fix(realtime): parse whole-number doubles in toInt transformer

packages/realtime_client/lib/src/transformers.dart

@@ -233,10 +233,42 @@ int? toInt(dynamic value) {
   if (value is int) {
     return value;
   }
+  if (value is double) {
+    return _wholeDoubleToInt(value);
+  }
   if (value == null) {
     return null;
   }
-  return int.tryParse(value.toString());
+  final stringValue = value.toString();
+  final parsedInt = int.tryParse(stringValue);
+  if (parsedInt != null) {
+    return parsedInt;
+  }
+  final match = _integerWithZeroFraction.firstMatch(stringValue);
+  if (match == null) {
+    return null;
+  }
+  return int.tryParse(match.group(1)!);
+}
+
+/// Matches an integer with a decimal fraction of only zeros, such as `10.0`
+/// or `-3.000`. Parsing the integer part directly keeps values above 2^53
+/// exact, which a round trip through [double] would not.
+final _integerWithZeroFraction = RegExp(r'^([+-]?\d+)\.0+$');
+
+/// The lowest and highest [double] values enclosing the native 64-bit
+/// integer range, -2^63 and 2^63. Both are exactly representable as doubles.
+const _minIntAsDouble = -9223372036854775808.0;
+const _maxIntExclusiveAsDouble = 9223372036854775808.0;
+
+int? _wholeDoubleToInt(double value) {
+  if (!value.isFinite || value.truncateToDouble() != value) {
+    return null;
+  }
+  if (value < _minIntAsDouble || value >= _maxIntExclusiveAsDouble) {
+    return null;
+  }
+  return value.toInt();
 }
 
 @internal

packages/realtime_client/test/transformers_test.dart

@@ -23,6 +23,22 @@ void main() {
   test('transformers toInt', () {
     expect(toInt(10), equals(10));
     expect(toInt('10'), equals(10));
+    expect(toInt(10.0), equals(10));
+    expect(toInt('10.0'), equals(10));
+    expect(toInt('-3.000'), equals(-3));
+    // Above 2^53, where a round trip through double would lose precision.
+    expect(toInt('9007199254740993.0'), equals(9007199254740993));
+    // Outside the 64-bit integer range.
+    expect(toInt('9223372036854775808.0'), isNull);
+    expect(toInt(1e19), isNull);
+    expect(toInt(-1e19), isNull);
+    expect(toInt('1e3'), isNull);
+    expect(toInt(10.5), isNull);
+    expect(toInt('10.5'), isNull);
+    expect(toInt(double.nan), isNull);
+    expect(toInt(double.infinity), isNull);
+    expect(toInt(double.negativeInfinity), isNull);
+    expect(toInt('NaN'), isNull);
     expect(toInt(null), isNull);
     expect(toInt(''), isNull);
     expect(toInt('not a number'), isNull);