/* relish: versatile lisp shell
 * Copyright (C) 2021 Aidan Hahn
 *
 * 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 3 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

use crate::sym::SymTable;
use crate::segment::{Seg, Ctr};

/* iterates over a syntax tree
 * returns a NEW LIST of values
 * representing the simplest possible form of the input
 */
pub fn eval (
    ast: &Seg,
    syms: &mut SymTable,
) -> Result<Box<Ctr>, String> {
    // data to return
    let mut ret = Box::from(Ctr::None);
    let mut first = true;

    // to be assigned from cloned/evaled data
    let mut car;
    let mut cdr = Box::from(Ctr::None);

    // lets me redirect the input
    let mut arg_car = &ast.car;
    let mut arg_cdr = &ast.cdr;

    // iterate over ast and build out ret
    let mut none = false;
    while !none {
        match &**arg_car {
            Ctr::Seg(ref inner) => {
                match eval(inner, syms) {
                    Ok(res) => car = res,
                    Err(e) => return Err(format!("evaluation error: {}", e)),
                }
            }

            Ctr::Symbol(ref tok) => {
                let outer_scope_seg_holder: Seg;
                let args: &Seg;
                if let Ctr::Seg(ref candidate_args) = **arg_cdr {
                    args = candidate_args;
                } else {
                    outer_scope_seg_holder = Seg::from_mono(arg_cdr.clone());
                    args = &outer_scope_seg_holder;
                }

                match syms.call_symbol(tok, args, first) {
                    Ok(s) => car = s,
                    Err(s) => return Err(format!("error in call to {}: {}", tok, s)),
                }
            }

            _ => {
                car = arg_car.clone();
            }
        }

        match **arg_cdr {
            Ctr::Symbol(ref tok) => {
                let blank = Seg::new();
                match syms.call_symbol(tok, &blank, false) {
                    Ok(res) => cdr = res,
                    Err(s) => return Err(format!("error fetching {}: {}", tok, s)),
                }
                none = true;
            }

            Ctr::Seg(ref next) => {
                if let Ctr::None = *ret {
                    *ret = Ctr::Seg(Seg::from(car, cdr.clone()))
                } else if let Ctr::Seg(ref mut s) = *ret {
                    s.append(Box::from(Ctr::Seg(Seg::from(car, cdr.clone()))))
                }
                arg_car = &next.car;
                arg_cdr = &next.cdr
            }

            _ => {
                cdr = ast.cdr.clone();
                none = true;
            }
        }

        if let Ctr::None = **arg_car {
            if let Ctr::None = **arg_cdr {
                none = true;
            }
        }

        first = false;
    }

    Ok(ret)
}