CODEWITHSUNDEEP
swift3xor
XerXeX
XerXeX

Reputation: 792

Swift 3 xor checksum calculation

I'm trying to calculate the checksum in an NMEA sentence, but i can't get the correct value. Here is my test code.

import UIKit

//$GPGLL,5300.97914,N,00259.98174,E,125926,A*28

let str = "GPGLL,5300.97914,N,00259.98174,E,125926,A"


var xor: UInt8 = 0
for i in 0..<str.characters.count {
    xor = xor ^ Array(str.utf8)[i]
}

print(xor)

This returns a checksum of 40, not the 28 i'd expected.

What am i doing wrong?

Upvotes: 1

Views: 838

Answers (1)

user3441734
user3441734

Reputation: 17572

The checksum is simple, just an XOR of all the bytes between the $ and the * (not including the delimiters themselves), and written in hexadecimal.

let str = "$GPGLL,5300.97914,N,00259.98174,E,125926,A*"

var xor: UInt8 = 0
for i in 1..<(str.characters.count - 1){
    xor = xor ^ Array(str.utf8)[i]
}
extension UnsignedInteger {
    var hex: String {
        var str = String(self, radix: 16, uppercase: true)
        while str.characters.count < 2 * MemoryLayout<Self>.size {
            str.insert("0", at: str.startIndex)
        }
        return str
    }
}

let strWithCheckSum = str + xor.hex
print(strWithCheckSum) // GPGLL,5300.97914,N,00259.98174,E,125926,A*28

Upvotes: 4

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