<?php


/**
 * Return answer, operator precedence must be retained. / * - + is the order.
 * @param unknown $s
 */
function calc($sum){

    // No requirement for handling parenthesis here,
    // but I'll do it anyway because I like recursion :)
    $sIX=0;
    $moreBrackets=true;
    // Always work out sums in brackets first, from deepest to most shallow
    while($moreBrackets){
        $l=strlen($sum);
        $moreBrackets=false;
        for($n=0;$n<$l;$n++){
            if($sum{$n}=='('){
                $moreBrackets=true;
                $sIX=$n;
            }
            if($sum{$n}==')'){
                if(!$moreBrackets){
                    return "Error, no opening bracket for closing bracket!";
                }
                // Could obviously combine these 3 lines into 1,
                // but I'll pay lip service to readability...
                $subSum=substr($sum,$sIX+1,$n-$sIX-1);
                $newSum=substr($sum,0,$sIX).calc($subSum).substr($sum,$n+1);
                $sum=$newSum;
                break;
            }
        }
    }

    $ops="/*-+";
    $result=0;
    $gotOperator=false;
    if($sum{0}=='-')
        $sum{0}='N';

    for($n=0;$n<strlen($ops);$n++)
        $sum=str_replace($ops{$n}.'-',$ops{$n}."N",$sum);

    for($n=0;$n<strlen($ops);$n++){
        $op=$ops{$n};
        $ix=1;
        while($ix!==false){
            $ix=strpos($sum,$op);
            $len=strlen($sum);
            if($ix!==false){
                $gotOperator=true;
                if($ix==0||$ix==$len-1){
                    return "Error, operator not enclosed by digits";
                }
                $s=$ix-1;
                $e=$ix+1;

                // Find start of lhs value
                while($s>-1 && (($sum{$s}>='0' && $sum{$s}<='9') || ($sum{$s}=='.' || $sum{$s}=='N')))
                    --$s;
                // Find end of rhs value
                while($e<$len && (($sum{$e}>='0' && $sum{$e}<='9') || ($sum{$e}=='.' || $sum{$e}=='N')))
                    ++$e;
                ++$s;
                --$e;

                // Check for negative values and decimals with no leading digit
                $v0=substr($sum,$s,$ix-$s);
                if($v0{0}=='.')
                    $v0='0'.$v0;
                else if($v0{0}=='N')
                    $v0{0}='-';
                $v1=substr($sum,$ix+1,$e-$ix);
                if($v1{0}=='.')
                    $v1='0'.$v1;
                else if($v1{0}=='N')
                    $v1{0}='-';

                switch($op){
                    case '/':
                        $result=$v0/$v1;
                        break;
                    case '*':
                        $result=$v0*$v1;
                        break;
                    case '-':
                        $result=$v0-$v1;
                        break;
                    case '+':
                        $result=$v0+$v1;
                        break;
                }
                if($result<0)
                    $result='N'.(-$result);
                $newSum=substr($sum,0,$s).$result.substr($sum,$ix+1+($e-$ix));
                $sum=$newSum;
            }
        }
    }
    if($sum{0}=='N')
        $sum{0}='-';
    return $sum;
}

header('Content-type:text/plain');

// Should print 100
print calc("10*10")."\n";

// Should print 19
print calc("20+-1")."\n";

// Should print 3
print calc("3*3/3")."\n";

// Should print -1180
print calc("-100*23/2+-30")."\n";

// Should print 8
print calc("2+3*2")."\n";

// Should print 6
print calc("2+2+2")."\n";

/** Bonus points :) **/

// Should print 25
print calc("5*(2+3)")."\n";

// Should print 2
print calc("((10/5)+(4/2))/(1+1+1-1)")."\n";