/* 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::eval::eval;
use crate::segment::{Ctr, Seg};
use crate::sym::{SymTable, Symbol, ValueType, Args};

pub fn if_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    let cond: bool;
    match *ast.car {
        Ctr::Seg(ref cond_form) => {
            if let Ctr::Bool(cond_from_eval) = *eval(cond_form, syms)? {
                cond = cond_from_eval;
            } else {
                return Err("first argument to if must evaluate to be a boolean".to_string());
            }
        }

        Ctr::Bool(cond_from_car) => cond = cond_from_car,
        _ => return Err("first argument to if must evaluate to be a boolean".to_string()),
    }

    let then_form: &Seg;
    if let Ctr::Seg(ref s) = *ast.cdr {
        then_form = s;
    } else {
        return Err("impossible condition: not enough args to if".to_string());
    }

    if cond {
        // then
        match *then_form.car {
            Ctr::Seg(ref first_arg) => Ok(*eval(first_arg, syms)?),
            _ => {
                let eval_tree = &Seg::from_mono(then_form.car.clone());
                let eval_res = *eval(eval_tree, syms)?;
                if let Ctr::Seg(ref s) = eval_res {
                    Ok(*s.car.clone())
                } else {
                    Err("impossible condition: list evals to non list".to_string())
                }
            }
        }
    } else {
        // else
        if let Ctr::Seg(ref else_form) = *then_form.cdr {
            match *else_form.car {
                Ctr::Seg(ref second_arg) => Ok(*eval(second_arg, syms)?),
                _ => {
                    let eval_tree = &Seg::from_mono(else_form.car.clone());
                    let eval_res = *eval(eval_tree, syms)?;
                    if let Ctr::Seg(ref s) = eval_res {
                        Ok(*s.car.clone())
                    } else {
                        Err("impossible condition: list evals to non list".to_string())
                    }
                }
            }
        } else {
            Err("impossible condition: args not in standard form".to_string())
        }
    }
}

pub fn let_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    let mut localsyms = syms.clone();
    let locals_form: &Seg;
    let eval_forms: &Seg;
    if let Ctr::Seg(ref locals_form_list) = *ast.car {
        locals_form = locals_form_list;
    } else {
        return Err("first element of let form must contain local variables".to_string())
    }

    if let Ctr::Seg(ref eval_forms_head) = *ast.cdr {
        eval_forms = eval_forms_head;
    } else {
        return Err("let form should contain one or more elements to evaluate".to_string())
    }

    // process locals forms
    if !locals_form.circuit(&mut |var_decl: &Ctr| -> bool {
        if let Ctr::Seg(ref var_form) = *var_decl {
            if let Ctr::Symbol(ref name) = *var_form.car {
                if let Ctr::Seg(ref var_val_form) = *var_form.cdr {
                    let var_val_res: Result<Box<Ctr>, String>;
                    if let Ctr::Seg(ref val_form) = *var_val_form.car {
                        var_val_res = eval(val_form, &mut localsyms);
                    } else {
                        let var_tree = Seg::from_mono(Box::new(*var_val_form.car.clone()));
                        let intermediate = eval(&var_tree, &mut localsyms);
                        if intermediate.is_err() {
                            var_val_res = intermediate;
                        } else if let Ctr::Seg(ref intermediate_result) = *intermediate.unwrap() {
                            var_val_res = Ok(intermediate_result.car.clone())
                        } else {
                            panic!()
                        }
                    }
                    if let Err(e) = var_val_res {
                        eprintln!("failed to evaluate definition of {}: {}", name, e);
                        return false
                    }

                    localsyms.insert(name.clone(), Symbol {
                        name: name.clone(),
                        args: Args::None,
                        conditional_branches: false,
                        value: ValueType::VarForm(Box::new(*var_val_res.unwrap().clone())),
                    });
                }
            } else {
                eprintln!("improper declaration of {}: not a list", var_decl);
                return false
            }
        } else {
            eprintln!("improper declaration form: {}", var_decl);
            return false
        }
        true
    }) {
        return Err("local variable declaration failure".to_string())
    }

    let mut result: Box<Ctr> = Box::new(Ctr::None);
    if !eval_forms.circuit(&mut |eval_form: &Ctr| -> bool {
        let res: Result<Box<Ctr>, String>;
        if let Ctr::Seg(ref eval_tree) = eval_form {
            res = eval(&eval_tree, &mut localsyms);
        } else {
            let eval_tree = Seg::from_mono(Box::new(eval_form.clone()));
            let intermediate = eval(&eval_tree, &mut localsyms);
            if intermediate.is_err() {
                res = intermediate;
            } else if let Ctr::Seg(ref intermediate_result) = *intermediate.unwrap() {
                res = Ok(intermediate_result.car.clone())
            } else {
                panic!()
            }
        }

        if let Err(e) = res {
            eprintln!("error evaluating let form: {}", e);
            return false
        }

        result = res.unwrap().clone();
        true
    }) {
        return Err("evaluation failure".to_string())
    }

    Ok((*result).clone())
}

pub fn while_callback(_ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
    todo!()
}

pub fn circuit_callback(_ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
    todo!()
}