discharge

#!/usr/bin/env bash

FILE=/tmp/discharge
TIME_NOW=$(date +%s)

START_TIME=$(date -d '-15min' +%s) # only consider entries less than 15 minutes old
MAX_DT=300 # only consider updates less than 5 minutes apart

case "$1" in
  update)
    # this is meant to be called by udev with the corresponding environment variables set
    if [[ "$POWER_SUPPLY_STATUS" == "Charging" ]]; then
      # we're charging - discard all previous entries
      > "$FILE"
      exit 0
    elif [[ "$POWER_SUPPLY_STATUS" == "Discharging" ]]; then
      # we're discharging - add a new entry
      echo "$TIME_NOW $POWER_SUPPLY_CHARGE_NOW" >> "$FILE"
    fi
    ;;
  *)
    if [[ "$(cat /sys/class/power_supply/BAT0/status)" != "Discharging" ]]; then
      exit
    fi
    SUM=0
    COUNT=0
    PREV_T=
    PREV_C=
    while IFS= read -r LINE; do
      IFS=" " read -r -a SPLIT <<< "$LINE"
      T="${SPLIT[0]}"
      C="${SPLIT[1]}"
      DT=$((T - PREV_T))
      DC=$((PREV_C - C))

      # perform update if
      # - there's a previous charge level
      # - the charge level has changed since last time
      # - we are within the time range we consider
      # - the time between the two steps isn't too big (e.g. due to system suspend)
      if [[ -n "$PREV_C" && "$C" != "$PREV_C" && "$T" -gt "$START_TIME" && "$DT" -lt "$MAX_DT" ]]; then
        RATE=$((DC / DT))
        SUM=$((SUM + (TIME_NOW-T) * RATE))
        COUNT=$((COUNT + (TIME_NOW-T)))
      fi

      # reset statistics if the charge _increased_
      if (( PREV_C < C )); then
        SUM=0
        COUNT=0
      fi

      PREV_T="$T"
      PREV_C="$C"
    done < "$FILE"

    if (( COUNT == 0 )); then
      echo "?"
      exit
    fi

    AVG=$((SUM / COUNT))
    CHARGE=$(cat /sys/class/power_supply/BAT0/charge_now)
    TIME=$((CHARGE / AVG))
    FORMAT=${1:-%H:%M}
    date -d@$TIME -u +"$FORMAT"
    ;;
esac