/**
* Parses a postfix expression read from the specified input stream
* and constructs the corresponding expression tree.
* The postfix expression consists of one-letter symbols,
* one-digit numbers, +, -, * and /.
*
* @param resource $in The input stream.
* @return object ExpressionTree An expression tree.
*/
public static function parsePostfix($in)
{
$stack = new StackAsLinkedList();
while (($c = fgetc($in)) != false) {
if (ord('0') <= ord($c) && ord($c) <= ord('9') || ord('a') <= ord($c) && ord($c) <= ord('z') || ord('A') <= ord($c) && ord($c) <= ord('Z')) {
$stack->push(new ExpressionTree($c));
} elseif ($c == '+' || $c == '-' || $c == '*' || $c == '/') {
$result = new ExpressionTree($c);
$result->attachRight($stack->pop());
$result->attachLeft($stack->pop());
$stack->push($result);
}
}
return $stack->pop();
}
/**
* A reverse-polish calculator.
* Reads RPN expression from the input stream
* and writes the computed results on the output stream.
* Uses a stack.
* Recognizes single-digit numbers, * +, * and =.
*
* @param resource $in The input stream.
* @param resource $out The output stream.
*/
public function calculator($in, $out)
{
$stack = new StackAsLinkedList();
while (($c = fgetc($in)) !== false) {
if (ord($c) >= ord('0') && ord($c) <= ord('9')) {
$stack->push(box(ord($c) - ord('0')));
} elseif ($c == '+') {
$arg2 = $stack->pop();
$arg1 = $stack->pop();
$stack->push(box($arg1->getValue() + $arg2->getValue()));
} elseif ($c == '*') {
$arg2 = $stack->pop();
$arg1 = $stack->pop();
$stack->push(box($arg1->getValue() * $arg2->getValue()));
} elseif ($c == '=') {
$arg = $stack->pop();
fprintf($out, "%s\n", str($arg));
}
}
}