Add ability to select distance unit

lib/services/distance_unit_service.dart

@@ -0,0 +1,211 @@
+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, {int decimals = 1}) {
+    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;
+  }
+}