PHP——表达式求值

表达式求值

设置一个操作数栈，设置一个操作符栈
分析字符串，操作数和操作符分别入栈，如果当前操作符优先级高于栈尾操作符优先级，则先计算栈尾操作符，再把当前操作符号入栈
最后把两个栈依次出栈

class StrToValue
{
    private static $operatorLevel = [
        '**' => 0,
        '++' => 1,
        '--' => 1,
        '~' => 1,
        '*' => 2,
        '/' => 2,
        '%' => 2,
        '+' => 3,
        '-' => 3,
        '<<' => 4,
        '>>' => 4,
        '&' => 5,
        '^' => 6,
        '|' => 7,
        '=' => 8,
        '+=' => 8,
        '-=' => 8,
        '*=' => 8,
        '**=' => 8,
        '/=' => 8,
        '.=' => 8,
        '%=' => 8,
        '&=' => 8,
        '|=' => 8,
        '^=' => 8,
        '<<=' => 8,
        '>>=' => 8,
    ];

    private static $full = [
        '~',
        '*',
        '/',
        '%',
        '+',
        '-',
    ];

    private static $nums = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];

    /**
     * @param $str
     * @return array
     * @throws Exception
     */
    public static function calculate($str)
    {
        $len = strlen($str);
        $numStack = $operatorStack = [];

        $num = 0;
        $operator = '';

        $isNum = $isOperator = false;
        $operators = array_keys(self::$operatorLevel);
        // 分析字符串入栈
        for ($i = 0; $i < $len; $i++) {
            if ($str[$i] === ' ') {
                continue;
            }
            if (in_array($str[$i], self::$nums, true)) {
                if ($isNum) {
                    $num .= $str[$i];
                } else {
                    $num = $str[$i];
                    if (count($operatorStack) > 0) {
                        $op = array_pop($operatorStack);
                        if (self::$operatorLevel[$op] > self::$operatorLevel[$operator]) {
                            $operatorStack[] = $op;
                            $operatorStack[] = $operator;
                        } else if (count($numStack) > 0) {
                            $right = array_pop($numStack);
                            $left = array_pop($numStack);
                            $aaa = 'return ' . ($left ?? '') . $op . $right . ';';
                            $numStack[] = eval($aaa);
                            $operatorStack[] = $operator;
                        }
                    } else if ($isOperator !== false) {
                        $operatorStack[] = $operator;
                    }
                }
                $isNum = true;
                $isOperator = false;
            } else if (in_array($str[$i], $operators, true)) {
                if ($isOperator) {
                    $operator .= $str[$i];
                } else {
                    $operator = $str[$i];
                    if ($isNum === false) {
                        $numStack[] = !in_array($operator, self::$full, true) ? '' : 0;
                    } else {
                        $numStack[] = $num;
                    }
                }

                $isNum = false;
                $isOperator = true;
            } else {
                throw  new Exception($str . '包含无效的字符' . $str[$i]);
            }
        }
        if ($isNum) {
            $numStack[] = $num;
        }
        if ($isOperator) {
            $operatorStack[] = $operator;
        }
        // 计算结果（出栈）
        while (!empty($operatorStack)) {
            $right = array_pop($numStack);
            $left = array_pop($numStack);
            $op = array_pop($operatorStack);
            $numStack[] = eval('return ' . $left . $op . $right . ';');
        }
        return $numStack[0];
    }
}