import 'dart:convert';

import 'package:flutter_dotenv/flutter_dotenv.dart';
import 'package:http/http.dart' as http;

import '../../domain/entities/food_product.dart';
import 'remote_food_data_source.dart';

/// Thin client for the USDA FoodData Central "foods/search" endpoint.
///
/// Acts as the automatic fallback when the local cache and Open Food Facts
/// cannot resolve a query. Nutrient values are mapped from the FDC nutrient
/// IDs used by both Foundation and Branded food records:
///
/// * 1008 — Energy (kcal)
/// * 1003 — Protein
/// * 1004 — Total lipid (fat)
/// * 1005 — Carbohydrates, by difference
/// * 1079 — Fiber, dietary
/// * 1093 — Sodium
///
/// The API key is read from `.env` (`USDA_FDC_API_KEY`) with a fallback to
/// the public `DEMO_KEY` so development builds work out of the box.
class UsdaFdcClient implements RemoteFoodDataSource {
  UsdaFdcClient({http.Client? client, String? apiKey})
      : _client = client ?? http.Client(),
        _apiKey = apiKey;

  static const String endpoint =
      'https://api.nal.usda.gov/fdc/v1/foods/search';

  /// Fallback key for light development traffic.
  static const String demoApiKey = 'DEMO_KEY';

  /// Nutrient IDs recognized by the mapper (see class docs).
  static const int nutrientIdEnergyKcal = 1008;
  static const int nutrientIdProtein = 1003;
  static const int nutrientIdFat = 1004;
  static const int nutrientIdCarbs = 1005;
  static const int nutrientIdFiber = 1079;
  static const int nutrientIdSodium = 1093;

  final http.Client _client;
  final String? _apiKey;

  /// Effective API key: constructor override > `.env` > [demoApiKey].
  String get apiKey {
    final fromEnv = _apiKey ??
        (dotenv.isInitialized ? dotenv.env['USDA_FDC_API_KEY'] : null);
    if (fromEnv == null || fromEnv.trim().isEmpty) return demoApiKey;
    return fromEnv.trim();
  }

  @override
  Future<List<FoodProduct>> searchByText(String query) =>
      search(query, isBarcode: false);

  @override
  Future<List<FoodProduct>> lookupBarcode(String barcode) =>
      search(barcode, isBarcode: true, pageSize: 5);

  /// Searches FDC for [query] and maps up to [pageSize] results.
  ///
  /// [isBarcode] switches the query into a GTIN/UPC lookup for branded foods.
  Future<List<FoodProduct>> search(
    String query, {
    bool isBarcode = false,
    int pageSize = 25,
  }) async {
    final trimmed = query.trim();
    if (trimmed.isEmpty) return const [];

    // The FDC API validates the key from the QUERY STRING only; an `apiKey`
    // field inside the JSON body is silently ignored and yields
    // HTTP 403 `API_KEY_MISSING`.
    final uri = Uri.parse(endpoint).replace(
      queryParameters: <String, String>{'api_key': apiKey},
    );

    final body = <String, dynamic>{
      'query': trimmed,
      'pageSize': pageSize,
      if (isBarcode) 'dataType': ['Branded'],
    };

    final response = await _client
        .post(
          uri,
          headers: const {
            'Content-Type': 'application/json',
            'User-Agent': 'GLP-1 Companion/0.1.0 (nutrition lookup)',
          },
          body: jsonEncode(body),
        )
        .timeout(const Duration(seconds: 8));

    if (response.statusCode != 200) {
      throw UsdaFdcException(
        'USDA FoodData Central responded with HTTP ${response.statusCode}.',
      );
    }

    final decoded = jsonDecode(response.body);
    if (decoded is! Map<String, dynamic>) return const [];
    final foods = decoded['foods'];
    if (foods is! List) return const [];

    final now = DateTime.now();
    final results = <FoodProduct>[];
    for (final food in foods) {
      if (food is! Map<String, dynamic>) continue;
      final product = _mapFood(
        food,
        now: now,
        fallbackBarcode: isBarcode ? trimmed : null,
      );
      if (product != null) results.add(product);
    }
    return results;
  }

  /// Maps a single FDC food payload to a [FoodProduct]; returns `null` when
  /// the payload has no usable identity or energy value.
  FoodProduct? _mapFood(
    Map<String, dynamic> food, {
    required DateTime now,
    String? fallbackBarcode,
  }) {
    final fdcId = food['fdcId'];
    final description = food['description'] as String? ?? '';
    if (fdcId == null || description.isEmpty) return null;

    final nutrients = <int, double>{};
    final rawNutrients = food['foodNutrients'];
    if (rawNutrients is List) {
      for (final entry in rawNutrients) {
        if (entry is! Map<String, dynamic>) continue;
        final id = entry['nutrientId'];
        final value = entry['value'];
        if (id is int && value is num) nutrients[id] = value.toDouble();
      }
    }

    // Energy is the gate: records without it cannot be logged meaningfully.
    final calories = nutrients[nutrientIdEnergyKcal];
    if (calories == null) return null;

    final barcode = (food['gtinUpc'] as String?)?.trim();
    final brand = (food['brandName'] as String?)?.trim();
    final servingSize = (food['servingSize'] as num?)?.toDouble() ?? 100;
    final servingUnit =
        (food['servingSizeUnit'] as String?)?.trim().toLowerCase();

    return FoodProduct(
      id: 'fdc:$fdcId',
      barcode: (barcode == null || barcode.isEmpty) ? fallbackBarcode : barcode,
      productName: description,
      brand: (brand == null || brand.isEmpty) ? null : brand,
      servingSizeGrams: servingSize,
      servingSizeUnit: _canonicalUnit(servingUnit),
      calories: calories,
      proteinGrams: nutrients[nutrientIdProtein] ?? 0,
      carbsGrams: nutrients[nutrientIdCarbs] ?? 0,
      fatGrams: nutrients[nutrientIdFat] ?? 0,
      fiberGrams: nutrients[nutrientIdFiber],
      sodiumMg: nutrients[nutrientIdSodium],
      imageUrl: null,
      lastUpdated: now,
    );
  }

  /// Normalizes the FDC serving unit (`"g"`, `"ml"`, `"count"` …) into the
  /// pantry-canonical unit keys. A missing unit defaults to grams: FDC
  /// nutrition values sit on a per-100 g basis when no serving is declared.
  String _canonicalUnit(String? unit) {
    switch (unit) {
      case 'g':
      case 'gram':
      case 'grams':
      case null:
      case '':
        return 'g';
      case 'ml':
      case 'milliliter':
      case 'milliliters':
        return 'ml';
      case 'kg':
        return 'kg';
      case 'oz':
        return 'oz';
      case 'count':
      case 'each':
        return 'count';
      default:
        return unit;
    }
  }

  /// Releases the underlying HTTP client.
  void close() => _client.close();
}

/// Non-fatal failure surfaced by [UsdaFdcClient]; the search repository
/// catches it and degrades to an empty result set.
class UsdaFdcException implements Exception {
  const UsdaFdcException(this.message);

  final String message;

  @override
  String toString() => 'UsdaFdcException($message)';
}