@indrajala/asn1der/src/bitwise.ts

parse ASN.1 DER structures

const one = Buffer.from('01', 'hex').readUInt8(0);

export const bitMask = (bits: Array<number>): number => {

    let byteMask = Buffer.from('00', 'hex').readUInt8(0);

    for (const bitNumber of bits) {
        const bitMask = one << (bitNumber - 1);
        byteMask = byteMask | bitMask;
    }

    return byteMask;
};

export const bitsAreSet = (b: number, bits: Array<number>): boolean => {

    //console.log(`bitsAreSet ${b} ${bits}`);

    const mask = bitMask(bits);
    //console.log(`mask ${mask}`);
    return (b & mask) == mask;
};

export const getStandAloneBitsValue = (b: number, bits: Array<number>): number =>
    (b & bitMask(bits)) >> (Math.min(...bits) - 1);