/**
 * Minimal, dependency-free BOLT11 Lightning invoice amount decoder.
 *
 * WHY THIS EXISTS
 * ---------------
 * nostr-tools removed `nip57.getSatoshisAmountFromBolt11` in a newer minor, so
 * we decode the amount ourselves. The amount is encoded in the invoice's
 * human-readable prefix (HRP), NOT the bech32 data — so we don't need a full
 * bech32 decode, just to parse the leading `ln<network><amount><multiplier>`.
 *
 * BOLT11 amount format (from the spec):
 *   - HRP looks like:  lnbc<amount?><multiplier?>1<data...>
 *   - <amount> is an integer; <multiplier> is one of:
 *       m (milli)  = 0.001 BTC
 *       u (micro)  = 0.000001 BTC
 *       n (nano)   = 0.000000001 BTC
 *       p (pico)   = 0.000000000001 BTC
 *   - No amount at all  → a "donation" invoice (0 / unspecified).
 *
 * 1 BTC = 100_000_000 sats.
 */

const MULTIPLIERS: Record<string, number> = {
  m: 0.001,
  u: 0.000001,
  n: 0.000000001,
  p: 0.000000000001,
};

const SATS_PER_BTC = 100_000_000;

/**
 * Decode the amount (in whole satoshis) from a BOLT11 invoice string.
 * Returns 0 for amountless invoices. Throws on a clearly malformed prefix so
 * callers can fall back to other amount sources (e.g. the zap `amount` tag).
 */
export function getSatoshisFromBolt11(invoice: string): number {
  if (!invoice || typeof invoice !== 'string') {
    throw new Error('Invalid BOLT11 invoice');
  }

  const lower = invoice.trim().toLowerCase();

  // The HRP is everything before the bech32 separator '1' that precedes the
  // data part. BOLT11 HRPs start with "ln". Grab from the start up to (but not
  // including) the final separator between HRP and data. In practice the amount
  // block sits right after the network prefix (lnbc / lntb / lntbs / lnbcrt).
  const match = lower.match(/^ln(?:bc|tb|tbs|bcrt|sb)(\d+)?([munp])?1/);
  if (!match) {
    throw new Error('Not a recognizable BOLT11 invoice');
  }

  const amountStr = match[1];
  const multiplier = match[2];

  // No amount encoded → amountless invoice.
  if (!amountStr) return 0;

  const amount = parseInt(amountStr, 10);
  if (Number.isNaN(amount)) {
    throw new Error('Invalid amount in BOLT11 invoice');
  }

  // btc value = amount * multiplier (multiplier absent means whole BTC units).
  const btc = multiplier ? amount * MULTIPLIERS[multiplier] : amount;
  return Math.round(btc * SATS_PER_BTC);
}

/** bech32 charset (the data-part alphabet). */
const CHARSET = 'qpzry9x8gf2tvdw0s3jn54khce6mua7l';

/**
 * Extract the **payment hash** (hex) from a BOLT11 invoice — the `p` tagged
 * field. Used to verify a Lightning **preimage receipt**: `sha256(preimage)`
 * must equal this hash. Dependency-free (we don't verify the bech32 checksum —
 * a bad invoice just yields a hash that fails the preimage check).
 */
export function getPaymentHashFromBolt11(invoice: string): string | null {
  try {
    const lower = invoice.trim().toLowerCase();
    const sep = lower.lastIndexOf('1');
    if (sep < 1 || sep + 7 > lower.length) return null;

    // Data words = everything after the separator, minus the 6-word checksum.
    const data = lower.slice(sep + 1, lower.length - 6);
    const words = [...data].map((c) => CHARSET.indexOf(c));
    if (words.some((w) => w < 0)) return null;

    // Tagged fields sit after the 35-bit timestamp (7 five-bit words).
    let i = 7;
    while (i + 3 <= words.length) {
      const tag = words[i];
      const len = (words[i + 1] << 5) | words[i + 2];
      const start = i + 3;
      if (start + len > words.length) break;
      if (tag === 1) {
        // `p` = payment hash: 32 bytes packed from `len` five-bit words.
        const bytes: number[] = [];
        let acc = 0;
        let bits = 0;
        for (let k = start; k < start + len; k += 1) {
          acc = (acc << 5) | words[k];
          bits += 5;
          if (bits >= 8) {
            bits -= 8;
            bytes.push((acc >>> bits) & 0xff);
          }
        }
        return bytes.slice(0, 32).map((b) => b.toString(16).padStart(2, '0')).join('');
      }
      i = start + len;
    }
    return null;
  } catch {
    return null;
  }
}