DALGLI0/crc.js

const Polynomial = require('./polynomial.js');

const L = new Polynomial.Class(2, new Array(16).fill(1));
const X16 = new Polynomial.Class(2, [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1]); //jako ze można to od razu wymnożyć
const G = new Polynomial.Class(2, [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1]); //jako ze można to od razu wymnożyć

const to_bin = a => {
    var result = "";
    for (var i = 0; i < a.length; i++) {
        var bin = a[i].charCodeAt().toString(2);
        result += Array(8 - bin.length + 1).join("0") + bin;
    }
    return result;
}
const to_ascii = a => {
    a = a.join('');
    a = parseInt(a, 2);
    //nie znalazłem innego sposobu w js na osiągniecie tego efektu..
    // if (a < 126) //ponieważ wieksze liczby nie należą do typowego Ascii//porzucone ze wzgledu na decode
    return String.fromCharCode(a);
    // else {
    //     return "0x" + a.toString(16);
    //     //  return "\\x" + a.toString(16); //escape \ nie wiem czemu dobrze nie działą i i tak wypisuje \\
    // }


}
const mod8format = array => {
    while (array.length % 8 != 0) {
        array.push(0);
    }
    return array;
}

const fcs = m => {
    let bits = m.map(to_bin); //message in binary
    bits = bits.join('').split('').reverse(); //reverse binary decoded message
    let M = new Polynomial.Class(2, bits);
    let fcs = Polynomial.multiply(X16, M);

    fcs = Polynomial.add(fcs,
        Polynomial.multiply(
            Polynomial.Mono(m.length * 8, 1, 2),
            L
        )
    )
    fcs = Polynomial.divide(fcs, G);
    for (let i = 0; i < 16 - fcs.coefficients.length; i++) fcs.coefficients.push(0);
    fcs.coefficients.reverse();
    return fcs.coefficients;
}



const check = m => {

    let bits = m.map(to_bin); //message in binary
    bits = bits.join('').split('').reverse(); //reverse binary decoded message

    let fcs = Polynomial.Mono(bits.length, 1, 2);


    let C = new Polynomial.Class(2, bits);
    C = Polynomial.multiply(X16, C);
    C.coefficients = mod8format(C.coefficients);
    let S = Polynomial.add(
        C,
        Polynomial.multiply(
            fcs,
            L
        )
    )

    S = Polynomial.divide(S, G)
    if (S.coefficients.length === 0) {
        return true;
    } else
        return false;
}

function encode(m) {
    m = m.split('');
    let res = fcs(m);

    let f1 = [];
    let f2 = [];
    for (let i = 0; i < res.length; i++) {
        if (i < 8) {
            f1.push(res[i]);
        } else {
            f2.push(res[i]);
        }
    }
    f1 = to_ascii(f1);
    f2 = to_ascii(f2);
    m.push(f1);
    m.push(f2);
    return {
        encoded: m,
        FCS: [f1, f2]

    };

}

exports.encode = encode;

function decode(m, fcs) {
    m = m.split('');
    to_check = fcs;
    for (let char in fcs) {
        m.push(fcs[char]);
    }
    return check(m)

}
exports.decode = decode;