fix(dex): resolve precision loss in wallet

assets/translations/en.json

@@ -468,6 +468,7 @@
   "nothingFound": "Nothing found",
   "half": "Half",
   "max": "Max",
+  "exact": "Exact",
   "reactivating": "Reactivating",
   "weFailedCoinActivate": "We failed to activate {} :(",
   "failedActivate": "Failed to activate",

lib/bloc/taker_form/taker_bloc.dart

@@ -233,6 +233,11 @@ class TakerBloc extends Bloc<TakerEvent, TakerState> {
       autovalidate: switchingCoin ? () => false : null,
     ));
 
+    // Auto-fill the exact maker amount when an order is selected
+    if (event.order != null) {
+      add(TakerSetSellAmount(event.order!.maxVolume));
+    }
+
     if (!state.autovalidate) add(TakerVerifyOrderVolume());
 
     await _autoActivateCoin(state.selectedOrder?.coin);

lib/generated/codegen_loader.g.dart

@@ -465,6 +465,7 @@ abstract class  LocaleKeys {
   static const nothingFound = 'nothingFound';
   static const half = 'half';
   static const max = 'max';
+  static const exact = 'exact';
   static const reactivating = 'reactivating';
   static const weFailedCoinActivate = 'weFailedCoinActivate';
   static const failedActivate = 'failedActivate';

lib/shared/utils/utils.dart

@@ -139,9 +139,11 @@ Rational? fract2rat(Map<String, dynamic>? fract, [bool willLog = true]) {
   if (fract == null) return null;
 
   try {
+    final String numerStr = fract['numer'].toString();
+    final String denomStr = fract['denom'].toString();
     final rat = Rational(
-      BigInt.from(double.parse(fract['numer'])),
-      BigInt.from(double.parse(fract['denom'])),
+      BigInt.parse(numerStr),
+      BigInt.parse(denomStr),
     );
     return rat;
   } catch (e) {

lib/views/dex/simple/form/taker/coin_item/taker_form_sell_item.dart

@@ -50,6 +50,8 @@ class _SellHeader extends StatelessWidget {
       actions: [
         Flexible(child: _AvailableGroup()),
         const SizedBox(width: 8),
+        _ExactButton(),
+        const SizedBox(width: 3),
         _MaxButton(),
         const SizedBox(width: 3),
         _HalfButton(),
@@ -85,3 +87,13 @@ class _MaxButton extends DexSmallButton {
           context.read<TakerBloc>().add(TakerAmountButtonClick(1));
         });
 }
+
+class _ExactButton extends DexSmallButton {
+  _ExactButton()
+      : super(LocaleKeys.exact.tr(), (context) {
+          final state = context.read<TakerBloc>().state;
+          final order = state.selectedOrder;
+          if (order == null) return;
+          context.read<TakerBloc>().add(TakerSetSellAmount(order.maxVolume));
+        });
+}

test_units/tests/utils/convert_fract_rat_test.dart

@@ -34,4 +34,25 @@ void testRatToFracAndViseVersa() {
     Rational? result = fract2rat(invalidFract, false);
     expect(result, isNull);
   });
+
+  test('fract2rat handles very large integers without precision loss', () {
+    // 10^50 / 10^20 = 10^30
+    final numer = '100000000000000000000000000000000000000000000000000';
+    final denom = '100000000000000000000';
+    final rat = fract2rat({'numer': numer, 'denom': denom}, false)!;
+    expect(rat.numerator, BigInt.parse(numer));
+    expect(rat.denominator, BigInt.parse(denom));
+    // Ensure round-trip
+    final back = rat2fract(rat, false)!;
+    expect(back['numer'], numer);
+    expect(back['denom'], denom);
+  });
+
+  test('fract2rat correctly parses strings that would overflow double', () {
+    final numer = '340282366920938463463374607431768211457'; // > 2^128
+    final denom = '1';
+    final rat = fract2rat({'numer': numer, 'denom': denom}, false)!;
+    expect(rat.numerator, BigInt.parse(numer));
+    expect(rat.denominator, BigInt.one);
+  });
 }