import { OTP_CODES, otpError } from "../errors/codes.js";

const MAX_U64 = (1n << 64n) - 1n;

/**
 * Encode an HOTP counter as the 8-byte big-endian block required by RFC 4226
 * §5.1. `bigint` is the canonical input type — `Number` loses precision past
 * `2^53` and the RFC permits counters up to `2^64 - 1`. Negative counters and
 * counters past `2^64 - 1` raise `OtpError(OTP.INVALID_OPTIONS)`.
 */
export function encodeCounter(counter: bigint): Uint8Array {
    if (counter < 0n) {
        throw otpError(
            OTP_CODES.INVALID_OPTIONS,
            "HOTP counter must be non-negative (RFC 4226 §5.1)"
        );
    }
    if (counter > MAX_U64) {
        throw otpError(
            OTP_CODES.INVALID_OPTIONS,
            "HOTP counter exceeds 2^64-1 (RFC 4226 §5.1 mandates an 8-byte big-endian block)"
        );
    }
    const out = new Uint8Array(8);
    let v = counter;
    for (let i = 7; i >= 0; i -= 1) {
        out[i] = Number(v & 0xffn);
        v >>= 8n;
    }
    return out;
}