#!/bin/bash
#Autor: Dominik Mańkowski

is_integer() {
    if [ "$1" -eq "$1" ] 2>/dev/null; then
        echo "YES"
    else
        echo "NO"
    fi
}

is_operator() {
    if [ ${#1} -ne 1 ]; then # wiecej niz 1 znak
        echo "NO"
    elif [[ $1 =~ [+-/\*] ]]; then
        echo "YES"
    else
        echo "NO"
    fi
}

is_left_brackt() {
    if [ ${#1} -ne 1 ]; then
        echo "NO"
    elif [[ $1 =~ [\(] ]]; then
        echo "YES"
    else
        echo "NO"
    fi
}

is_right_brackt() {
    if [ ${#1} -ne 1 ]; then
        echo "NO"
    elif [[ $1 =~ [\)] ]]; then
        echo "YES"
    else
        echo "NO"
    fi
}

#######################################

args="$@"

if [ "$#" -lt 3 ]; then
    echo "Not enough arguments - less than 3."
    echo "Consider using --help"
    exit 1
fi

if [ "$1" = "--help" ]; then
    echo "
DESCRIPTION
Script checks if given formula is syntactically correct, and if so, then prints the result.
Allowed operators: + - / *
Only one level of nested brackets is allowed.
Only integers are allowed.

USAGE
script_name arg1 op arg2 op ( arg3 op arg 4 op ...) op ...
arg1, arg2, ..., argN - integers
op - operator (+,-,/,*)

EXAMPLES OF USAGE
./1.sh 1 + 2 + 3
./1.sh 1 + 2 \* 3
./1.sh 1 + \( 2 + 3 \* 4 \) / 2

EXIT CODE
0                   No errors
1                   Number is not placed after operator or left bracket
2                   Script's first argument is not a number
3                   No number or bracket after operator
4                   There must be at least 4 args after left bracket
5                   Left bracket must be placed after operator
6                   Right bracket must be placed after number
7                   Missing left bracket
8                   Incorrect operator (different than allowed) or number is not an integer
9                   There is missing at least one bracket

    "
fi

counter=0
arg="start"
left_bracket=0

while [ "$1" ]; do
    flag=0

    status=$(is_integer "$1")                           # sprawdz, czy argument to liczba calkowita
    if [ "$status" = "YES" ]; then
        if [ ! "$arg" = "operator" ] && [ ! "$arg" = "left_bracket" ] && [ ! "$arg" = "start" ]; then
            echo "Incorrect usage - wrongly placed number."
            echo "Must be placed after operator or left bracket."
            echo "Consider using --help"
            echo ""
            exit 1
        fi
        flag=1
        counter=1
        arg="number"
        shift
    fi

    if [ $flag -eq 0 ]; then
        if [ $counter -eq 0 ]; then                     # przypadek typu ./skrypt.sh + + 3
            echo "Incorrect first argument."
            echo "Consider using --help"
            echo ""
            exit 2
        fi
        status=$(is_operator "$1")                      # sprawdz, czy argument to operator

        if [ "$status" = "YES" ]; then
            flag=1
            arg="operator"
            shift
            if [ "$#" -eq 0 ]; then                     # jesli nic nie ma po operatorze, to wyswietl blad
                echo "Not enough arguments - emptiness after operator."
                echo "Consider using --help"
                echo ""
                exit 3
            fi
        fi
    fi

    if [ $flag -eq 0 ]; then
        status=$(is_left_brackt "$1")
        if [ "$status" = "YES" ]; then
            if [ "$#" -lt 5 ]; then                     # ( 1 + 2 ) - musi byc przynajmniej 5 znakow
                echo "Incorrect usage - not enough arguments after left bracket."
                echo "Consider using --help"
                echo ""
                exit 4
            fi
            if [ ! "$arg" = "operator" ]; then
                echo "Incorrect usage - left bracket can not be placed after a number."
                echo "Consider using --help"
                echo ""
                exit 5
            fi
            shift
            arg="left_bracket"
            flag=1
            left_bracket=1;
        fi
    fi

    if [ $flag -eq 0 ]; then
        status=$(is_right_brackt "$1")
        if [ "$status" = "YES" ]; then
            if [ ! "$arg" = "number" ]; then
                echo "Incorrect usage - right bracket must be placed after a number."
                echo "Consider using --help"
                echo ""
                exit 6
            fi
            shift
            arg="bracket"
            flag=1
            if [ $left_bracket -ne 1 ]; then
                echo "Incorrect usage - missing left bracket."
                echo "Consider using --help"
                echo ""
                exit 7
            fi
            left_bracket=0;
            shift
        fi
    fi

    if [ $flag -eq 0 ]; then
        echo "Incorrect usage - numbers are not integers or incorrect operator."
        echo "Consider using --help"
        echo ""
        exit 8
    fi
done

if [ $left_bracket -eq 1 ]; then
    echo "Brackets are not closed."
    echo "Consider using --help"
    echo ""
    exit 9
fi

result=$(($args))
echo "$args = $result"