mileograph_flutter/lib/services/distance_unit_service.dart

import 'package:flutter/foundation.dart';
import 'package:intl/intl.dart';
import 'package:shared_preferences/shared_preferences.dart';

enum DistanceUnit {
  milesDecimal,
  milesChains,
  kilometers,
}

extension DistanceUnitLabels on DistanceUnit {
  String get label {
    switch (this) {
      case DistanceUnit.milesDecimal:
        return 'Miles (decimal)';
      case DistanceUnit.milesChains:
        return 'Miles & chains';
      case DistanceUnit.kilometers:
        return 'Kilometers';
    }
  }

  String get shortLabel {
    switch (this) {
      case DistanceUnit.milesDecimal:
        return 'mi';
      case DistanceUnit.milesChains:
        return 'm.ch';
      case DistanceUnit.kilometers:
        return 'km';
    }
  }

  String get _prefsValue => toString().split('.').last;

  static DistanceUnit fromPrefs(String raw) {
    return DistanceUnit.values.firstWhere(
      (u) => u._prefsValue == raw,
      orElse: () => DistanceUnit.milesDecimal,
    );
  }
}

class DistanceUnitService extends ChangeNotifier {
  static const _prefsKey = 'distance_unit';
  static const double kmPerMile = 1.609344;
  static const double chainsPerMile = 80.0;

  DistanceUnitService() {
    _load();
  }

  DistanceUnit _unit = DistanceUnit.milesDecimal;
  bool _loaded = false;

  DistanceUnit get unit => _unit;
  bool get isLoaded => _loaded;

  Future<void> _load() async {
    final prefs = await SharedPreferences.getInstance();
    final saved = prefs.getString(_prefsKey);
    if (saved != null && saved.trim().isNotEmpty) {
      _unit = DistanceUnitLabels.fromPrefs(saved.trim());
    }
    _loaded = true;
    notifyListeners();
  }

  Future<void> setUnit(DistanceUnit unit) async {
    _unit = unit;
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString(_prefsKey, unit._prefsValue);
    notifyListeners();
  }

  double? milesFromInput(String input) =>
      DistanceFormatter(_unit).parseInputMiles(input);

  String format(double miles,
          {int decimals = 1, bool includeUnit = true}) =>
      DistanceFormatter(_unit)
          .format(miles, decimals: decimals, includeUnit: includeUnit);

  double toDisplay(double miles, {int decimals = 1}) =>
      DistanceFormatter(_unit).convertMiles(miles, decimals: decimals);
}

class DistanceFormatter {
  DistanceFormatter(this.unit);

  final DistanceUnit unit;

  String format(double miles,
      {int decimals = 1, bool includeUnit = true}) {
    decimals = decimals.clamp(1, 2);
    if (unit == DistanceUnit.milesChains) {
      // Always show chains with two decimals.
      decimals = 2;
    }
    switch (unit) {
      case DistanceUnit.milesDecimal:
        final value = _numberFormat(decimals).format(miles);
        return includeUnit ? '$value mi' : value;
      case DistanceUnit.kilometers:
        final kms = miles * DistanceUnitService.kmPerMile;
        final value = _numberFormat(decimals).format(kms);
        return includeUnit ? '$value km' : value;
      case DistanceUnit.milesChains:
        final value = _formatMilesChains(miles);
        return includeUnit ? '$value mi' : value;
    }
  }

  double convertMiles(double miles, {int decimals = 1}) {
    decimals = decimals.clamp(1, 2);
    switch (unit) {
      case DistanceUnit.milesDecimal:
        return double.parse(miles.toStringAsFixed(decimals));
      case DistanceUnit.kilometers:
        final kms = miles * DistanceUnitService.kmPerMile;
        return double.parse(kms.toStringAsFixed(decimals));
      case DistanceUnit.milesChains:
        // Return miles again; chains representation handled by format.
        return double.parse(miles.toStringAsFixed(decimals));
    }
  }

  double milesFromDisplayValue(double value) {
    switch (unit) {
      case DistanceUnit.milesDecimal:
        return value;
      case DistanceUnit.kilometers:
        return value / DistanceUnitService.kmPerMile;
      case DistanceUnit.milesChains:
        // Value already represents miles when parsed via parseMilesChains.
        return value;
    }
  }

  double? parseInputMiles(String input) {
    final trimmed = input.trim();
    if (trimmed.isEmpty) return null;
    switch (unit) {
      case DistanceUnit.milesDecimal:
        return double.tryParse(trimmed.replaceAll(',', ''));
      case DistanceUnit.kilometers:
        final km = double.tryParse(trimmed.replaceAll(',', ''));
        if (km == null) return null;
        return km / DistanceUnitService.kmPerMile;
      case DistanceUnit.milesChains:
        return _parseMilesChains(trimmed);
    }
  }

  NumberFormat _numberFormat(int decimals) {
    final pattern =
        decimals == 1 ? '#,##0.0' : '#,##0.00';
    return NumberFormat(pattern);
  }

  String _formatMilesChains(double miles) {
    final totalChains = miles * DistanceUnitService.chainsPerMile;
    var milesPart = totalChains ~/ DistanceUnitService.chainsPerMile;
    final chainRemainder =
        totalChains - (milesPart * DistanceUnitService.chainsPerMile);

    // Always show chains as two digits (00-79), rounded to the nearest chain.
    var roundedChains = chainRemainder.roundToDouble();
    if (roundedChains >= DistanceUnitService.chainsPerMile) {
      milesPart += 1;
      roundedChains -= DistanceUnitService.chainsPerMile;
    }
    final chainText = NumberFormat('00').format(roundedChains);
    return '$milesPart.$chainText';
  }

  double? _parseMilesChains(String raw) {
    final cleaned = raw
        .toLowerCase()
        .replaceAll(',', '')
        .replaceAll('m', '.')
        .replaceAll('c', '.')
        .replaceAll(RegExp(r'\s+'), '.')
        .replaceAll(RegExp(r'\.+'), '.')
        .trim();
    if (cleaned.isEmpty) return null;

    final parts = cleaned.split('.');
    if (parts.isEmpty) return null;

    final milesPart =
        int.tryParse(parts[0].isEmpty ? '0' : parts[0]);
    if (milesPart == null) return null;
    double chainsPart = 0;
    if (parts.length >= 2) {
      final chainRaw = parts
          .sublist(1)
          .join()
          .trim();
      if (chainRaw.isNotEmpty) {
        final parsedChains = double.tryParse(chainRaw);
        if (parsedChains == null) return null;
        chainsPart = parsedChains;
      }
    }
    final totalMiles =
        milesPart +
        (chainsPart / DistanceUnitService.chainsPerMile);
    return totalMiles;
  }
}