[keichi]

Problem

bitN assigned options.length = size directly on the caller's options object, so reusing one options object across bit fields of different sizes made all of them use the last size. string() similarly wrote options.encoding onto the caller's object.

Repro

const o = {};
new Parser().bit4("a", o).bit8("b", o).bit4("c").parse(Buffer.from([0xab, 0xcd]));
// RangeError (reads 24 bits) instead of { a: 0xa, b: 0xbc, c: 0xd }

Fix

Copy the options object ({ ...options, length: size } / { ...options, encoding }) instead of mutating it. Added a regression test.