#!/usr/bin/env bash
set -euo pipefail

# This is a script which can be kept running as a cronjob or systemd timer to
# notify the user when the battery is low.

# $BATTERY env var can be set manually, being the path of the device
# (in /sys/class/power_supply/) i.e. BATTERY=/sys/class/power_supply/BAT0.
#
# If left unset, the script will automatically pick the battery device
# with the most recent change time. (This provides some limited support
# for systems with multiple batteries.)
#
# If the battery reaches below $NOTIFY_THRESHOLD (defaults to 5%),
# a notification will be shown. Note that only a single notification
# will be shown per charge cycle.

NOTIFY_THRESHOLD="${NOTIFY_THRESHOLD:-5}"

if [ -z "${BATTERY:-}" ]; then
  BATTERY="$(find /sys/class/power_supply -maxdepth 1 -name 'BAT*' -exec stat -c '%Y %n' {} + | sort -k1,1nr | head -n 1 | cut -d' ' -f2-)"
fi

NOTIFY_LOCK_FILE=/tmp/battery-notif

get_bat_info() {
  cat "$BATTERY"/"$1"
}

capacity="$(get_bat_info capacity)"
status="$(get_bat_info status)"

reset_notify() {
  if [ -f "$NOTIFY_LOCK_FILE" ]; then
    echo "Resetting notification flag"
    rm -f "$NOTIFY_LOCK_FILE"
  fi
}

maybe_notify() {
  if [ -f "$NOTIFY_LOCK_FILE" ]; then
    exit 0
  fi

  echo "Sending notification"
  notify-send \
    -a battery-notification \
    -u critical \
    -i "$(pick_battery_icon)" \
    "Battery low" "Battery is below $NOTIFY_THRESHOLD%"
  touch "$NOTIFY_LOCK_FILE"
}

pick_battery_icon() {
  local icon_candidate
  local data_dirs="${XDG_DATA_DIRS:-/usr/local/share:/usr/share}" # fallback
  local IFS=':'

  for icon_candidate in battery-caution battery-low battery; do
    for dir in $data_dirs; do
      if find "$dir/icons" -type f -iname "${icon_candidate}.*" 2>/dev/null | grep -q .; then
        echo "$icon_candidate"
        return 0
      fi
    done
  done

  echo ""
}

# The battery is not discharging (i.e. charging or full),
# we don't need to notify the user.
if [ "$status" != "Discharging" ]; then
  reset_notify
  exit 0
fi

if [ "$capacity" -gt "$NOTIFY_THRESHOLD" ]; then
  reset_notify
  exit 0
fi

maybe_notify