/* 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        Err("do not quote more than one thing at a time".to_string())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())
        }

        /* this bit removed because it was determined eval shouldnt do things twice
         * kept here for reference purposes since I have gone back and forth on this
         * a bit
         *
         * thanks you for your patience (ava)

        match arguments {
            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(arguments)
        }*/
    }
}

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 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())
    }
}

pub const GETDOC_DOCSTRING: &str = "accepts an unevaluated symbol, returns the doc string.
Returns an error if symbol is undefined";

pub fn getdoc_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() != 1 {
        Err("get-doc only takes a single argument".to_string())
    } else {
        if let Ctr::Symbol(ref symbol) = *ast.car {
            if let Some(sym) = syms.get(symbol) {
                Ok(Ctr::String(sym.docs.clone()))
            } else {
                Err("undefined symbol".to_string())
            }

        } else {
            Err("get-doc should only be called on a symbol".to_string())
        }
    }
}

pub const SETDOC_DOCSTRING: &str = "accepts a symbol and a doc string.
Returns an error if symbol is undefined, otherwise sets the symbols docstring to the argument.";

pub fn setdoc_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
    if ast.len() != 2 {
        Err("set-doc only takes two arguments".to_string())
    } else {
        if let Ctr::Symbol(ref symbol) = *ast.car {
            if let Some(mut sym) = syms.remove(symbol) {
                if let Ctr::Seg(ref doc_node) = *ast.cdr {
                    if let Ctr::String(ref doc) = *doc_node.car {
                        sym.docs = doc.clone();
                        syms.insert(sym.name.clone(), sym);
                        Ok(Ctr::None)
                    } else {
                        syms.insert(sym.name.clone(), sym);
                        Err("second arg must be a string".to_string())
                    }
                } else {
                    Err("impossible: not a second arg".to_string())
                }
            } else {
                Err("undefined symbol".to_string())
            }

        } else {
            Err("first argument must be a symbol".to_string())
        }
    }
}

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)

Additionally, passing a tree as a name will trigger def to evaluate the tree and try to derive
a value from it. If it does not return a ";


pub fn store_callback(ast: &Seg, syms: &mut SymTable, env_cfg: bool) -> Result<Ctr, String> {
    let is_var = ast.len() == 3;
    let name: String;
    let docs: String;

    match *ast.car {
        Ctr::String(ref s) => name = s.clone(),
        Ctr::Symbol(ref s) => name = s.clone(),
        Ctr::Seg(ref s) => match *eval(s, syms)? {
            Ctr::String(ref s) => name = s.clone(),
            Ctr::Symbol(ref s) => name = s.clone(),
            _ => {
                println!("{}", *eval(s, syms)?);
                return Err("evaluated symbol name doesnt make sense".to_string());
            },
        },
        _ => return Err("symbol name doesnt make sense".to_string()),
    }

    // remove var case
    if ast.len() == 1 {
        syms.remove(&name);
        if env_cfg {
            env::remove_var(name);
        }

        return Ok(Ctr::None)
    } else {
        if ast.len() < 3 || ast.len() > 4 {
            return Err("expected 3 or 4 args".to_string())
        }
    }

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

    match *iter.car {
        Ctr::String(ref s) => docs = s.clone(),
        Ctr::Symbol(ref s) => {
            if let Ctr::String(doc) = *syms.call_symbol(&s, &Seg::new(), true)? {
                docs = doc.clone();
            } else {
                return Err("docs argument does not evaluate to a string".to_string())
            }
        },
        _ => return Err("docs argument does not evaluate to a string".to_string())
    }

    if let Ctr::Seg(ref s) = *iter.cdr {
        iter = s;
    } else {
        return Err("not enough args".to_string())
    }

    let mut outer_scope_val: Seg = Seg::new();
    let noseg = Seg::new(); // similarly, rust shouldnt need this either
    let mut args = &noseg;
    let mut var_val_form: &Seg = &outer_scope_val;
    let mut expand = false;
    match *iter.car {
        Ctr::Seg(ref s) if !is_var => args = s,
        Ctr::Seg(ref s) if is_var => var_val_form = s,
        _ if is_var => {
            expand = true;
            outer_scope_val = Seg::from_mono(Box::new(*iter.car.clone()));
            var_val_form = &outer_scope_val;
        },
        _ if !is_var => return Err("arg list must at least be a list".to_string()),
        _ => unimplemented!(), // rustc is haunted
    }

    if is_var {
        let var_val: Ctr;
        let var_eval_result = *eval(var_val_form, syms)?;
        match var_eval_result {
            Ctr::Seg(ref s) if expand => var_val = *s.car.clone(),
            Ctr::Seg(ref s) if !expand => var_val = Ctr::Seg(s.clone()),
            _ => var_val = var_eval_result,
        }

        let outer_seg = Seg::from_mono(Box::new(var_val.clone()));
        syms.insert(
            name.clone(),
            Symbol::from_ast(&name, &docs, &outer_seg, None),
        );
        if env_cfg {
            env::set_var(name.clone(), var_val.to_string());
        }
        return Ok(Ctr::None)
    }

    let mut arg_list = vec![];
    if !args.circuit(&mut |c: &Ctr| -> bool {
        if let Ctr::Symbol(s) = c {
            arg_list.push(s.clone());
            true
        } else if let Ctr::None = c {
            // no args case
            true
        } else {
            false
        }
    }) {
        return Err("all arguments defined for function must be of type symbol".to_string())
    }

    if let Ctr::Seg(ref eval_bodies) = *iter.cdr {
        syms.insert(
            name.clone(),
            Symbol::from_ast(
                &name, &docs,
                eval_bodies,
                Some(arg_list),
            ),
        );
        Ok(Ctr::None)
    } else {
        Err("expected one or more bodies to evaluate in function".to_string())
    }
}