#!/bin/bash
set -e
function _help() {
  echo '
Usage: file-age {file} [units]

  Checks file modification date, and the current time. Prints the age of
  the file in [units].

  [units] is one of S, M, H, d, w, m, y, full, human
          seconds is the default. All values floored.
          month = 30 days
          year  = 365 days
          full  = YY-MM-DD hh:mm:ss
          human = 3 months 2 days
  '
  exit
}
for (( i=1; i<=$#; i++ ))
do  [[ "${!i}" = "-h" ]] && _help
done

function _human {
  file_date=( $( date -r "$STATFILE" +"%Y %m %d %H %M %S" ) )
  current_date=( $( date +"%Y %m %d %H %M %S" ) )
  local values=()
  for (( i=0; $i<${#file_date[@]}; i++ )); do
    values+=( $(( ${current_date[$i]#0} - ${file_date[$i]#0})) )
  done

  if [[ ${values[5]} -lt 0 ]]; then
    values[5]=$(( 60 + ${values[5]} )); values[4]=$(( ${values[4]}  -1 ))
  fi
  if [[ ${values[4]} -lt 0 ]]; then
    values[4]=$(( 60 + ${values[4]} )); values[3]=$(( ${values[3]} -1 ))
  fi
  if [[ ${values[3]} -lt 0 ]]; then
    values[3]=$(( 24 + ${values[3]} )); values[2]=$(( ${values[2]} -1 ))
  fi
  if [[ ${values[2]} -lt 0 ]]; then
    case ${file_date[1]#0} in
      1|3|5|7|8|10|12) mlen=31;;
      2) mlen=28;;
      *) mlen=30;;
    esac
    values[2]=$(( $mlen + ${values[2]} ));
    values[1]=$(( ${values[1]} -1 ))
  fi
  if [[ ${values[1]} -lt 0 ]]; then
    values[1]=$(( 12 + ${values[1]} )); values[0]=$(( ${values[0]} -1 ))
  fi
  if [[ "$1" = "full" ]]; then
    printf '%02d-%02d-%02d %02d:%02d:%02d\n' ${values[@]}
    return
  fi
  local units=( yr mo day hr min sec )
  if [[ "${file_date[@]}" = "${current_date[@]}" ]]; then
     printf " 0 sec\n"; return
  fi
  for (( i=0; $i<${#values[@]}; i++ )); do
    if [[ ${values[$i]} -gt 0 ]]; then
      j=$(( i + 1 ))
      printf "%2s %-3s" ${values[$i]} ${units[$i]}
      if [[ $j -lt ${#values[@]} ]]; then
        printf " %2s %-3s" ${values[$j]} ${units[$j]}
        break
      fi
    fi
  done
  printf "\n"
}

STATFILE="$1"
[[ -z "$STATFILE" ]] && _help
test -e "$STATFILE" || { echo File "$STATFILE" does not exist >&2; exit 1; }

UNITS=${2:-S}
case $OSTYPE in
  darwin*) created=$( stat -f %c -t %s "$STATFILE" ) ;;
  *) created=$( stat -c %Y "$STATFILE" ) ;;
esac
now=$( date +%s )
age=$(( $now - $created ))

case $UNITS in
  S)
    echo $age ;;
  M)
    echo $(( $age / 60 )) ;;
  H)
    echo $(( $age / 3600 )) ;;
  d)
    echo $(( $age / 86400 )) ;;
  w)
    echo $(( $age / 604800 )) ;;
  m)
    echo $(( $age / 2592000 )) ;;
  y)
    echo $(( $age / 31536000 )) ;;
  full)
    _human full;;
  human)
    _human;;
  *)
    echo Unit $UNITS not recognized.
    _help ;;
esac