--- /dev/null
+#!/bin/bash
+
+set -euo pipefail
+
+# "script-and-regex.regex": "/^(?P<severity>.*?)\n(?P<message>.*?)\n(?P<line>\\d),(?P<char>\\d)(\n(?P<original>.*?)>>>>\n(?P<replacement>.*?)<<<<?)$/s",
+
+# Arcanist linter that invokes clang-format.
+# stdout from this script is parsed into a regex and used by Arcanist.
+# https://secure.phabricator.com/book/phabricator/article/arcanist_lint_script_and_regex/
+
+# To skip running all linters when creating/updating a diff, use `arc diff --nolint`.
+
+if ! hash clang-format-diff >/dev/null; then
+ # advice severity level is completely non-disruptive.
+ # switch to warning or error if you want to prompt the user.
+ echo "advice"
+ echo "clang-format-diff not found in user's PATH; not linting file."
+ echo "===="
+ exit 0
+fi
+
+src_file="${1}"
+original_file="$(mktemp)"
+formatted_file="$(mktemp)"
+readonly src_file
+readonly original_file
+readonly formatted_file
+cp -p "${src_file}" "${original_file}"
+cp -p "${src_file}" "${formatted_file}"
+
+cleanup() {
+ rc=$?
+ rm "${formatted_file}" "${original_file}"
+ exit ${rc}
+}
+trap 'cleanup' INT HUP QUIT TERM EXIT
+
+# Arcanist can filter out lint messages for unchanged lines, but for that, we
+# need to generate line by line lint messages. Instead, we generate one lint
+# message on line 1, char 1 with file content edited using clang-format-diff.
+if git rev-parse --git-dir >/dev/null; then
+ arc_base_commit=$(arc which --show-base)
+ # An alternative is to use git-clang-format.
+ git diff -U0 --no-color "${arc_base_commit}"| clang-format-diff -style LLVM -i -p1
+else
+ svn diff --diff-cmd=diff -x -U0 "${src_file}" | clang-format-diff -style LLVM -i
+fi
+
+cp -p "${src_file}" "${formatted_file}"
+cp -p "${original_file}" "${src_file}"
+if ! diff -q "${src_file}" "${formatted_file}" > /dev/null ; then
+ echo "autofix"
+ echo "clang-format suggested style edits found:"
+ echo "1,1" # line,char of start of replacement.
+ cat "${src_file}"
+ echo ">>>>"
+ cat "${formatted_file}"
+ echo "<<<<"
+fi