decode

pub fn decode(bytes: []const u8, index: Index) DecodeError!Element {
    var stream = std.io.fixedBufferStream(bytes[index..]);
    var reader = stream.reader();

    const tag = try Tag.decode(reader);
    const size_or_len_size = try reader.readByte();

    var start = index + 2;
    var end = start + size_or_len_size;
    // short form between 0-127
    if (size_or_len_size < 128) {
        if (end > bytes.len) return error.InvalidLength;
    } else {
        // long form between 0 and std.math.maxInt(u1024)
        const len_size: u7 = @truncate(size_or_len_size);
        start += len_size;
        if (len_size > @sizeOf(Index)) return error.InvalidLength;

        const len = try reader.readVarInt(Index, .big, len_size);
        if (len < 128) return error.InvalidLength; // should have used short form

        end = std.math.add(Index, start, len) catch return error.InvalidLength;
        if (end > bytes.len) return error.InvalidLength;
    }

    return Element{ .tag = tag, .slice = Slice{ .start = start, .end = end } };
}