/*
 * Copyright 2022 Arnaud Golfouse
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

use ielr::{
    input::{Grammar, Node, Token},
    output::{Action, Lookahead, Table},
};

pub struct Runtime<'a> {
    pub table: Table,
    pub grammar: &'a Grammar,
}

impl Runtime<'_> {
    pub fn is_valid(&self, start: Node, tokens: &[Token]) -> bool {
        let mut stack = Vec::new();
        let start_state = self.table.get_starting_state(start).unwrap();
        stack.push(start_state);
        let mut tokens = tokens.iter().copied().peekable();
        loop {
            let top = *stack.last().unwrap();
            let state = &self.table.states[top];
            let lookahead = match tokens.peek() {
                Some(token) => Lookahead::Token(*token),
                None => Lookahead::Eof,
            };
            match state.action_table().get_action(lookahead) {
                Some(Action::Shift(to_state)) => {
                    tokens.next();
                    stack.push(to_state);
                }
                Some(Action::Reduce(production)) => {
                    let rhs = self.grammar.get_rhs(production).unwrap();
                    for _ in 0..rhs.len() {
                        stack.pop();
                    }
                    let top = *stack.last().unwrap();
                    let state = &self.table.states[top];
                    match state.goto_table().get_goto(production.lhs) {
                        Some(to_state) => {
                            stack.push(to_state);
                        }
                        None => return false,
                    }
                }
                Some(Action::Accept(node)) => {
                    if node == start {
                        break;
                    } else {
                        return false;
                    }
                }
                None => return false,
            }
        }
        stack.len() == 2
    }
}