pub mod op;
pub mod frame;

use crate::{
    obj::prelude::*,
    vm::{
        op::Op,
        frame::Frame,
    }
};
use std::rc::Rc;

pub struct State {
    frames: Vec<Frame>,
}

impl State {
    pub fn new() -> Self {
        State { frames: Default::default(), }
    }

    fn frame(&self) -> &Frame {
        self.frames
            .last()
            .unwrap()
    }

    fn frame_mut(&mut self) -> &mut Frame {
        self.frames
            .last_mut()
            .unwrap()
    }

    fn push_stack(&mut self, value: DynRef) {
        self.frame_mut()
            .push(value);
    }

    fn pop_stack(&mut self) -> Option<DynRef> {
        self.frame_mut()
            .pop()
    }

    fn ip(&self) -> usize {
        self.frame().ip()
    }

    fn set_ip(&mut self, ip: usize) {
        self.frame_mut().set_ip(ip);
    }

    fn get_local(&self, sym: Sym) -> Option<DynRef> {
        self.frame().get_local(sym)
    }

    fn set_local(&mut self, sym: Sym, obj_ref: DynRef) -> Option<DynRef> {
        self.frame_mut().set_local(sym, obj_ref)
    }

    pub fn run(&mut self, ops: Rc<Vec<Op>>) {
        self.frames.push(Default::default());
        while self.ip() < ops.len() {
            let mut next_ip = self.ip() + 1;
            match &ops[self.ip()] {
                Op::Push(sym) => {
                    // TODO - local not found
                    let obj_ref = self.get_local(*sym)
                        .expect("TODO - local not found");
                    self.push_stack(obj_ref);
                }
                Op::Pop(sym) => {
                    // stack should not underflow
                    let obj_ref = self.pop_stack()
                        .expect("misaligned stack for pop");
                    if let Some(sym) = *sym {
                        self.set_local(sym, obj_ref);
                    }
                }
                Op::GetAttr(sym) => {
                    let top = self.pop_stack()
                        .expect("misaligned stack for getattrs");
                    let obj_ref = {
                        let top_ref = top.borrow();
                        let attrs = top_ref.attrs();
                        let attrs_ref = attrs.borrow();
                        // TODO - local not found
                        attrs_ref.get(*sym)
                            .expect("TODO - local not found")
                    };
                    self.push_stack(obj_ref);
                }
                Op::Call(argc) => {
                    let fun = self.pop_stack()
                        .expect("misaligned stack for function call");
                    let mut argv = Vec::with_capacity(*argc);
                    for i in 0 .. *argc {
                        let arg = self.pop_stack()
                            .expect("misaligned stack for argv");
                        argv.push(arg);
                    }
                    // reverse since arguments are pushed in order of being passed
                    argv.reverse();

                    // call function
                    // TODO - call the function indirectly?
                    //
                    // problem: downcast_ref returns a reference bound by the lifetime of the
                    // function (because of the ref cell). This keeps the fun object borrowed,
                    // which means any attempts to modify the function while it is running will
                    // cause the VM to panic.
                    //
                    // possible solution:
                    //  pop the function object, and then determine which function to call on the
                    //  object (possibly after downcasting?)
                    // 
                    // TODO
                    // Figure out a way to either call a native function with NativeFun::call, or
                    // call a user function with Fun::code()
                    if let Some(fun) = fun.borrow().as_any().downcast_ref::<Fun>() {
                    } else if let Some(fun) = fun.borrow().as_any().downcast_ref::<NativeFun>() {
                    } else {
                        todo!("TODO - not a function")
                    }
                    todo!()
                }
            }
            self.set_ip(next_ip);
        }
        self.frames.pop();
    }
}