From ac64008db66b2b1b3af0ef72217de6e39474b96d Mon Sep 17 00:00:00 2001
From: santidev21 <orsantiago21@gmail.com>
Date: Thu, 1 Oct 2026 17:04:16 -0500
Subject: [PATCH] chore(ci): add Stryker.NET mutation testing for the split
 math

- stryker-config.json: mutate SplitIt.Infrastructure/Services/SplitCalculator.cs and run only the SplitCalculator tests (pure, no DB). Thresholds high=80 / low=70 / break=65.
- .github/workflows/mutation.yml: runs on PRs touching the calculator, its tests or the config, on a weekly schedule and on demand; fails below break and uploads the report.
- SplitCalculatorBoundaryTests: boundary cases (null/empty input, zero total, 0/100 percentages, rounding drift) that raise the mutation score from 69% to 82%.
---
 .github/workflows/mutation.yml                | 51 ++++++++++++++++
 .gitignore                                    |  3 +
 SplitIt.Tests/SplitCalculatorBoundaryTests.cs | 59 +++++++++++++++++++
 stryker-config.json                           | 15 +++++
 4 files changed, 128 insertions(+)
 create mode 100644 .github/workflows/mutation.yml
 create mode 100644 SplitIt.Tests/SplitCalculatorBoundaryTests.cs
 create mode 100644 stryker-config.json

diff --git a/.github/workflows/mutation.yml b/.github/workflows/mutation.yml
new file mode 100644
index 0000000..b6bc4ff
--- /dev/null
+++ b/.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/
diff --git a/.gitignore b/.gitignore
index f25fcce..6b4ad82 100644
--- a/.gitignore
+++ b/.gitignore
@@ -201,3 +201,6 @@ Desktop.ini
 
 # SonarScanner for .NET working directory
 .sonarqube/
+
+# Mutation testing output (Stryker.NET)
+StrykerOutput/
diff --git a/SplitIt.Tests/SplitCalculatorBoundaryTests.cs b/SplitIt.Tests/SplitCalculatorBoundaryTests.cs
new file mode 100644
index 0000000..b56ee45
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/stryker-config.json b/stryker-config.json
new file mode 100644
index 0000000..bb23aba
--- /dev/null
+++ b/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
+  }
+}