import 'nostr_event.dart';
import 'offer.dart';

/// Maps [Offer] ↔ Nostr NIP-99 "Classified Listing" events (kind 30402), the
/// first concrete `OfferTransport` backend (sharing-model.md §2).
///
/// This codec is where the privacy contract is *enforced on the wire*: the only
/// location that ever leaves the device is a coarse geohash, truncated here to
/// [maxGeohashChars]. There is no field that could carry the inventory or an
/// exact address, because [Offer] itself has none — the seam does the work.
class Nip99Codec {
  /// NIP-99 addressable classified listing.
  static const kindActive = 30402;

  /// A geohash of 5 chars ≈ ±2.4 km cell — the coarse "near you" of the mocks.
  /// Anything finer is refused so a slip in the caller can't leak a precise fix.
  static const maxGeohashChars = 5;

  /// Builds an UNSIGNED event from [offer]. Caller signs with the derived key.
  NostrEvent encode(Offer offer, {required int createdAt}) {
    final coarse = _coarsen(offer.approxGeohash);

    final tags = <List<String>>[
      ['d', offer.id],
      ['title', offer.summary],
      ['status', offer.status == OfferStatus.active ? 'active' : 'sold'],
      // Reciprocity mode is a Tanemaki concept NIP-99 lacks; carry it explicitly.
      ['offer_type', offer.type.name],
    ];
    // NIP-52 geohash ladder: one `g` tag per prefix length so an area query
    // matches by *exact* tag (how Nostr relays filter) yet still finds coarser
    // searches. Coarse only — NO "location"/address/exact-fix tag ever.
    for (var i = 1; i <= coarse.length; i++) {
      tags.add(['g', coarse.substring(0, i)]);
    }
    if (offer.category != null) tags.add(['t', offer.category!]);
    if (offer.radiusKm != null) {
      tags.add(['radius_km', '${offer.radiusKm}']);
    }
    if (offer.type == OfferType.sale && offer.priceAmount != null) {
      tags.add([
        'price',
        '${offer.priceAmount}',
        offer.priceCurrency ?? 'EUR',
      ]);
    }
    if (offer.type == OfferType.exchange && offer.exchangeTerms != null) {
      tags.add(['exchange_terms', offer.exchangeTerms!]);
    }
    if (offer.imageUrl != null) tags.add(['image', offer.imageUrl!]);
    if (offer.expiresAt != null) {
      tags.add([
        'expiration',
        '${offer.expiresAt!.millisecondsSinceEpoch ~/ 1000}',
      ]);
    }

    return NostrEvent(
      pubkey: offer.authorPubkeyHex,
      createdAt: createdAt,
      kind: kindActive,
      tags: tags,
      content: offer.summary,
    );
  }

  /// Reconstructs an [Offer] from a received event.
  Offer decode(NostrEvent event) {
    final priceTag = _fullTag(event, 'price');
    return Offer(
      id: event.tag('d') ?? event.id,
      authorPubkeyHex: event.pubkey,
      summary: event.tag('title') ?? event.content,
      type: _parseType(event.tag('offer_type')),
      status: event.tag('status') == 'sold'
          ? OfferStatus.closed
          : OfferStatus.active,
      category: event.tag('t'),
      approxGeohash: _longestGeohash(event),
      radiusKm: int.tryParse(event.tag('radius_km') ?? ''),
      priceAmount: priceTag != null && priceTag.length > 1
          ? num.tryParse(priceTag[1])
          : null,
      priceCurrency: priceTag != null && priceTag.length > 2
          ? priceTag[2]
          : null,
      exchangeTerms: event.tag('exchange_terms'),
      imageUrl: event.tag('image'),
    );
  }

  String _coarsen(String geohash) => geohash.length > maxGeohashChars
      ? geohash.substring(0, maxGeohashChars)
      : geohash;

  /// The finest `g` tag present (top of the prefix ladder).
  String _longestGeohash(NostrEvent event) {
    var best = '';
    for (final t in event.tags) {
      if (t.isNotEmpty && t[0] == 'g' && t.length > 1 && t[1].length > best.length) {
        best = t[1];
      }
    }
    return best;
  }

  List<String>? _fullTag(NostrEvent event, String name) {
    for (final t in event.tags) {
      if (t.isNotEmpty && t[0] == name) return t;
    }
    return null;
  }

  OfferType _parseType(String? name) => OfferType.values.firstWhere(
    (t) => t.name == name,
    orElse: () => OfferType.gift,
  );
}