use std::assert_matches::assert_matches;
use std::collections::HashMap;

use crate::value::Value;
use crate::{ast, prims};

pub(crate) fn eval(t: &ast::Tree, env: &mut Env) -> Result<Value, RTError> {
    Ok(match t {
        ast::Tree::Leaf(l) => match l {
            ast::Literal::Sym(s) => match env.lookup(s) {
                Some(v) => v,
                None => return err(format!("Undefined variable `{}`", s)),
            },
            ast::Literal::Num(v) => Value::Num(*v),
            ast::Literal::Bool(b) => Value::Bool(*b),
        },
        ast::Tree::Lambda(l) => Value::Lambda(l.clone()),
        ast::Tree::Branch(args) => {
            let Some(fun) = args.get(0) else { return err("No argument given".to_owned()) };
            let fun = eval(fun, env)?;
            let fun = fun.as_func()?;
            let args = args
                .iter()
                .skip(1)
                .map(|a| eval(a, env))
                .collect::<Result<Vec<_>, _>>()?;
            // return fun(&args);
            match fun {
                Callable::Func(f) => f(&args)?,
                Callable::Lambda(l) => {
                    if l.0.len() == args.len() {
                        let mut env = env.child();
                        for (x, y) in l.0.iter().zip(args) {
                            env.define(x.clone(), y)
                        }

                        let [main @ .., tail] = &l.1[..] else { unreachable!("Body has 1+ element by parser") };
                        for i in main {
                            eval(i, &mut env)?;
                        }

                        eval(tail, &mut env)?
                    } else {
                        return err(format!("Need {} args, got {}", l.0.len(), args.len()));
                    }
                }
            }
        }
        ast::Tree::Define(name, to) => {
            let val = eval(to, env)?;
            env.define(name.to_owned(), val);
            Value::Trap
        }
        ast::Tree::If(box [cond, tcase, fcase]) => {
            let b = eval(cond, env)?.as_bool()?;
            let body = if b { tcase } else { fcase };
            eval(body, env)?
        }
    })
}

#[derive(Debug)]
crate struct RTError(crate String);

pub(crate) fn err<T>(s: String) -> Result<T, RTError> {
    Err(RTError(s))
}

pub(crate) enum Callable<'a> {
    Func(prims::Func),
    Lambda(&'a ast::Lambda),
}

pub(crate) struct Env<'a> {
    pub(crate) vars: HashMap<String, Value>,
    pub(crate) enclosing: Option<&'a Env<'a>>,
}

impl<'a> Env<'a> {
    pub(crate) fn child(&'a self) -> Env<'a> {
        Env {
            vars: HashMap::new(),
            enclosing: Some(self),
        }
    }

    pub(crate) fn lookup(&self, s: &str) -> Option<Value> {
        if let Some(v) = self.vars.get(s) {
            Some(v.clone())
        } else if let Some(parent) = self.enclosing {
            parent.lookup(s)
        } else {
            None
        }
    }

    pub(crate) fn define(&mut self, name: String, val: Value) {
        self.vars.insert(name, val);
    }
}

pub(crate) fn default_env() -> Env<'static> {
    let mut vars = HashMap::new();

    for (name, fun) in prims::prims() {
        assert_matches!(vars.insert((*name).to_owned(), Value::Func(*fun)), None)
    }

    Env {
        vars,
        enclosing: None,
    }
}