() as [UInt8]                   // [64, 16, 0, 0, 0, 0, 0, 0]
                                                    //
let value_1 = UInt64(0x40_10_00_00_00_00_00_00)     // 4616189618054758400
let flp_2 = unsafeBitCast(value_1, to: Double.self) // 4.0
                                                    //
let value_2: UInt64 = UInt64(flp_1.bytes)!          // 4616189618054758400
let flp_3 = unsafeBitCast(value_2, to: Double.self) // 4.0
                                                    //
let flp_4: Float64 = 123456.123456                  //
flp_4.bytes                                         // [168, 255, 172, 249, 1, 36, 254, 64]
Float64(flp_4.bytes)                                // 123456.123456