#!/usr/bin/env bash

# Unofficial Bash Strict Mode
set -euo pipefail
IFS='
'

number='^-?[0-9]+$'

do_add () {
    echo $(($1 + $2))
}

do_sub () {
    echo $(($1 - $2))
}

do_mult () {
    echo $(($1 * $2))
}

do_divide () {
    echo $(($1 / $2))
}


if [ $# != 3 ]
then
    >&2 echo "Error: expect 3 arguments"
    exit 1
elif ! [[ $1 =~ $number && $3 =~ $number ]]
then
    >&2 echo "Error: invalid number"
    exit 4
else
    case $2 in

        "+")
            echo $(do_add $1 $3)
            ;;
        
        "-")
            echo $(do_sub $1 $3)
            ;;
        
        "*")
            echo $(do_mult $1 $3)
            ;;
        
        "/")
            if [ $3 == 0 ]
            then
                >&2 echo "Error: division by 0"
                exit 2
            fi
            echo $(do_divide $1 $3)
            ;;
        
        *)
            >&2 echo "Error: invalid operator"
            exit 3
            ;;

    esac

fi