CI: Add format check

Add --check flag to scripts/format.sh rather than calling clang-format
directly in ci.yml, so the set of matched files stays consistent between
local formatting and CI verification.

Replace rg with find in the script to avoid requiring ripgrep in CI.

Rename workflow from "build" to "ci" to match the filename and reflect
that it now covers more than building.

The format job runs in a debian:trixie container to pin clang-format 19.
The script checks the version at startup so local dev catches mismatches
early.

Author: michalhosna

.github/workflows/ci.yml

@@ -1,4 +1,4 @@
-name: build
+name: ci
 
 on:
   pull_request:
@@ -11,6 +11,18 @@ permissions:
   checks: write
 
 jobs:
+  format:
+    runs-on: ubuntu-latest
+    container: debian:trixie
+    steps:
+      - name: Install tools
+        run: apt-get update && apt-get install -y --no-install-recommends git clang-format
+
+      - uses: actions/checkout@v4
+
+      - name: Check formatting
+        run: bash scripts/format.sh --check
+
   build:
     strategy:
       fail-fast: false

scripts/format.sh

@@ -1,12 +1,30 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-FILES=$(rg --files -g '*.{h,hpp,cc,cpp,cxx}' include src tests tools)
+REQUIRED_CF_VERSION=19
+
+CF_VERSION=$(clang-format --version | sed -n 's/.*version \([0-9]*\)\..*/\1/p' | head -1)
+if [[ -z "${CF_VERSION}" ]]; then
+  echo "error: could not determine clang-format version" >&2
+  exit 1
+fi
+if [[ "${CF_VERSION}" -ne "${REQUIRED_CF_VERSION}" ]]; then
+  echo "error: clang-format ${REQUIRED_CF_VERSION} required, found ${CF_VERSION}" >&2
+  echo "  apt install clang-format-${REQUIRED_CF_VERSION}  (or see https://apt.llvm.org)" >&2
+  exit 1
+fi
+
+FILES=$(find include src tests tools -name '*.h' -o -name '*.hpp' -o -name '*.cc' -o -name '*.cpp' -o -name '*.cxx')
 
 if [[ -z "${FILES}" ]]; then
   echo "No source files found."
   exit 0
 fi
 
-echo "Formatting files..."
-clang-format -i ${FILES}
+if [[ "${1:-}" == "--check" ]]; then
+  echo "Checking formatting..."
+  clang-format --dry-run -Werror ${FILES}
+else
+  echo "Formatting files..."
+  clang-format -i ${FILES}
+fi