fix(math): bound fraction regex execution

OpenNest.Tests/Math/FractionTests.cs

@@ -1,3 +1,5 @@
+using System.Globalization;
+using System.Text.RegularExpressions;
 using OpenNest.Math;
 using Xunit;
 
@@ -5,6 +7,103 @@ namespace OpenNest.Tests.Math;
 
 public class FractionTests
 {
+    // Calibrated on .NET 8: unbounded no-match takes seconds, well above the 250 ms budget.
+    private static string TimeoutInput => new string('1', 50000);
+
+    [Fact]
+    public void FractionRegex_HasExplicitTimeout()
+    {
+        Assert.Equal(TimeSpan.FromMilliseconds(250), Fraction.FractionRegex.MatchTimeout);
+    }
+
+    [Theory]
+    [InlineData("abc 3/8 xyz", 0.375)]
+    [InlineData("abc 1 3/4 xyz", 1.75)]
+    [InlineData("abc 1-3/4 xyz", 1.75)]
+    public void Fraction_PreservesSubstringMatchingAndMixedSeparators(string input, double expected)
+    {
+        Assert.True(Fraction.IsValid(input));
+        Assert.Equal(expected, Fraction.Parse(input), 8);
+    }
+
+    [Theory]
+    [InlineData("en-US")]
+    [InlineData("fr-FR")]
+    public void Parse_UsesCurrentCultureForOrdinaryIntegerComponents(string culture)
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = CultureInfo.GetCultureInfo(culture);
+            // The implementation uses current-culture double.Parse, not invariant parsing.
+            // Ordinary digits are invariant-parseable too; exotic numeric syntax is not this contract.
+            var expected = double.Parse("1") + System.Math.Round(double.Parse("3") / double.Parse("4"), 8);
+            Assert.Equal(expected, Fraction.Parse("1 3/4"), 8);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
+    [Fact]
+    public void ReplaceFractionsWithDecimals_PreservesOrderAndUnmatchedText()
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
+            Assert.Equal("first 0.5, then 1.75, finally 0.375!",
+                Fraction.ReplaceFractionsWithDecimals("first 1/2, then 1-3/4, finally 3/8!"));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
+    [Fact]
+    public void Parse_OnTimeout_ThrowsRatherThanReturningValue()
+    {
+        Assert.Throws<RegexMatchTimeoutException>(() => Fraction.Parse(TimeoutInput));
+    }
+
+    [Fact]
+    public void IsValid_OnTimeout_ThrowsRatherThanReturningValidity()
+    {
+        Assert.Throws<RegexMatchTimeoutException>(() => Fraction.IsValid(TimeoutInput));
+    }
+
+    [Fact]
+    public void ReplaceFractionsWithDecimals_OnTimeout_ThrowsRatherThanReturningText()
+    {
+        Assert.Throws<RegexMatchTimeoutException>(() => Fraction.ReplaceFractionsWithDecimals(TimeoutInput));
+    }
+
+    [Fact]
+    public void ReplaceFractionsWithDecimals_AfterEarlierMatchTimeout_ReturnsNoPartialText()
+    {
+        var input = "3/8 " + TimeoutInput;
+        var matches = Fraction.FractionRegex.Matches(input).GetEnumerator();
+        Assert.True(matches.MoveNext());
+        Assert.Equal("3/8", ((Match)matches.Current).Value);
+        Assert.Throws<RegexMatchTimeoutException>(() => matches.MoveNext());
+
+        // MatchCollection is lazy; even after an earlier match, the method's local
+        // StringBuilder never escapes if enumeration fails (including its sorting step).
+        Assert.Throws<RegexMatchTimeoutException>(() => Fraction.ReplaceFractionsWithDecimals(input));
+        Assert.StartsWith("3/8 ", input);
+    }
+
+    [Fact]
+    public void TryParse_OnTimeout_ReturnsFalseAndZero()
+    {
+        var input = TimeoutInput;
+        Assert.Throws<RegexMatchTimeoutException>(() => Fraction.Parse(input));
+        Assert.False(Fraction.TryParse(input, out var fraction));
+        Assert.Equal(0, fraction);
+    }
+
     [Theory]
     [InlineData("3/8", 0.375)]
     [InlineData("1 3/4", 1.75)]