数组 – Swift位数组到字节数组(UInt8数组)

func bitsToBytes(bits: [Bit]) -> [UInt8] {
    let numBits = bits.count
    let numBytes = (numBits + 7)/8
    var bytes = [UInt8](count : numBytes,repeatedValue : 0)

    for (index,bit) in enumerate(bits) {
        if bit == .One {
            bytes[index / 8] += 1 << (7 - index % 8)
        }
    }

    return bytes
}

主要思想是对于位数组中的给定索引,
index / 8是字节数组中的对应索引,
和索引％8是字节中的位位置.您可以
使用指数％8或7 – 指数％8作为转移金额,具体取决于
所需的位顺序.

例：

// 0110 0100  0000 1001
let bits : [Bit] = [.Zero,.One,.Zero,.One]
let bytes = bitsToBytes(bits)
println(bytes) // [100,9]

或者,您可以“内联”每个组的计算
8位您必须检查哪种解决方案表现更好
在你的情况下.

func bitsToBytes(bits: [Bit]) -> [UInt8] {
    let numBits = bits.count
    let numBytes = numBits/8
    var bytes = [UInt8](count : numBytes,repeatedValue : 0)
    for pos in 0 ..< numBytes {
        let val = 128 * bits[8 * pos].toIntMax() + 
            64 * bits[8 * pos + 1].toIntMax() +
            32 * bits[8 * pos + 2].toIntMax() +
            16 * bits[8 * pos + 3].toIntMax() +
            8 * bits[8 * pos + 4].toIntMax() +
            4 * bits[8 * pos + 5].toIntMax() +
            2 * bits[8 * pos + 6].toIntMax() +
            1 * bits[8 * pos + 7].toIntMax()
        bytes[pos] = UInt8(val)
    }
    return bytes
}

这里,为简单起见,如果位数不是8的倍数,则忽略任何多余的位.相同的代码也可以写一点
“斯威夫特”为

func bitsToBytes(bits: [Bit]) -> [UInt8] {
    return map(0 ..< bits.count/8) {
        pos in
        let val = 128 * bits[8 * pos].toIntMax() + 
            64 * bits[8 * pos + 1].toIntMax() +
            32 * bits[8 * pos + 2].toIntMax() +
            16 * bits[8 * pos + 3].toIntMax() +
            8 * bits[8 * pos + 4].toIntMax() +
            4 * bits[8 * pos + 5].toIntMax() +
            2 * bits[8 * pos + 6].toIntMax() +
            1 * bits[8 * pos + 7].toIntMax()
        return (UInt8(val))
    }
}

基准：现在是一个快速而肮脏的基准测试应用程序(下面的代码),比较各种解决方案.
它测量转换长度为256的10,000位数组的时间.
测试是在MacBook Pro 2,3 GHz Intel Core i7上完成的,
以及使用“Release”配置编译的代码.

Swift 1.1 / Xcode 6.2(6C131e)的结果：

Martin1: 0.0460730195045471
Martin2: 0.0280380249023438
Martin3: 0.0374950170516968
Antonio: 5.85363000631332
Nate   : 4.86936402320862

Swift 1.2 / Xcode 6.3(6D532l)的结果：

Martin1: 0.0228430032730103
Martin2: 0.00573796033859253
Martin3: 0.00732702016830444
Antonio: 0.515677988529205
Nate   : 0.634827971458435

码：

protocol BitsToBytesConverter {
    var ident : String { get }
    func bitsToBytes(bits: [Bit]) -> [UInt8]
}

class MR1 : BitsToBytesConverter {

    let ident = "Martin1"
    func bitsToBytes(bits: [Bit]) -> [UInt8] {
        let numBits = bits.count
        let numBytes = (numBits + 7)/8
        var bytes = [UInt8](count : numBytes,repeatedValue : 0)

        for (index,bit) in enumerate(bits) {
            if bit == .One {
                bytes[index / 8] += UInt8(1 << (7 - index % 8))
            }
        }

        return bytes
    }
}

class MR2 : BitsToBytesConverter {

    let ident = "Martin2"

    func bitsToBytes(bits: [Bit]) -> [UInt8] {
        let numBits = bits.count
        let numBytes = numBits/8
        var bytes = [UInt8](count : numBytes,repeatedValue : 0)
        for pos in 0 ..< numBytes {
            let val = 128 * bits[8 * pos].toIntMax() + 
                64 * bits[8 * pos + 1].toIntMax() +
                32 * bits[8 * pos + 2].toIntMax() +
                16 * bits[8 * pos + 3].toIntMax() +
                8 * bits[8 * pos + 4].toIntMax() +
                4 * bits[8 * pos + 5].toIntMax() +
                2 * bits[8 * pos + 6].toIntMax() +
                1 * bits[8 * pos + 7].toIntMax()
            bytes[pos] = UInt8(val)
        }
        return bytes
    }
}

class MR3 : BitsToBytesConverter {

    let ident = "Martin3"

    func bitsToBytes(bits: [Bit]) -> [UInt8] {
        return map(0 ..< bits.count/8) {
            pos in
            let val = 128 * bits[8 * pos].toIntMax() + 
                64 * bits[8 * pos + 1].toIntMax() +
                32 * bits[8 * pos + 2].toIntMax() +
                16 * bits[8 * pos + 3].toIntMax() +
                8 * bits[8 * pos + 4].toIntMax() +
                4 * bits[8 * pos + 5].toIntMax() +
                2 * bits[8 * pos + 6].toIntMax() +
                1 * bits[8 * pos + 7].toIntMax()
            return (UInt8(val))
        }
    }
}

class AB : BitsToBytesConverter {

    let ident = "Antonio"

    typealias IntegerType = UInt8

    func bitsToBytes(bits: [Bit]) -> [UInt8] {

        let initial = [IntegerType]()

        return reduce(enumerate(bits),initial) { array,element in
            // The size in bits of a UInt8
            let size = sizeof(IntegerType) * 8

            // Create a mutable copy of the array returned at the previous iteration
            var next = array

            // If it's the first iteration,or an iteration divisible by the size of UInt8,// append a new element to the array
            if element.index % size == 0 {
                next.append(0x00)
            }

            // Shift all bits of the last element to the left
            next[next.count - 1] <<= 1

            // If the current bit is one,add 1 to the rightmost bit
            // Using a logical OR
            if element.element == .One {
                next[next.count - 1] |= 0x01
            }

            return next
        }
    }
}

class NC : BitsToBytesConverter {

    let ident = "Nate   "

    func group<T>(array: [T],byCount groupCount: Int) -> [Slice<T>] {
        // get a list of the start indices
        let startIndices = stride(from: 0,to: array.count,by: groupCount)
        // add `groupCount` to each to get the end indices
        let endIndices = lazy(startIndices).map { advance($0,groupCount,array.count) }

        // zip those together & map onto an array of slices of the input array
        return map(Zip2(startIndices,endIndices)) {
            array[$0.0 ..< $0.1]
        }
    }

    func bitsToByte(bits: Slice<Bit>) -> UInt8 {
        return bits.reduce(0) { accumulated,current in
            accumulated << 1 | (current == .One ? 1 : 0)
        }
    }

    func bitsToBytes(bits: [Bit]) -> [UInt8] {
        return group(bits,byCount: 8).map(bitsToByte)
    }
}


let numBits = 256 // Bits per bit array
let numBitArrays = 10000 // Number of bit arrays

func randomBits() -> [Bit] {
    return map(0 ..< numBits) { _  in
        Bit(rawValue: Int(arc4random_uniform(2)))!
    }
}

func randomBitsArray() -> [[Bit]] {
    return map(0 ..< numBitArrays) { _ in
        randomBits()
    }
}

let bitsArray = randomBitsArray()

func test(conv : BitsToBytesConverter) {
    let x = conv.bitsToBytes([])
    let startTime = NSDate()
    for bits in bitsArray {
        let bytes = conv.bitsToBytes(bits)
    }
    let duration = -startTime.timeIntervalSinceNow
    println("(conv.ident): (duration)")
}

test(MR1())
test(MR2())
test(MR3())
test(AB())
test(NC())