somthing wrong in bitvec codec

[Mohsen]

In the BitVec codec, I see the following comment:

 * @name BitVec
 * @description
 * A BitVec that represents an array of bits. The bits are however stored encoded. The difference between this
 * and a normal Bytes would be that the length prefix indicates the number of bits encoded, not the bytes
 */

However, in the decoding section, we have:

function decodeBitVecU8a (value?: Uint8Array): [number, Uint8Array] {
  if (!value?.length) {
    return [0, new Uint8Array()];
  }

  // handle all other Uint8Array inputs, these do have a length prefix which is the number of bits encoded
  const [offset, length] = compactFromU8aLim(value);
  const total = offset + Math.ceil(length / 8);

  if (total > value.length) {
    throw new Error(`BitVec: required length less than remainder, expected at least ${total}, found ${value.length}`);
  }

  return [length, value.subarray(offset, total)];
}

Can you please double-check the encoding and decoding logic for BitVec? Thanks!

[Tarik Gul]

Do you mind explaining what exactly is wrong with this implementation?

[Mohsen]

Do you mind explaining what exactly is wrong with this implementation?

The current implementation of the BitVec class contains a discrepancy in how the length is encoded. According to the class comment, the length prefix should indicate the number of bits encoded, not the number of bytes.

/**
 * A BitVec that represents an array of bits. The bits are however stored encoded. The difference between this
 * and a normal Bytes would be that the length prefix indicates the number of bits encoded, not the bytes.
 */

The method responsible for calculating the encoded length currently returns:

return this.length + compactToU8a(this.#decodedLength).length;

The length should be encoded correctly as the number of bits. Therefore, the correct implementation should be:

return this.length + compactToU8a(this.#decodedLength * 8).length;

decode section

function decodeBitVecU8a (value?: Uint8Array): [number, Uint8Array] {
  if (!value?.length) {
    return [0, new Uint8Array()];
  }

  // handle all other Uint8Array inputs, these do have a length prefix which is the number of bits encoded
  const [offset, length] = compactFromU8aLim(value);
  const total = offset + Math.ceil(length / 8);

  if (total > value.length) {
    throw new Error(`BitVec: required length less than remainder, expected at least ${total}, found ${value.length}`);
  }

  return [length, value.subarray(offset, total)];
}

/** @internal */
function decodeBitVec (value?: AnyU8a): [number, Uint8Array] {
  if (Array.isArray(value) || isString(value)) {
    const u8a = u8aToU8a(value);

    return [u8a.length / 8, u8a];
  }

  return decodeBitVecU8a(value);
}

constructor (registry: Registry, value?: AnyU8a, isMsb = false) {
    const [decodedLength, u8a] = decodeBitVec(value);

    super(registry, u8a);

    this.#decodedLength = decodedLength;
    this.#isMsb = isMsb;
  }