/* 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 std::rc::Rc;
use std::cell::RefCell;
use std::convert::TryInto;
use std::collections::HashMap;
use crate::segment::{Ctr, Type, circuit, list_len, list_idx, Ast};
use crate::vars::{VTable};
use crate::eval::eval;

pub type FTable = HashMap<String, Rc<RefCell<Function>>>;

// Standardized function signature for stdlib functions
pub type InternalOperation = fn(Ast, Rc<RefCell<VTable>>, Rc<RefCell<FTable>>) -> Ast;
pub struct ExternalOperation {
    // Un-evaluated abstract syntax tree
    // TODO: Intermediate evaluation to simplify branches with no argument in them
    //       Simplified branches must not have side effects.
    // TODO: Apply Memoization?
    ast: Ast,
    // list of argument string tokens
    arg_syms: Vec<String>
}

/* A stored function may either be a pointer to a function
 * or a syntax tree to eval with the arguments
 */
pub enum Operation {
    Internal(InternalOperation),
    External(ExternalOperation)
}

/* Function Args
 * If Lazy, is an integer denoting number of args
 * If Strict, is a list of type tags denoting argument type.
 */
pub enum Args {
    // signed: -1 denotes infinite args
    Lazy(i128),
    Strict(Vec<Type>)
}

// function which does not need args checked
pub struct Function {
    pub function: Operation,
    pub name: String,
    pub args: Args,

    // dont fail on undefined symbol (passed to eval)
    pub loose_syms: bool,

    // dont evaluate args at all. leave that to the function
    pub eval_lazy: bool
}

/* call
 * routine is called by eval when a function call is detected
 */
pub fn func_call(
    function: Rc<RefCell<Function>>,
    args: Ast,
    vars: Rc<RefCell<VTable>>,
    funcs: Rc<RefCell<FTable>>
) -> Result<Ast, String> {
    let called_func = function.borrow_mut();
    let mut n_args: Ast = args.clone();
    if !called_func.eval_lazy {
        match eval(args, vars.clone(), funcs.clone(), called_func.loose_syms) {
            Ok(rc_seg) => n_args = rc_seg.clone(),
            Err(s) => return Err(
                format!(
                    "error evaluating args to {}: {}",
                    called_func.name,
                    s
                )
            )
        }
    }

    match &called_func.args {
        Args::Lazy(num) => {
            if *num < 0 {
            }

            if !(*num == (list_len(n_args.clone()) as i128 - 1)) {
                return Err(format!("expected {} args in call to {}", num, called_func.name))
            }
        },

        Args::Strict(arg_types) => {
            let mut idx: usize = 0;
            let passes = circuit(n_args.clone(), &mut |c: &Ctr| {
                if idx >= arg_types.len() {
                    return false;
                }

                if let Ctr::None = c {
                    return false;
                }

                let ret = arg_types[idx] == c.to_type();
                if ret {
                    idx += 1;
                }
                return ret;
            });

            if passes && idx < (arg_types.len() - 1) {
                return Err(format!(
                    "{} too little arguments in call to {}",
                    arg_types.len() - (idx + 1),
                    called_func.name
                ));
            }

            if !passes {
                if idx < (arg_types.len() - 1) {
                    return Err(format!(
                        "argument {} in call to {} is of wrong type (expected {})",
                        idx + 1,
                        called_func.name,
                        arg_types[idx].to_str()
                    ));
                }

                if idx == (arg_types.len() - 1) {
                    return Err(format!(
                        "too many arguments in call to {}",
                        called_func.name
                    ));
                }
            }
        }
    }

    match &called_func.function {
        Operation::Internal(f) => Ok((f)(n_args, vars, funcs)),
        Operation::External(f) => {
            let mut temp = vars.borrow().clone();
            for n in 0..f.arg_syms.len() {
                temp.insert(
                    f.arg_syms[n].clone(),
                    Rc::new(list_idx(n_args.clone(), n as u128))
                );
            }

            eval(f.ast.clone(), Rc::new(RefCell::new(temp)), funcs, called_func.loose_syms)
        }
    }
}

pub fn func_declare(
    ft: Rc<RefCell<FTable>>,
    f: Rc<RefCell<Function>>
) -> Option<String> {
    let func = f.borrow();
    let name = func.name.clone();
    if let Operation::External(fun) = &func.function {
        if let Args::Lazy(i) = func.args {
            if fun.arg_syms.len() != i.try_into().unwrap() {
                return Some(
                    "external function must have lazy args equal to declared arg_syms length"
                        .to_string()
                );
            }
        } else {
            return Some(
                "external function must have lazy args"
                    .to_string()
            );
        }
    }

    drop(func);
    ft.borrow_mut().insert(name, f);
    None
}