crc16 algorithm from C++ to bash

I am trying to implement a CRC16 checksum in bash. I'm porting from an existing piece of C++ code. I'm almost there, but I am getting different answers.

I don't quite see why the checksums between the C++ code and the bash script are different.

Another set of eyes would be a big help.

Here is the C++ code:

uint16_t Encoder::checksum(std::string thestring)
{
    uint8_t d, e, f;
    uint16_t c, r, crccalc;
    c = 0xffff;

    for (unsigned int i = 0; i < thestring.length(); i++)
    {
        d = thestring[i];
        e = c ^ d;
        f = e ^ (e << 4);
        r = (c >> 8) ^ (f << 8) ^ (f << 3) ^ (f >> 4);
        c = r;
    }
    c ^= 0xffff;
    crccalc = c;
    return crccalc;
}

And here is my bash code:

function calc_crc16()
{
    string=$1
    while read -d "" -n 1 ; do astring+=( "$reply" ) ; done <<< "$string"
    
    cnt=${#astring[@]}
    c=0xffff
    
    for ((x=0;x<$cnt;x++)); do
        char=${astring[$x]}
        e=$(($c ^ $char))
        s=$(($e << 4))
        f=$(($e ^ $s))
        t1=$(($c >> 8))
        t2=$(($f << 8))
        t3=$(($f << 3))
        t4=$(($f >> 4))
        r1=$(($t1 ^ $t2 ^ $t3 ^ $t4))
        c=$r1
    done
    c=$c ^ 0xffff
    echo "checksum = $c"
}

Is it going to have something to do with the size of the ints? I'm guessing there's not much I can do about that in bash.

I'm getting an actual number, but it doesn't match the C++, which I know works correctly. does anyone see anything where I may be screwing things up?

ok. with Sorpigal's help, i've got a working version.

i suspect this could all be done within an awk script, which may run a lot faster. i may try that next.

thank you all for the help. i don't mean to steal the solution here, but i worked on it, and figure it is worth putting up.

anyway, here is a working version:

function calc_crc16()
{
    while read -r -d "" -n 1 ; do astring+=( "$REPLY" ) ; done <<< "$1"

    cnt=${#1}
    c=65535

    for ((x=0;x<$cnt;x++)); do
        char=$(printf '%d' \'"${1:$x:1}")
        e=$(((c ^ char) & 0x00FF))
        s=$(((e << 4) & 0x00FF))
        f=$(((e ^ s) & 0x00FF))
        r1=$(((c >> 8) ^ (f << 8) ^ (f << 3) ^ (f >> 4)))
        c=$r1
    done
    c=$((c ^ 0xffff))
    echo "checksum = $c"
}