badvpn/lime/parse_engine.php

<?php
/*
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU Library General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */


define('LIME_CALL_PROTOCOL', '$tokens, &$result');

abstract class lime_parser {
}

class parse_error extends Exception {}	# If this happens, the input doesn't match the grammar.
class parse_bug extends Exception {}	# If this happens, I made a mistake.

class parse_unexpected_token extends parse_error {
	function __construct($type, $state) {
		parent::__construct("Unexpected token of type ($type)");
		$this->type = $type;
		$this->state = $state;
	}
}
class parse_premature_eof extends parse_error {
	function __construct() {
		parent::__construct("Premature EOF");
	}
}


class parse_stack {
	function __construct($qi) {
		$this->q = $qi;
		$this->qs = array();
		$this->ss = array();
	}
	function shift($q, $semantic) {
		$this->ss[] = $semantic;
		$this->qs[] = $this->q;
		$this->q = $q;
		# echo "Shift $q -- $semantic<br/>\n";
	}
	function top_n($n) {
		if (!$n) return array();
		return array_slice($this->ss, 0-$n);
	}
	function pop_n($n) {
		if (!$n) return array();
		$qq = array_splice($this->qs, 0-$n);
		$this->q = $qq[0];
		return array_splice($this->ss, 0-$n);
	}
	function occupied() { return !empty($this->ss); }
	function index($n) {
		if ($n) $this->q = $this->qs[count($this->qs)-$n];
	}
	function text() {
		return $this->q." : ".implode(' . ', array_reverse($this->qs));
	}
}
class parse_engine {
	function __construct($parser) {
		$this->parser = $parser;
		$this->qi = $parser->qi;
		$this->rule = $parser->a;
		$this->step = $parser->i;
		#$this->prepare_callables();
		$this->reset();
		#$this->debug = false;
	}
	function reset() {
		$this->accept = false;
		$this->stack = new parse_stack($this->qi);
	}
	private function enter_error_tolerant_state() {
		while ($this->stack->occupied()) {
			if ($this->has_step_for('error')) return true;
			$this->drop();
		};
		return false;
	}
	private function drop() { $this->stack->pop_n(1); }
	function eat_eof() {
		{/*
		
		So that I don't get any brilliant misguided ideas:
		
		The "accept" step happens when we try to eat a start symbol.
		That happens because the reductions up the stack at the end
		finally (and symetrically) tell the parser to eat a symbol
		representing what they've just shifted off the end of the stack
		and reduced. However, that doesn't put the parser into any
		special different state. Therefore, it's back at the start
		state.
		
		That being said, the parser is ready to reduce an EOF to the
		empty program, if given a grammar that allows them.
		
		So anyway, if you literally tell the parser to eat an EOF
		symbol, then after it's done reducing and accepting the prior
		program, it's going to think it has another symbol to deal with.
		That is the EOF symbol, which means to reduce the empty program,
		accept it, and then continue trying to eat the terminal EOF.
		
		This infinte loop quickly runs out of memory.
		
		That's why the real EOF algorithm doesn't try to pretend that
		EOF is a terminal. Like the invented start symbol, it's special.
		
		Instead, we pretend to want to eat EOF, but never actually
		try to get it into the parse stack. (It won't fit.) In short,
		we look up what reduction is indicated at each step in the
		process of rolling up the parse stack.
		
		The repetition is because one reduction is not guaranteed to
		cascade into another and clean up the entire parse stack.
		Rather, it will instead shift each partial production as it
		is forced to completion by the EOF lookahead.
		*/}
		
		# We must reduce as if having read the EOF symbol
		do {
			# and we have to try at least once, because if nothing
			# has ever been shifted, then the stack will be empty
			# at the start.
			list($opcode, $operand) = $this->step_for('#');
			switch ($opcode) {
				case 'r': $this->reduce($operand); break;
				case 'e': $this->premature_eof(); break;
				default: throw new parse_bug(); break;
			}
		} while ($this->stack->occupied());
		{/*
		If the sentence is well-formed according to the grammar, then
		this will eventually result in eating a start symbol, which
		causes the "accept" instruction to fire. Otherwise, the
		step('#') method will indicate an error in the syntax, which
		here means a premature EOF.
		
		Incedentally, some tremendous amount of voodoo with the parse
		stack might help find the beginning of some unfinished
		production that the sentence was cut off during, but as a
		general rule that would require deeper knowledge.
		*/}
		if (!$this->accept) throw new parse_bug();
		return $this->semantic;
	}
	private function premature_eof() {
		$seen = array();
		while ($this->enter_error_tolerant_state()) {
			if (isset($seen[$this->state()])) {
				// This means that it's pointless to try here.
				// We're guaranteed that the stack is occupied.
				$this->drop();
				continue;
			}
			$seen[$this->state()] = true;
			
			$this->eat('error', NULL);
			if ($this->has_step_for('#')) {
				// Good. We can continue as normal.
				return;
			} else {
				// That attempt to resolve the error condition
				// did not work. There's no point trying to
				// figure out how much to slice off the stack.
				// The rest of the algorithm will make it happen.
			}
		}
		throw new parse_premature_eof();
	}
	private function current_row() { return $this->step[$this->state()]; }
	private function step_for($type) {
		$row = $this->current_row();
		if (!isset($row[$type])) return array('e', $this->stack->q);
		return explode(' ', $row[$type]);
	}
	private function has_step_for($type) {
		$row = $this->current_row();
		return isset($row[$type]);
	}
	private function state() { return $this->stack->q; }
	function eat($type, $semantic) {
		# assert('$type == trim($type)');
		# if ($this->debug) echo "Trying to eat a ($type)\n";
		list($opcode, $operand) = $this->step_for($type);
		switch ($opcode) {
			case 's':
			# if ($this->debug) echo "shift $type to state $operand\n";
			$this->stack->shift($operand, $semantic);
			# echo $this->stack->text()." shift $type<br/>\n";
			break;
			
			case 'r':
			$this->reduce($operand);
			$this->eat($type, $semantic);
			# Yes, this is tail-recursive. It's also the simplest way.
			break;
			
			case 'a':
			if ($this->stack->occupied()) throw new parse_bug('Accept should happen with empty stack.');
			$this->accept = true;
			#if ($this->debug) echo ("Accept\n\n");
			$this->semantic = $semantic;
			break;
			
			case 'e':
			# This is thought to be the uncommon, exceptional path, so
			# it's OK that this algorithm will cause the stack to
			# flutter while the parse engine waits for an edible token.
			# if ($this->debug) echo "($type) causes a problem.\n";
			if ($this->enter_error_tolerant_state()) {
				$this->eat('error', NULL);
				if ($this->has_step_for($type)) $this->eat($type, $semantic);
			} else {
				# If that didn't work, give up:
				throw new parse_error("Parse Error: ($type)($semantic) not expected");
			}
			break;
			
			default:
			throw new parse_bug("Bad parse table instruction ".htmlspecialchars($opcode));
		}
	}
	private function reduce($rule_id) {
		$rule = $this->rule[$rule_id];
		$len = $rule['len'];
		$semantic = $this->perform_action($rule_id, $this->stack->top_n($len));
		#echo $semantic.br();
		if ($rule['replace']) $this->stack->pop_n($len);
		else $this->stack->index($len);
		$this->eat($rule['symbol'], $semantic);
	}
	private function perform_action($rule_id, $slice) {
		# we have this weird calling convention....
		$result = null;
		$method = $this->parser->method[$rule_id];
		#if ($this->debug) echo "rule $id: $method\n";
		$this->parser->$method($slice, $result);
		return $result;
	}
}