我正在为大型视频文件创建MD5校验和。我当前正在使用代码:
extension NSData { func MD5() -> NSString { let digestLength = Int(CC_MD5_DIGEST_LENGTH) let md5Buffer = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: digestLength) CC_MD5(bytes, CC_LONG(length), md5Buffer) let output = NSMutableString(capacity: Int(CC_MD5_DIGEST_LENGTH * 2)) for i in 0..<digestLength { output.appendFormat("%02x", md5Buffer[i]) } return NSString(format: output) } }
但这会创建一个内存缓冲区,并且对于大型视频文件而言并不理想。Swift中是否有一种方法可以计算读取文件流的MD5校验和,从而使内存占用量最小?
您可以分块计算MD5校验和,例如在?中有没有一个MD5库不需要同时输入全部内容?。
这是使用Swift的可能实现(现已针对Swift 5更新)
import CommonCrypto func md5File(url: URL) -> Data? { let bufferSize = 1024 * 1024 do { // Open file for reading: let file = try FileHandle(forReadingFrom: url) defer { file.closeFile() } // Create and initialize MD5 context: var context = CC_MD5_CTX() CC_MD5_Init(&context) // Read up to `bufferSize` bytes, until EOF is reached, and update MD5 context: while autoreleasepool(invoking: { let data = file.readData(ofLength: bufferSize) if data.count > 0 { data.withUnsafeBytes { _ = CC_MD5_Update(&context, $0.baseAddress, numericCast(data.count)) } return true // Continue } else { return false // End of file } }) { } // Compute the MD5 digest: var digest: [UInt8] = Array(repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH)) _ = CC_MD5_Final(&digest, &context) return Data(digest) } catch { print("Cannot open file:", error.localizedDescription) return nil } }
需要自动释放池来释放所返回的内存 file.readData(),否则,整个文件(可能很大)将被加载到内存中。感谢Abhi Beckert注意到这一点并提供了实现。
file.readData()
如果您需要摘要作为十六进制编码的字符串,则将返回类型更改为String?并替换
String?
return digest
通过
let hexDigest = digest.map { String(format: "%02hhx", $0) }.joined() return hexDigest