Add pre-commit configuration

.github/scripts/clang-format-hook

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Thin wrapper around clang-format for easier to parse output from the
+# pre-commit hook.
+#
+# Needs to work with multiple input files as pre-commit passes multiple files to
+# the "executables"
+
+# Make sure that diff is actually recent enough (diffutils >= 3.4) to support
+# colored output
+COLOR_OUTPUT=$(diff --color=always <(echo) <(echo) > /dev/null 2>&1 && echo "--color=always")
+
+success=0
+for file in ${@}; do
+    if ! $(clang-format --style=file --Werror --dry-run ${file} > /dev/null 2>&1); then
+        echo "Necessary changes for: '${file}' (run 'clang-format --style=file -i ${file}' to fix it)"
+        diff ${COLOR_OUTPUT} -u ${file} <(clang-format --style=file ${file}) | tail -n +3
+        success=1
+    fi
+done
+exit ${success}

.github/workflows/pre-commit.yml

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+name: pre-commit
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  pre-commit:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cvmfs-contrib/github-action-cvmfs@v4
+    - uses: aidasoft/run-lcg-view@v4
+      with:
+        container: el9
+        view-path: /cvmfs/sw-nightlies.hsf.org/key4hep
+        run: |
+          echo "::group::Setup pre-commit"
+          # Newer versions of git are more cautious around the github runner
+          # environment and without this git rev-parse --show-cdup in pre-commit
+          # fails
+          git config --global --add safe.directory $(pwd)
+          python -m venv /root/pre-commit-venv
+          source /root/pre-commit-venv/bin/activate
+          pip install pre-commit
+          echo "::endgroup::"
+          echo "::group::Run pre-commit"
+          pre-commit run --show-diff-on-failure \
+            --color=always \
+            --all-files
+          echo "::endgroup::"

.pre-commit-config.yml

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+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v3.4.0
+    hooks:
+      - id: mixed-line-ending
+      - id: trailing-whitespace
+        exclude: (doc/ReleaseNotes.md)
+  - repo: local
+    hooks:
+      - id: clang-format
+        name: clang-format
+        entry: .github/scripts/clang-format-hook
+        types: [c++]
+        language: system