/* 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::{Seg, Ctr, Type};
use crate::vars::VTable;
use std::collections::HashMap;
use std::convert::TryInto;

pub struct FTable<'a>(HashMap<String, Box<Function<'a>>>);

impl<'a> FTable<'a> {
    pub fn declare(&mut self, func: Box<Function<'a>>) -> Option<String> {
        if let Operation::External(ref fun) = &func.function {
            if let Args::Lazy(ref i) = func.args {
                if fun.arg_syms.len() != i.clone().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());
            }
        }

        self.0.insert(func.name, func);
        None
    }

    pub fn get(&self, id: String) -> Option<&Box<Function<'a>>> {
        self.0.get(&id)
    }

    pub fn remove(&self, id: String) {
        self.0.remove(&id);
    }

    pub fn new() -> FTable<'a> {
        FTable{0: HashMap::<String, Box<Function>>::new()}
    }
}


// Standardized function signature for stdlib functions
//pub type InternalOperation = impl Fn(Box<Seg>, Box<VTable>, Box<FTable>) -> Box<Ctr>;

#[derive(Debug)]
pub struct ExternalOperation<'a> {
    // 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?
    pub ast: Box<Seg<'a>>,
    // list of argument string tokens
    pub 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<'a> {
    Internal(Box<dyn Fn(&'a Seg, &'a mut VTable, &'a mut FTable) -> Ctr<'a>>),
    External(ExternalOperation<'a>),
}

/* 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>),
}

pub struct Function<'a> {
    pub function: Operation<'a>,
    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<'a> Function<'a> {
    /* call
     * routine is called by eval when a function call is detected
     */
    pub fn func_call(
        &self,
        args: &'a Seg<'a>,
        vars: &'a mut VTable<'a>,
        funcs: &'a mut FTable<'a>,
    ) -> Result<Box<Ctr<'a>>, String> {
        let mut evaluated_args = args;
        if !self.eval_lazy {
            match eval(args, vars, funcs, self.loose_syms) {
                Ok(arg_data) => {
                    if let Ctr::Seg(ast) = *arg_data {
                        evaluated_args = &ast;
                    } else {
                        return Err("Panicking: eval returned not a list for function args.".to_string());
                    }
                }
                Err(s) => {
                    return Err(format!(
                        "error evaluating args to {}: {}",
                        self.name, s
                    ))
                }
            }
        }

        match self.args {
            Args::Lazy(ref num) => {
                let called_arg_count = evaluated_args.len() as i128;
                if *num == 0 {
                    if let Ctr::None = *evaluated_args.car {
                        //pass
                    } else {
                        return Err(format!(
                            "expected 0 args in call to {}. Got one or more.",
                            self.name,
                        ));
                    }
                } else if *num > -1 && (*num != called_arg_count) {
                    return Err(format!(
                        "expected {} args in call to {}. Got {}.",
                        num, self.name, called_arg_count
                    ));
                }
            }

            Args::Strict(ref arg_types) => {
                let mut idx: usize = 0;
                let passes = evaluated_args.circuit(&mut |c: &Ctr| -> bool {
                    if idx >= arg_types.len() {
                        return false;
                    }

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

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

                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_string()
                        ));
                    }

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

        /* corecursive with eval.
         * essentially calls eval on each body in the function.
         * result of the final body is returned.
         */
        match &self.function {
            Operation::Internal(ref f) => return Ok(Box::new(f(evaluated_args, vars, funcs))),
            Operation::External(ref f) => {
                for n in 0..f.arg_syms.len() {
                    let iter_arg = evaluated_args[n];
                    vars.insert(f.arg_syms[n].clone(), Box::new(iter_arg));
                }

                let mut result: Box<Ctr>;
                let iterate = &*(f.ast);
                loop {
                    if let Ctr::Seg(ref data) = *iterate.car {
                        match eval(data, vars, funcs, self.loose_syms) {
                            Ok(ctr) => result = ctr,
                            Err(e) => return Err(e),
                        }
                    } else {
                        panic!("function body not in standard form!")
                    }

                    match *iterate.cdr {
                        Ctr::Seg(ref next) => iterate = next,
                        Ctr::None => break,
                        _ => panic!("function body not in standard form!"),
                    }
                }
                for n in 0..f.arg_syms.len() {
                    vars.remove(f.arg_syms[n]);
                }

                return Ok(result);
            }
        }
    }
}