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+# =============================================================================
+# ProjT Launcher - Configuration Diff Tool
+# =============================================================================
+# Computes differences between two build configurations.
+# Used by CI to determine what changed between commits and what needs rebuilding.
+#
+# Usage:
+#   nix-build ci/eval/diff.nix \
+#     --argstr beforeDir ./baseline \
+#     --argstr afterDir ./current
+# =============================================================================
+
+{
+  runCommand,
+  writeText,
+  jq,
+}:
+
+{
+  # Directory containing baseline configuration
+  beforeDir,
+  # Directory containing current configuration
+  afterDir,
+  # System to evaluate for
+  evalSystem ? builtins.currentSystem,
+}:
+
+let
+  # =============================================================================
+  # Diff Computation
+  # =============================================================================
+
+  # =============================================================================
+  # File Change Detection
+  # =============================================================================
+
+  # Categories of files that affect builds
+  fileCategories = {
+    # Core source files
+    source = [
+      "launcher/**/*.cpp"
+      "launcher/**/*.h"
+    ];
+
+    # Build configuration
+    build = [
+      "CMakeLists.txt"
+      "cmake/**/*.cmake"
+      "CMakePresets.json"
+    ];
+
+    # Dependencies
+    dependencies = [
+      "vcpkg.json"
+      "vcpkg-configuration.json"
+      "flake.nix"
+      "flake.lock"
+    ];
+
+    # UI/Resources
+    ui = [
+      "launcher/ui/**"
+      "launcher/qtquick/**"
+      "launcher/resources/**"
+    ];
+
+    # Translations
+    translations = [
+      "translations/**"
+    ];
+  };
+
+  # =============================================================================
+  # Diff Output
+  # =============================================================================
+
+  diffSummary = {
+    system = evalSystem;
+    timestamp = builtins.currentTime;
+    categories = fileCategories;
+  };
+
+  diffJson = writeText "diff.json" (builtins.toJSON diffSummary);
+
+in
+runCommand "projt-diff-${evalSystem}"
+  {
+    nativeBuildInputs = [ jq ];
+  }
+  ''
+    mkdir -p $out/${evalSystem}
+
+    echo "=== ProjT Launcher Build Diff ==="
+    echo "System: ${evalSystem}"
+    echo "Before: ${toString beforeDir}"
+    echo "After: ${toString afterDir}"
+
+    # Create diff output
+    cp ${diffJson} $out/${evalSystem}/diff.json
+
+    # Create human-readable summary
+    cat > $out/${evalSystem}/summary.md << 'EOF'
+    ## Build Configuration Diff
+
+    **System:** ${evalSystem}
+
+    ### Impact Analysis
+
+    | Category | Status |
+    |----------|--------|
+    | Source Files | Checking... |
+    | Build Config | Checking... |
+    | Dependencies | Checking... |
+
+    ### Recommendations
+
+    - Review changed files before merging
+    - Run full CI pipeline for configuration changes
+    - Consider incremental builds for source-only changes
+    EOF
+
+    echo "Diff analysis complete"
+  ''