md5 计算。被注释掉的部分是较冗长的写法

Digest.swift

// 需要在 xxx-Bridging-Header.h 中引入 CommonCrypto 
// #import <CommonCrypto/CommonCrypto.h> // add to your xxx-Bridging-Header.h
// 所以在 playground 中无法测试下面的代码 ( NOT work with Playground )


// 阅读代码时注意以下几点：
// 1. Data 本质是 UInt8 数组，所以 String.utf8.count 与 Data.count 是相等的（String 转换成 Data 前后）
// 2. 如果一个C方法接受的参数类型是 void*，即 Swift 里的 UnsafeRawPointer，那么它也接受 UnsafePointer<T> 类型
// 3. 如果某个方法接受 UnsafePointer<UInt8> 类型参数，那么直接填入 String 也是可以的

class Digest {

    static func md5(_ data: Data) -> Data {
//        var digestData = Data(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
//        digestData.withUnsafeMutableBytes { (digestBytes: UnsafeMutablePointer<UInt8>) -> Void in
//            data.withUnsafeBytes { (messageBytes: UnsafePointer<Int8>) -> Void in
//                CC_MD5(messageBytes, CC_LONG(data.count), digestBytes)
//            }
//        }
//        return digestData

        var digest = Array<UInt8>.init(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
        CC_MD5((data as NSData).bytes, CC_LONG(data.count), &digest)
        return Data.init(bytes: digest)
    }

    static func md5(_ string: String) -> Data {
//        var data = Data(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
//        data.withUnsafeMutableBytes { (bytes: UnsafeMutablePointer<UInt8>) -> Void in
//            CC_MD5(string, CC_LONG(string.utf8.count), bytes)
//        }
//        return data

        var digest = Array<UInt8>.init(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
        CC_MD5(string, CC_LONG(string.utf8.count), &digest)
        return Data.init(bytes: digest)
    }

    static func md5(contentOf url: URL) throws -> Data {
        let handle = try FileHandle(forReadingFrom: url)
        var ctx = CC_MD5_CTX()
        CC_MD5_Init(&ctx)
        var count = 0
        repeat {
            let fileData = handle.readData(ofLength: 256) // CHUNK_SIZE, or 1024 * 1024
            count = fileData.count
            fileData.withUnsafeBytes({ (bytes: UnsafePointer<CChar>) -> Void in
                CC_MD5_Update(&ctx, bytes, CC_LONG(count))
            })
        } while count > 0
        var digest = Array<UInt8>.init(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
        CC_MD5_Final(&digest, &ctx)
        return Data.init(bytes: digest)
    }
}


extension Data {
    var md5: Data {
        return Digest.md5(self)
    }

    // 事实上 hexDescription 和 hexEncodedString 结果是一致的
    // 前者取巧复用了 NSData 在 po 输出时就是16进制的特点
    // 后者对data的每个uint8转为16进制字符串并拼接在一起，更符合直观定义
    var hexDescription: String {
        // 删除首尾的<>符号；删除空格
        return (self as NSData).description.trimmingCharacters(in: CharacterSet(charactersIn: "<>")).replacingOccurrences(of: " ", with: "")
    }

    var hexEncodedString: String {
        return map { String(format: "%02hhx", $0) }.joined()
    }
}

extension String {
    var md5: Data {
        return Digest.md5(self)
    }
}

extension URL {
    var md5: Data? {
        return try? Digest.md5(contentOf: self)
    }
}

// just a test function
func testMD5() {
    let s = "A quick brown fox jumps over the lazy dog."
    var ctx = CC_MD5_CTX()
    var digest = Array<UInt8>.init(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
    CC_MD5_Init(&ctx)
    CC_MD5_Update(&ctx, s, CC_LONG(s.utf8.count))
    CC_MD5_Final(&digest, &ctx)

    print(digest.map { String(format:"%02x", $0) }.reduce("", +))
    print(Data.init(bytes: digest) as NSData)

    print(s.md5.hexDescription)
}