Add git clang format workflow

Author: mcbarton

.github/workflows/clang-format.yml

@@ -0,0 +1,50 @@
+name: clang-format
+
+on:
+  pull_request:
+    paths:
+      - '**.h'
+      - '**.cc'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  precheckin:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout PR branch
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install clang-format
+        run: |
+          curl https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+          os_codename="`cat /etc/os-release | grep UBUNTU_CODENAME | cut -d = -f 2`"
+          echo "deb https://apt.llvm.org/${os_codename}/ llvm-toolchain-${os_codename}-18 main" | sudo tee -a /etc/apt/sources.list
+          sudo apt update
+          sudo apt install -y clang-format-18
+
+      - name: Run git-clang-format
+        run: |
+          PR_BASE=$(git rev-list ${{ github.event.pull_request.head.sha }} ^${{ github.event.pull_request.base.sha }} | tail --lines 1 | xargs -I {} git rev-parse {}~1)
+          echo "running git clang-format against $PR_BASE commit"
+          git \
+          -c color.ui=always \
+          -c diff.wsErrorHighlight=all \
+          -c color.diff.whitespace='red reverse' \
+          clang-format-18 --diff --binary clang-format-18 --commit $PR_BASE -- src/ || \
+          (echo "Please run the following git-clang-format locally to fix the formatting: \n
+                git-clang-format HEAD~\n
+                for multiple commits we should place the formatting changes in the related commit with:\n
+                \t\tgit rebase -i -x \"git-clang-format-18 main && git commit -a --allow-empty --fixup=HEAD\" --strategy-option=theirs origin/main\n
+                \t\t Then inspect the results with: git log --oneline\n
+                \t\t Then squash without poluting the history with: git rebase --autosquash -i main\n" && exit 1)