/**
 * Verifying a payment claim that CAN be checked: a **NIP-57 zap receipt**.
 *
 * A `31871` may carry `["proof","zap","<receiptId>"]`. A zap receipt (kind 9735)
 * is signed by the recipient's **LNURL server** after it received the payment,
 * so — unlike a hand-typed invoice hash — it is independently checkable by
 * anyone: we fetch the receipt, verify its signature, that it pays the expected
 * seller, and what it paid.
 *
 * ⚠️ Honest ceiling: the receipt is only as trustworthy as the seller's own
 * LNURL provider. It proves *that provider* acknowledged a payment to that
 * seller — not that goods changed hands. Still, it is a real cryptographic
 * signal, a tier above a free-text claim.
 */
import { verifyEvent } from 'nostr-tools';
import type { NostrEvent } from '@nostrify/nostrify';
import { getSatoshisFromBolt11 } from '@/lib/bolt11';

export interface ZapReceiptCheck {
  valid: boolean;
  sats: number;
  reason?: string;
}

/**
 * Verify a kind-9735 zap receipt pays `expectedRecipient`.
 * Never throws — returns `{ valid: false, reason }` on any problem.
 */
export function verifyZapReceipt(receipt: NostrEvent, expectedRecipient: string): ZapReceiptCheck {
  if (receipt.kind !== 9735) return { valid: false, sats: 0, reason: 'not a zap receipt' };
  if (!verifyEvent(receipt)) return { valid: false, sats: 0, reason: 'bad receipt signature' };

  const recipient = receipt.tags.find(([n]) => n === 'p')?.[1];
  if (recipient !== expectedRecipient) return { valid: false, sats: 0, reason: 'recipient mismatch' };

  const bolt11 = receipt.tags.find(([n]) => n === 'bolt11')?.[1];
  if (!bolt11) return { valid: false, sats: 0, reason: 'missing invoice' };

  let sats = 0;
  try {
    sats = getSatoshisFromBolt11(bolt11);
  } catch {
    return { valid: false, sats: 0, reason: 'undecodable invoice' };
  }
  if (!Number.isFinite(sats) || sats <= 0) return { valid: false, sats: 0, reason: 'no amount' };

  // The embedded `description` is the original zap request (kind 9734); when
  // present it must also name the same recipient.
  const description = receipt.tags.find(([n]) => n === 'description')?.[1];
  if (description) {
    try {
      const request = JSON.parse(description) as NostrEvent;
      if (request.kind && request.kind !== 9734) {
        return { valid: false, sats, reason: 'description is not a zap request' };
      }
      const requestRecipient = request.tags?.find(([n]) => n === 'p')?.[1];
      if (requestRecipient && requestRecipient !== expectedRecipient) {
        return { valid: false, sats, reason: 'zap request recipient mismatch' };
      }
    } catch {
      return { valid: false, sats, reason: 'unparsable description' };
    }
  }

  return { valid: true, sats };
}