#! /usr/bin/bash

set -euo pipefail
IFS=$'\n\t'

default_editor() {
    if [ -n "${SUDO_EDITOR+set}" ]; then
        echo "${SUDO_EDITOR}"
    elif [ -n "${VISUAL+set}" ]; then
        echo "${VISUAL}"
    elif [ -n "${EDITOR+set}" ]; then
        echo "${EDITOR}"
    else
        echo "No editor configured" 1>&2
        exit 1
    fi
}

try_command() {
    if "$@" 2> /dev/null; then
        return
    elif run0 "$@" > /dev/null; then
        return
    else
        exit 2
    fi
}

make_editable_copy() {
    orig_file="$1"
    tempfile="$2"

    try_command cp "${orig_file}" "${tempfile}"
}

elevate_permissions() {
    tempfile="$1"
    orig_file="$2"

    try_command chown --reference "${orig_file}" "${tempfile}"
    try_command chmod --reference "${orig_file}" "${tempfile}"
}

update_file() {
    tempfile="$1"
    orig_file="$2"

    try_command mv --force "${tempfile}" "${orig_file}"
}

clean_tempfiles() {
    tempfile="$1"

    rm --force "${tempfile}" "${tempfile}.orig"
}

if [ "$#" -ne "1" ]; then
    echo "Expected exactly one input file. Usage:" 1>&2
    echo "   $0 FILE" 1>&2
    exit 1
fi

editor_cmd="$(default_editor)"
echo "Editing using the command \"${editor_cmd}\""

IFS=' ' read -r -a editor <<< "${editor_cmd}"

orig_file="$1"
tempfile="$(mktemp)"
echo "Using temporary file ${tempfile}"
make_editable_copy "${orig_file}" "${tempfile}"
cp "${tempfile}" "${tempfile}.orig"
"${editor[@]}" "${tempfile}"
if cmp --quiet "${tempfile}" "${tempfile}.orig"; then
    echo "No change to file"
else
    elevate_permissions "${tempfile}" "${orig_file}"
    update_file "${tempfile}" "${orig_file}"
fi

clean_tempfiles "${tempfile}"