Swift 4 CC_SHA1 and fast toHexString extensions

Digest.swift

import Foundation

private func loadCSymbol<T>(named name: String, of _: T.Type = T.self) -> T {
    let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
    guard let sym = dlsym(RTLD_DEFAULT, name) else { preconditionFailure(String(cString: dlerror())) }
    return unsafeBitCast(sym, to: T.self)
}

private let CC_SHA1_DIGEST_LENGTH = 20
private let CC_SHA1: @convention(c) (UnsafeRawPointer, Int32, UnsafeMutablePointer<UInt8>) -> UnsafeMutablePointer<UInt8> = loadCSymbol(named: "CC_SHA1")

extension Data {

    public func toHexString() -> String {
        return lazy.map { (byte) in
            (byte <= 0xF ? "0" : "") + String(byte, radix: 16)
        }.joined()
    }

}

extension String {

    public func SHA1Sum() -> String {
        return withCString { (ptr) in
            var data = Data(repeating: 0, count: CC_SHA1_DIGEST_LENGTH)
            _ = data.withUnsafeMutableBytes { CC_SHA1(ptr, numericCast(strlen(ptr)), $0) }
            return data.toHexString()
        }
    }

}