#!/bin/bash
#set -x

CALENDAR=(31 28 31 30 31 30 31 31 30 31 30 31)

#############################
### FUNCTION-is_leap_year ###
#############################

is_leap_year() {
	local PART1=$(expr ${1} % 4)
	local PART2=$(expr ${1} % 100)
	local RESULT="false"

	if [[ 0 -eq ${PART1} ]] && [[ 0 -ne ${PART2} ]] || [[ 0 -eq $(expr ${1} % 400) ]]; then #{
		RESULT="true"
	fi #}

	echo ${RESULT}
}

####################
### END FUNCTION ###
####################

########################
### FUNCTION-get_day ###
########################

get_day() {
	local MM=${1}
	local DD=${2}
	local FIRSTDIGIT=${MM:0:1}
	local SECONDDIGIT=${MM:1:1}
	local DAY="BAD"

	if [[ ${FIRSTDIGIT} -eq 0 ]]; then #{
		if [[ ${DD} -gt 0 ]] && [[ ${DD} -le ${CALENDAR[SECONDDIGIT-1]} ]]; then #{
			DAY="GOOD"
		fi #}
	else #} {
		if [[ ${DD} -gt 0 ]] && [[ ${DD} -le ${CALENDAR[MM-1]} ]]; then #{
			DAY="GOOD"
		fi #}
	fi #}

	echo ${DAY}
}

####################
### END FUNCTION ###
####################

KEEPGOING=1
while [[ ${KEEPGOING} -eq 1 ]]; do #{
	read -p "Enter a date for validation: " DATE

	# establish the varible LEN to hold the number of characters in Date, 8 is the only valid number
	LEN=$(echo ${#DATE})

	if [[ "${DATE}" == exit ]]; then #{
		$KEEPGOING=0
	elif [ $LEN -eq 8 ]; then # } {
		# set date dariables MM, DD, & YY
		MM=${DATE:0:2}
		DD=${DATE:2:2}
		YY=${DATE:4:4}

		if [ ${YY} -gt 0 ]; then #{
			if [ ${MM} -gt 0 ] && [ ${MM} -lt 13 ]; then #{
				if [ ${MM} -eq 02 ]; then #{
					result=$(is_leap_year ${YY})
					if [ "$result" == "true" ]; then #{
						if [ ${DD} -gt 0 ] && [ ${DD} -le 29 ]; then #{
							echo "${DATE} is a valid date!"
						else #} {
							echo "${DD} is invalid for this date!"
						fi #}
					else #} {
						DAY=$(get_day ${MM} ${DD})
						if [ "${DAY}" == "GOOD" ]; then #{
							echo "${DATE} is a valid date!"
						else #} {
							echo "${DD} is invalid for this date!"
						fi #}
					fi #}
				else #} {
					DAY=$(get_day ${MM} ${DD})
					if [ "${DAY}" == "GOOD" ]; then #{
						echo "${DATE} is a valid date!"
					else # } {
						echo "${DD} is invalid for this date!"
					fi #}
				fi #}

			else # } {
				echo "${MM} is invalid for this date!"
			fi #}

		else # } {
			echo "${YY} is invalid for this date!"
		fi #}

	else # } {
		echo "Invalid number of digits for a date!"
	fi #}
done #}
#END#