#!/bin/bash
# datediff  -  render time interval in human readable form
# Using bash's integers.             (C) 2021 - F-Hauri.ch
# Licensed by GNU GENERAL PUBLIC LICENSE Version 3
# shellcheck disable=SC2154 # referenced but not assigned (eyear ??)
# shellcheck disable=SC2034 # appears unused (week ??)

start="${1:-$(LANG=C ps ho lstart 1)}"		end="${2:-now}"

declare -A trad='( [year]="an"  [month]="mois"  [week]="semaine"  [day]="jour"
    [nothing]=rien  [and]=et  [hour]="heure"  [min]="minute"  [sec]="seconde" )'
declare -i {,s,e}{year,month,week,day,hour,min,sec}

{ read -r s{year,month,day,hour,min,sec}
  read -r e{year,month,day,hour,min,sec}
} < <(date -f - <<<"$start"$'\n'"$end" +'%Y 10#%m 10#%d 10#%H 10#%M 10#%S')

year=eyear-syear	month=emonth-smonth	day=eday-sday
hour=ehour-shour	min=emin-smin		sec=esec-ssec

((sec<0))	&&  min+=-1	sec+=60
((min<0))	&&  hour+=-1	min+=60
((hour<0))	&&  day+=-1	hour+=24
((day<0))	&&  month+=-1	day+=$( date -d "$eyear-$emonth-01 -1 day" +%d )
((month<0))	&&  year+=-1	month+=12
week=day/7	day=day%7

out=()
for w in  year month week day hour min sec ;do
    (( ${!w} )) && {
	(( ${!w} > 1 )) && [ "${trad[$w]: -1}" != "s" ] && s=s || s=
	out+=("${!w} ${trad[$w]}$s")
    }
done

case ${#out[@]} in
    0 ) echo "${trad[nothing]}" ;;
    1 ) echo "${out[0]}." ;;
    * ) printf -v str '%s, ' "${out[@]::${#out[@]}-1}"
	echo "${str%, } ${trad[and]} ${out[*]: -1}." ;;
esac