Validates HGD string

hgd-validate.bash

function validate() {
    let -i e=0

    let -i balance=0
    let -i opset=0

    local ch
    for (( i=0; i<${#1}; i++ )); do
        ch="${1:$i:1}"
        case "${ch}" in
            '(')
                if [ ${opset} -eq 1 ]; then
                    ((balance++))
                    opset=0
                else
                    e=1
                    break
                fi
            ;;
            ')') ((balance--));;
            '|'|'!'|'&') opset=1;;
        esac
        if [ ${balance} -lt 0 ]; then
            e=2
            break
        fi
    done

    if [ $e -eq 0 ]; then
        if [ ${balance} -ne 0 ]; then
            e=3
        fi

        if [ ${ch} != ')' ]; then
            e=4
        fi
    fi

    return $e
}