chore(ci): add Stryker.NET mutation testing for the split math

.github/workflows/mutation.yml

@@ -0,0 +1,51 @@
+name: Mutation testing
+
+# Stryker.NET on the pure split math (SplitCalculator): the only unit-testable
+# logic that is fully DB-free, so the job is fast and hermetic (no SQL Server).
+#
+# Runs when the mutated code, its tests or the config change; nightly-style on a
+# schedule; and on demand. Fails when the mutation score drops below the `break`
+# threshold in stryker-config.json.
+on:
+  pull_request:
+    branches: [main]
+    paths:
+      - 'SplitIt.API/SplitIt.Infrastructure/Services/SplitCalculator.cs'
+      - 'SplitIt.Tests/SplitCalculator*'
+      - 'stryker-config.json'
+      - '.github/workflows/mutation.yml'
+  schedule:
+    - cron: '0 4 * * 1'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  stryker:
+    name: Stryker.NET (SplitCalculator)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 8.0.x
+
+      - name: Install Stryker.NET
+        run: dotnet tool install --global dotnet-stryker
+
+      - name: Restore
+        run: dotnet restore SplitIt.API/SplitIt.Back.sln
+
+      - name: Run mutation tests
+        run: dotnet stryker --config-file stryker-config.json
+
+      - name: Upload mutation report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: mutation-report
+          path: StrykerOutput/**/reports/

.gitignore

@@ -201,3 +201,6 @@ Desktop.ini
 
 # SonarScanner for .NET working directory
 .sonarqube/
+
+# Mutation testing output (Stryker.NET)
+StrykerOutput/

SplitIt.Tests/SplitCalculatorBoundaryTests.cs

@@ -0,0 +1,59 @@
+using SplitIt.Infrastructure.Services;
+
+namespace SplitIt.Tests;
+
+/// <summary>
+/// Boundary and validation cases for the split math, complementing
+/// <see cref="SplitCalculatorTests"/> so mutation testing has fewer survivors.
+/// </summary>
+public class SplitCalculatorBoundaryTests
+{
+    [Fact]
+    public void EqualSplit_ZeroTotal_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.EqualSplit(0m, new[] { 1, 2 }));
+
+    [Fact]
+    public void EqualSplit_NullParticipants_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.EqualSplit(100m, null!));
+
+    [Fact]
+    public void EqualSplit_EmptyParticipants_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.EqualSplit(100m, Array.Empty<int>()));
+
+    [Fact]
+    public void ByAmount_NullEntries_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.ByAmount(null!, 100m));
+
+    [Fact]
+    public void ByAmount_EmptyEntries_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.ByAmount(Array.Empty<(int, decimal)>(), 100m));
+
+    [Fact]
+    public void ByPercentage_NullEntries_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.ByPercentage(null!, 100m));
+
+    [Fact]
+    public void ByPercentage_ZeroTotal_Throws() =>
+        Assert.Throws<ArgumentException>(() => SplitCalculator.ByPercentage(new[] { (1, 100m) }, 0m));
+
+    [Fact]
+    public void ByPercentage_AcceptsBoundaryPercentagesZeroAndHundred()
+    {
+        var result = SplitCalculator.ByPercentage(new[] { (1, 0m), (2, 100m) }, 100m);
+
+        Assert.Equal(0m, result[0].AmountOwed);
+        Assert.Equal(100m, result[1].AmountOwed);
+    }
+
+    [Fact]
+    public void ByPercentage_AbsorbsRoundingDriftIntoLastParticipant()
+    {
+        var result = SplitCalculator.ByPercentage(
+            new[] { (1, 33.33m), (2, 33.33m), (3, 33.33m) },
+            100m);
+
+        Assert.Equal(100m, result.Sum(r => r.AmountOwed));
+        Assert.Equal(33.33m, result[0].AmountOwed);
+        Assert.Equal(33.34m, result[2].AmountOwed);
+    }
+}

stryker-config.json

@@ -0,0 +1,15 @@
+{
+  "stryker-config": {
+    "project": "SplitIt.API/SplitIt.Infrastructure/SplitIt.Infrastructure.csproj",
+    "test-projects": ["SplitIt.Tests/SplitIt.Tests.csproj"],
+    "mutate": ["Services/SplitCalculator.cs"],
+    "test-case-filter": "FullyQualifiedName~SplitCalculator",
+    "thresholds": {
+      "high": 80,
+      "low": 70,
+      "break": 65
+    },
+    "reporters": ["progress", "html", "json"],
+    "concurrency": 4
+  }
+}