[DRAFT] Indent-checking `git commit` hook

git-commit-indent.pre-commit

#!/bin/sh
# git config hooks.indent "indent -linux -li0"
# NOTE: this code is horrible, you have been warned

indent_cmd="$(git config hooks.indent)"
if test -z "$indent_cmd"; then
	exit 0
fi

set -e

exec 1>&2

tmpdir="$(mktemp -d)"
trap "rm -rf '$tmpdir'" 0

filenames="$@"
if test -z "$filenames"; then
	filenames="$(git diff --cached --name-only --diff-filter M)"
fi
filenames="$(echo $filenames | grep '.[ch]$')"

if test -z "$filenames"; then
	exit 0
fi

mkdir -p $tmpdir/orig $tmpdir/indent $tmpdir/newindent

cp --parents $filenames $tmpdir/newindent
cp --parents $filenames $tmpdir/orig
cp --parents $filenames $tmpdir/indent

# TODO write_original_files() {...
for filename in $filenames; do
	git show HEAD:$filename > $tmpdir/orig/$filename
	cp $tmpdir/orig/$filename $tmpdir/indent/$filename
done

${indent_cmd} $(echo $filenames | sed "s,^,$tmpdir/newindent/,g")	\
	      $(echo $filenames | sed "s,^,$tmpdir/indent/,g")

failed=""
for filename in $filenames; do
	diff -u0 $tmpdir/newindent/$filename $filename >$tmpdir/newindent/$filename.diff || :
	diff -u0 $tmpdir/indent/$filename $tmpdir/orig/$filename >$tmpdir/indent/$filename.diff || :

	sed -i '1,2d' $tmpdir/indent/$filename.diff $tmpdir/newindent/$filename.diff
	diff -u0 $tmpdir/indent/$filename.diff $tmpdir/newindent/$filename.diff > $tmpdir/orig/$filename.diff || :
	#echo "diff -u0 $tmpdir/indent/$filename.diff $tmpdir/newindent/$filename.diff"
	awk '\
BEGIN { status = 0; }
/^+@@/ || /^-@@/ {
    current = substr($0, 0, 1);
    if (current == previous) {
	split(prevline, arr);
	print arr[2];
	print code;
	status = 1;
    }
    previous = current;
    code = "";
    prevline = $0;
}
/^@@/ {
    previous = current = 0;
    code = "";
}
!/^[+-]?@@/ && !/^(---|\+\+\+)/ {
    prefix = substr($0, 1, 2);
    if (prefix == "++" || prefix == "-0") {
	    if (code)
		code = code"\n"substr($0, 3);
	    else
		code = substr($0, 3);
    }
}
END { 
    if (code) {
        split(prevline, arr);
        print arr[2];
        print code;
	status = 1;
    }
    exit status;
}' $tmpdir/orig/$filename.diff > $tmpdir/orig/$filename.log || {
		failed="$failed $filename"
	}
done

if test -n "$failed"; then
	for fail in $failed; do
		echo "FAILED indent check for $fail" >&2
		cat $tmpdir/orig/$filename.log >&2
	done
	exit 1
fi
exit 0

Use "indent -linux -il0" instead