Compare integer with byteslice

Question

I have a process that computes and checks the checksum in AIS messages, the checksum is the xor of all the bytes in the message expressed in hexadecimal format. At the moment I am turning both the calculated and included checksums into strings and comparing them, which seems a bit compute heavy. Is there a better way to compare the single byte calculation with its 2 byte hexadecimal representation? bytdata is the AIS message in a []byte{}.

j := i + 1
for ; j < leng ; j++ {
    checkSum ^= (int)(bytdata[j])
}

checkBytes := fmt.Sprintf("%02X", checkSum)
if checkBytes == string(bytdata[(j+1):(j+3)]) {
    // valid checksum......
    etc
}

Saman Ebrahimpoor · Accepted Answer · 2025-09-14 05:45:47Z

2

AIS checksum is one byte encoded as two ASCII hex character. you can parse these two and compare them directly instead of fmt.Sprintf:

import (
    "encoding/hex"
)

j := i + 1
checkSum := 0
for ; j < leng; j++ {
    checkSum ^= int(bytdata[j])
}

bytes := bytdata[j+1 : j+3]
msgCs, err := hex.DecodeString(string(bytes))
if err != nil || len(msgCs) != 1 {
    return
}

if byte(checkSum) == msgCs[0] {}

answered Sep 14 at 5:45

Saman Ebrahimpoor

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1 Comment

Konstantin Makarov · Accepted Answer · 2025-09-17 03:55:34Z

2

It follows from the code in the question: fmt.Sprintf("%02X", checkSum) that the checksum is written in uppercase. Here is the fastest way to check the checksum without any string conversions:

const hexDigits = "0123456789ABCDEF"

// ...

var checkSum byte = 0
for ; j < leng; j++ {
    checkSum ^= bytdata[j]
}

if hexDigits[checkSum >>  4] == bytdata[j + 1] &&
   hexDigits[checkSum & 0xF] == bytdata[j + 2] {
    // valid checksum......
}

edited Sep 17 at 3:55

answered Sep 16 at 10:39

Konstantin Makarov

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Comments

Hoodoo · Accepted Answer · 2025-09-19 05:48:14Z

-1

Thanks for the suggestions, this also avoids any string conversions and doesn't use the hex lookup table.

    dcode := make([]byte, 1)
...
            checkSum = 0
            for j = i+1 ; j < leng ; j++ {
                checkSum ^= (int)(bytdata[j])
            }
            _, err := hex.Decode(dcode, bytdata[(j+1):(j+3)])
            if err == nil && (int)(dcode[0]) == checkSum {
                // valid checksum, will it fit in one packet
...

edited Sep 19 at 5:48

answered Sep 15 at 4:17

Hoodoo

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1 Comment

Konstantin Makarov Sep 18 at 2:47

if err == nil && (int)(hsum[0]) == checkSum -> if err == nil && (int)(dcode[0]) == checkSum ?

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