#!/bin/bash

is_leap_year () {
    [[ "$1" =~ ^[0-9]+$ ]] \
	&& (( (($1 % 4) == 0) && ((($1 % 100) != 0) || (($1 % 400) == 0)) ))
}

is_ddmmyyyy () {
    local dd="${1:0:2}" mm="${1:2:2}" yyyy="${1:4:4}"
    local ds=(31 28 31 30 31 30 31 31 30 31 30 31)
    is_leap_year $yyyy && ds[1]=29
    [[ "$1" =~ [0123][0-9][01][0-9][0-9]{4} ]] \
	&& (( mm > 0 && mm < 13 && dd > 0 && dd <= ds[mm - 1] ))
}

is_finnish_id () (
    ddmm=${1:0:4} yy=${1:4:2} a=${1:6:1} n=${1:7:3} c=${1:10:1}
    checksum () {
	local s='0123456789ABCDEFHJKLMNPRSTUVWXYZ'
	echo ${s:$(( $1 % 31)):1}
    }
    case "$a" in '+') yyyy=18$yy;; '-') yyyy=19$yy;; 'A') yyyy=20$yy;; esac
    [[ "$a$n$c" =~ [-+A][0-9]{3}[0-9A-Z] ]] \
	&& is_ddmmyyyy $ddmm$yyyy \
	&& [[ "$c" = $(checksum $ddmm$yy$n) ]]
)