/* 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, UserFn, ValueType};
use std::env;

pub const QUOTE_DOCSTRING: &str = "takes a single unevaluated tree and returns it as it is: unevaluated.";

pub fn quote_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() > 1 {
        Err("do not quote more than one thing at a time".to_string())
    } else {
        Ok(*ast.car.clone())
    }
}

pub const EVAL_DOCSTRING: &str = "takes an unevaluated argument and evaluates it.
Specifically, does one pass of the tree simplification algorithm.
If you have a variable referencing another variable you will get the
referenced variable.";

pub fn eval_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() > 1 {
        Err("do not eval more than one thing at a time".to_string())
    } else {
        match *ast.car {
            Ctr::Seg(ref s) => Ok(*eval(s, syms)?.clone()),
            Ctr::Symbol(ref sym) => {
                let intermediate = syms.call_symbol(sym, &Seg::new(), true)?;
                if let Ctr::Seg(ref s) = *intermediate {
                    Ok(*eval(s, syms)?.clone())
                } else {
                    Ok(*intermediate)
                }
            },
            _ => Ok(*ast.car.clone())
        }
    }
}

pub const HELP_DOCSTRING: &str = "prints help text for a given symbol. Expects only one argument.";

pub fn help_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() != 1 {
        return Err("help only takes a single argument".to_string());
    }
    if let Ctr::Symbol(ref symbol) = *ast.car {
        if let Some(ref sym) = syms.get(symbol) {
            let args_str: String;
            if let ValueType::VarForm(_) = sym.value {
                args_str = "(its a variable)".to_string();
            } else {
                args_str = sym.args.to_string();
            }
            println!(
                "NAME: {0}\n
ARGS: {1}\n
DOCUMENTATION:\n
{2}\n
CURRENT VALUE AND/OR BODY:
{3}",
                sym.name, args_str, sym.docs, sym.value
            );
        } else {
            return Err("undefined symbol".to_string());
        }
    } else {
        return Err("help should only be called on a symbol".to_string());
    }

    Ok(Ctr::None)
}

pub const STORE_DOCSTRING: &str = "allows user to define functions and variables.
  A call may take one of three forms:
  1. variable declaration:
     Takes a name, doc string, and a value.
     (def myvar 'my special variable' 'my var value')
  2. function declaration:
     Takes a name, doc string, list of arguments, and one or more bodies to evaluate.
     Result of evaluating the final body is returned.
     (def myfunc 'does a thing' (myarg1 myarg2) (dothing myarg1 myarg2) (add myarg1 myarg2))
  3. symbol un-definition:
     Takes just a name. Removes variable from table.
     (def useless-var)";

pub fn store_callback(ast: &Seg, syms: &mut SymTable, env_cfg: bool) -> Result<Ctr, String> {
    let is_var = ast.len() == 3;
    if let Ctr::Symbol(ref identifier) = *ast.car {
        match &*ast.cdr {
            // define a symbol
            Ctr::Seg(doc_tree) => {
                if let Ctr::String(ref doc) = *doc_tree.car {
                    match &*doc_tree.cdr {
                        // define a variable
                        Ctr::Seg(data_tree) if is_var => {
                            let eval_arg: &Seg;
                            let outer_maybe_eval_seg: Seg;
                            let mut expand = false;
                            if let Ctr::Seg(ref eval_me) = *data_tree.car {
                                eval_arg = eval_me;
                            } else {
                                outer_maybe_eval_seg = Seg::from_mono(data_tree.car.clone());
                                eval_arg = &outer_maybe_eval_seg;
                                expand = true;
                            }
                            match eval(eval_arg, syms) {
                                Ok(ctr) => {
                                    let mut body = ctr;
                                    if expand {
                                        if let Ctr::Seg(ref s) = *body {
                                            body = s.car.clone();
                                        } else {
                                            return Err("impossible expansion".to_string())
                                        }
                                    }
                                    syms.insert(
                                        identifier.clone(),
                                        Symbol::from_ast(
                                            identifier, doc,
                                            &Seg::from_mono(body.clone()), None
                                        ),
                                    );
                                    if env_cfg {
                                        env::set_var(identifier.clone(), body.to_string());
                                    }
                                }
                                Err(e) => return Err(format!("couldnt eval symbol: {}", e)),
                            }
                        },

                        // define a function
                        Ctr::Seg(data_tree) if !is_var => {
                            if let Ctr::Seg(ref args) = *data_tree.car {
                                let mut arg_list = vec![];
                                if !args.circuit(&mut |c: &Ctr| -> bool {
                                    if let Ctr::Symbol(ref arg) = c {
                                        arg_list.push(arg.clone());
                                        true
                                    } else if let Ctr::None = c {
                                        // a user cannot type a None
                                        // this case represents no args
                                        true
                                    } else {
                                        false
                                    }
                                }) {
                                    return Err(
                                        "all arguments defined for function must be of type symbol"
                                            .to_string(),
                                    );
                                };

                                if let Ctr::Seg(ref bodies) = *data_tree.cdr {
                                    syms.insert(
                                        identifier.clone(),
                                        Symbol::from_ast(
                                            identifier, doc, bodies,
                                            Some(arg_list),
                                        ),
                                    );
                                } else {
                                    return Err(
                                        "expected one or more function bodies in function definition"
                                            .to_string(),
                                    );
                                }
                            } else {
                                return Err(
                                    "expected list of arguments in function definition".to_string()
                                );
                            }
                        }

                        // theres a name and a doc string but nothing else
                        _ => return Err("have name and doc string, but no body.".to_string()),
                    }
                } else {
                    return Err("doc string is a required argument".to_string());
                }
            }

            // undefine a symbol
            Ctr::None => {
                syms.remove(&identifier.to_string());
                if env_cfg {
                    env::remove_var(identifier);
                }
            }

            _ => return Err("arguments not in standard form".to_string()),
        }
    } else {
        return Err("first argument to export must be a symbol".to_string());
    }
    Ok(Ctr::None)
}

pub const ISSET_DOCSTRING: &str = "accepts a single argument: a symbol.
returns true or false according to whether or not the symbol is found in the symbol table.";

pub fn isset_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() != 1 {
        Err("help only takes a single argument".to_string())
    } else {
        if let Ctr::Symbol(ref symbol) = *ast.car {
            if let Some(_) = syms.get(symbol) {
                Ok(Ctr::Bool(true))
            } else {
                Ok(Ctr::Bool(false))
            }
        } else {
            Err("help should only be called on a symbol".to_string())
        }
    }
}

pub const ENV_DOCSTRING: &str = "takes no arguments
prints out all available symbols and their associated values";

pub fn env_callback(_ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    let mut functions = vec![];
    println!("VARIABLES:");
    for (name, val) in syms.iter() {
        match val.value {
            ValueType::VarForm(_) => {
                println!("  {}: {}", &name, val.value);
            }
            _ => functions.push(name.clone()),
        }
    }
    println!("FUNCTIONS:");
    for func in functions {
        println!("  {}", func);
    }
    Ok(Ctr::None)
}

pub const LAMBDA_DOCSTRING: &str = "Takes two arguments of any type.
No args are evaluated when lambda is called.
Lambda makes sure the first argument is a list of symbols (or 'arguments') to the lambda function.
The next arg is stored in a tree to evaluate on demand.

Example: (lambda (x y) (add x y))
This can then be evaluated like so:
((lambda (x y) (add x y)) 1 2)
which is functionally equivalent to:
(add 1 2)";

pub fn lambda_callback(
    ast: &Seg,
    _syms: &mut SymTable
) -> Result<Ctr, String> {
    let mut args = vec![];
    if let Ctr::Seg(ref arg_head) = *ast.car {
        if !arg_head.circuit(&mut |arg: &Ctr| -> bool {
            if let Ctr::Symbol(ref s) = *arg {
                args.push(s.clone());
                true
            } else if let Ctr::None = *arg {
                // no args case
                true
            } else {
                false
            }
        }) {
            Err("all elements of first argumnets must be symbols".to_string())
        } else {
            if let Ctr::Seg(ref eval_head) = *ast.cdr {
                if let Ctr::Seg(_) = *eval_head.car {
                    Ok(Ctr::Lambda(UserFn{
                        ast: Box::new(eval_head.clone()),
                        arg_syms: args,
                    }))
                } else {
                    Err("function body must be in list form".to_string())
                }
            } else {
                Err("not enough args".to_string())
            }
        }
    } else {
        Err("first argument should be a list of symbols".to_string())
    }
}