/* 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::boxed::Box;
use std::convert::TryInto;
use std::collections::HashMap;
use crate::cell::{Ctr, Cell, Type};
use crate::vars::{VTable};
use crate::eval::eval;

pub type FTable = HashMap<String, Box<Function>>;

// Standardized function signature for stdlib functions
pub type InternalOperation = fn(&Box<Cell>, &mut Box<VTable>, &mut Box<FTable>) -> Box<Cell>;
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: Box<Cell>,
    // 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
}

impl Function {
    /* call
     * routine is called by eval when a function call is detected
     */
    pub fn call(
        &self,
        args: &Box<Cell>,
        vars: &mut Box<VTable>,
        funcs: &mut Box<FTable>
    ) -> Result<Box<Cell>, String> {
        let n_args: &Box<Cell>;
        let outer_owner: Box<Cell>;
        if !self.eval_lazy {
            match eval(args, vars, funcs, self.loose_syms) {
                Ok(box_cell) => outer_owner = box_cell,
                Err(s) => return Err(
                    format!(
                        "error evaluating args to {}: {}",
                        self.name,
                        s
                    )
                )
            }
            n_args = &outer_owner;
        } else {
            n_args = args;
        }

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

                if !(*num == (n_args.len() - 1)) {
                    return Err(format!("expected {} args in call to {}", num, self.name))
                }
            },

            Args::Strict(arg_types) => {
                let mut idx: usize = 0;
                let passes = n_args.circuit(&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),
                        self.name
                    ));
                }

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

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

        match &self.function {
            Operation::Internal(f) => Ok((f)(&n_args, vars, funcs)),
            Operation::External(f) => {
                // copy var table and add args
                let mut temp = vars.clone();
                for n in 0..f.arg_syms.len() {
                    temp.insert(
                        f.arg_syms[n].clone(),
                        Box::new(n_args.index(n))
                    );
                }
                eval(&f.ast, &temp, funcs, self.loose_syms)
            }
        }
    }
}

pub fn declare(
    ft: &mut Box<FTable>,
    f: Box<Function>
) -> Option<String> {
    if let Operation::External(fun) = &f.function {
        if let Args::Lazy(i) = f.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()
            );
        }
    }

    ft.insert(f.name.clone(), f);
    None
}