How to convert an Obj-C method with NSData bytes using bitwise operators to Swift

Question

I'm trying to convert an Objective C method to Swift but in order to do so I need some help understanding what's going on.

For context, the input to this method begins as an NSString which is then converted into an NSData object with utf8 encoding. The bytes of this data object are then passed into the method (the message parameter).

The return value of this method is then sent via writeData to a certain characteristic of a CBPeripheral.

Here's what I understand (from a Swift perspective).

• The message passed in is a UInt8 byte array: [UInt8]
• 3 variables are created as UInt32 values; crcVal, byteVal and mask
• crcVal is then set to the max value of UInt32 type
• each byte from the UInt8 byte array passed in is then looped through and some kind of operation is performed to produce and append to the final outcome of the crcVal value which is ultimately used to send via CoreBluetooth's writeData command to a CBPeripheral.

I'm not very solid on bitwise operators or why the below method is doing what it's doing as there's no documentation. Can somebody help clear up any or all parts of this? I'm hoping to write a Swift equivalent version of this method. Thank you.

- (uint32_t) computeCRC:(uint8_t *)message len:(NSUInteger)msgLen {

    uint32_t crcVal = 0, byteVal, mask;

    crcVal = 0xFFFFFFFF;
    for (int i=0; i<msgLen; i++) {
        byteVal = message[i];
        crcVal ^= byteVal;
        for (int j=7; j>=0; j--) {
            mask = -(crcVal&1);
            crcVal = (crcVal>>1)^(0xEDB88320 & mask);
        }
    }

    return ~crcVal;
}

Answer 1

Instead of writing the implementation from scratch, why don't you use the zlib implementation of crc32?:

import zlib

func crc32(from data: Data) -> UInt {
    return data.withUnsafeBytes { (buffer: UnsafePointer<Bytef>) -> UInt in
       return zlib.crc32(0, buffer, uInt(data.count))
    }
}

However, to help you understand the bitwise operations:

func computeCRC(message: [UInt8]) -> UInt32 {
    var crc: UInt32 = 0xFFFFFFFF

    for byte in message {
        crc ^= UInt32(byte)
        for _ in 0 ..< 8 {
            // negation using 2-complement: -x = ~x + 1
            // use &+ addition with overflow
            let mask = ~(crc & 1) &+ 1
            crc = (crc >> 1) ^ (0xEDB88320 & mask)
        }
    }

    return ~crc
}

Note that we don't need to pass the number of bytes in Swift. The implementations have different signatures and return a slightly different type but they both give the same result.