refactor: improve parseFixed logic and add comprehensive unit tests for edge cases (#2661)

cw_core/lib/parse_fixed.dart

@@ -1,14 +1,13 @@
-BigInt parseFixed(String value, int? decimals) {
-  decimals ??= 0;
+BigInt parseFixed(String value, int decimals) {
   final multiplier = getMultiplier(decimals);
 
-// Is it negative?
-  final negative = (value.substring(0, 1) == "-");
+  final negative = value.startsWith("-");
   if (negative) value = value.substring(1);
 
   if (value == ".") throw Exception("missing value, value, $value");
 
-// Split it into a whole and fractional part
+  if (value.startsWith(".")) value = "0$value";
+
   final comps = value.split(".");
   if (comps.length > 2) {
     throw Exception("too many decimal points, value, $value");
@@ -17,7 +16,6 @@ BigInt parseFixed(String value, int? decimals) {
   var whole = comps.isNotEmpty ? comps[0] : "0";
   var fraction = (comps.length == 2 ? comps[1] : "0").padRight(decimals, "0");
 
-  // Check the fraction doesn't exceed our decimals size
   if (fraction.length > multiplier.length - 1) {
     throw Exception(
         "fractional component exceeds decimals, underflow, parseFixed");

cw_core/test/parse_fixed_test.dart

@@ -0,0 +1,38 @@
+import 'package:cw_core/parse_fixed.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('parseFixed', () {
+    group('parseFixed, positive', () {
+      test('should parse 1.000001 as 1000001',
+          () => expect(parseFixed("1.000001", 6), BigInt.from(1000001)));
+
+      test('should parse 1 as 1000000', () => expect(parseFixed("1", 6), BigInt.from(1000000)));
+
+      test('should parse 1. as 1000000', () => expect(parseFixed("1.", 6), BigInt.from(1000000)));
+
+      test('should parse 1.1 as 1100000', () => expect(parseFixed("1.1", 6), BigInt.from(1100000)));
+
+      test('should parse 01.1 as 1100000',
+          () => expect(parseFixed("01.1", 6), BigInt.from(1100000)));
+
+      test('should parse 1100000 as 11000000',
+          () => expect(parseFixed("1100000", 1), BigInt.from(11000000)));
+    });
+
+    group('parseFixed, negative', () {
+      test('should parse -1.000001 as -1000001',
+          () => expect(parseFixed("-1.000001", 6), BigInt.from(-1000001)));
+
+      test('should parse -1 as 1000000', () => expect(parseFixed("-1", 6), BigInt.from(-1000000)));
+    });
+
+    group('parseFixed, no leading 0', () {
+      test('should parse .000001 as 1', () => expect(parseFixed(".000001", 6), BigInt.from(1)));
+
+      test('should parse .00002 as 20', () => expect(parseFixed(".00002", 6), BigInt.from(20)));
+
+      test('should parse -.00002 as -20', () => expect(parseFixed("-.00002", 6), BigInt.from(-20)));
+    });
+  });
+}