use crate::*;
use ::tengri_dsl::{Dsl, DslExpr, DslWord, DslNs};

pub trait View<O, U> {
    fn view_expr <'a> (&'a self, output: &mut O, expr: &'a impl DslExpr) -> Usually<U> {
        Err(format!("View::view_expr: no exprs defined: {expr:?}").into())
    }
    fn view_word <'a> (&'a self, output: &mut O, word: &'a impl DslWord) -> Usually<U> {
        Err(format!("View::view_word: no words defined: {word:?}").into())
    }
    fn view <'a> (&'a self, output: &mut O, dsl: &'a impl Dsl) -> Usually<U> {
        if let Ok(Some(expr)) = dsl.expr() {
            self.view_expr(output, &expr)
        } else if let Ok(Some(word)) = dsl.word() {
            self.view_word(output, &word)
        } else {
            panic!("{dsl:?}: invalid")
        }
    }
}

pub fn evaluate_output_expression <'a, O: Out + 'a, S> (
    state: &S, output: &mut O, expr: &'a impl DslExpr
) -> Usually<bool> where
    S: View<O, ()>
        + for<'b>DslNs<'b, bool>
        + for<'b>DslNs<'b, O::Unit>
{
    // First element of expression is used for dispatch.
    // Dispatch is proto-namespaced using separator character
    let head = expr.head()?;
    let mut frags = head.src()?.unwrap_or_default().split("/");
    // The rest of the tokens in the expr are arguments.
    // Their meanings depend on the dispatched operation
    let args = expr.tail();
    let arg0 = args.head();
    let tail0 = args.tail();
    let arg1 = tail0.head();
    let tail1 = tail0.tail();
    let arg2 = tail1.head();
    // And we also have to do the above binding dance
    // so that the Perhaps<token>s remain in scope.
    match frags.next() {

        Some("when") => output.place(&When::new(
            state.from(arg0?)?.unwrap(),
            Thunk::new(move|output: &mut O|state.view(output, &arg1).unwrap())
        )),

        Some("either") => output.place(&Either::new(
            state.from(arg0?)?.unwrap(),
            Thunk::new(move|output: &mut O|state.view(output, &arg1).unwrap()),
            Thunk::new(move|output: &mut O|state.view(output, &arg2).unwrap())
        )),

        Some("bsp") => output.place(&{
            let a = Thunk::new(move|output: &mut O|state.view(output, &arg0).unwrap());
            let b = Thunk::new(move|output: &mut O|state.view(output, &arg1).unwrap());
            match frags.next() {
                Some("n") => Bsp::n(a, b),
                Some("s") => Bsp::s(a, b),
                Some("e") => Bsp::e(a, b),
                Some("w") => Bsp::w(a, b),
                Some("a") => Bsp::a(a, b),
                Some("b") => Bsp::b(a, b),
                frag => unimplemented!("bsp/{frag:?}")
            }
        }),

        Some("align") => output.place(&{
            let a = Thunk::new(move|output: &mut O|state.view(output, &arg0).unwrap());
            match frags.next() {
                Some("n")  => Align::n(a),
                Some("s")  => Align::s(a),
                Some("e")  => Align::e(a),
                Some("w")  => Align::w(a),
                Some("x")  => Align::x(a),
                Some("y")  => Align::y(a),
                Some("c")  => Align::c(a),
                frag => unimplemented!("align/{frag:?}")
            }
        }),

        Some("fill") => output.place(&{
            let a = Thunk::new(move|output: &mut O|state.view(output, &arg0).unwrap());
            match frags.next() {
                Some("xy") | None => Fill::XY(a),
                Some("x") => Fill::X(a),
                Some("y") => Fill::Y(a),
                frag => unimplemented!("fill/{frag:?}")
            }
        }),

        Some("fixed") => output.place(&{
            let axis = frags.next();
            let arg  = match axis { Some("x") | Some("y") => arg1, Some("xy") | None => arg2, _ => panic!("fixed: unsupported axis {axis:?}") };
            let cb   = Thunk::new(move|output: &mut O|state.view(output, &arg).unwrap());
            match axis {
                Some("xy") | None => Fixed::XY(state.from(arg0?)?.unwrap(), state.from(arg1?)?.unwrap(), cb),
                Some("x") => Fixed::X(state.from(arg0?)?.unwrap(), cb),
                Some("y") => Fixed::Y(state.from(arg0?)?.unwrap(), cb),
                frag => unimplemented!("fixed/{frag:?} ({expr:?}) ({head:?}) ({:?})",
                    head.src()?.unwrap_or_default().split("/").next())
            }
        }),

        Some("min") => output.place(&{
            let axis = frags.next();
            let arg  = match axis { Some("x") | Some("y") => arg1, Some("xy") | None => arg2, _ => panic!("fixed: unsupported axis {axis:?}") };
            let cb   = Thunk::new(move|output: &mut O|state.view(output, &arg).unwrap());
            match axis {
                Some("xy") | None => Min::XY(state.from(arg0?)?.unwrap(), state.from(arg1?)?.unwrap(), cb),
                Some("x") => Min::X(state.from(arg0?)?.unwrap(), cb),
                Some("y") => Min::Y(state.from(arg0?)?.unwrap(), cb),
                frag => unimplemented!("min/{frag:?}")
            }
        }),

        Some("max") => output.place(&{
            let axis = frags.next();
            let arg  = match axis { Some("x") | Some("y") => arg1, Some("xy") | None => arg2, _ => panic!("fixed: unsupported axis {axis:?}") };
            let cb   = Thunk::new(move|output: &mut O|state.view(output, &arg).unwrap());
            match axis {
                Some("xy") | None => Max::XY(state.from(arg0?)?.unwrap(), state.from(arg1?)?.unwrap(), cb),
                Some("x") => Max::X(state.from(arg0?)?.unwrap(), cb),
                Some("y") => Max::Y(state.from(arg0?)?.unwrap(), cb),
                frag => unimplemented!("max/{frag:?}")
            }
        }),

        Some("push") => output.place(&{
            let axis = frags.next();
            let arg  = match axis { Some("x") | Some("y") => arg1, Some("xy") | None => arg2, _ => panic!("fixed: unsupported axis {axis:?}") };
            let cb   = Thunk::new(move|output: &mut O|state.view(output, &arg).unwrap());
            match axis {
                Some("xy") | None => Push::XY(state.from(arg0?)?.unwrap(), state.from(arg1?)?.unwrap(), cb),
                Some("x") => Push::X(state.from(arg0?)?.unwrap(), cb),
                Some("y") => Push::Y(state.from(arg0?)?.unwrap(), cb),
                frag => unimplemented!("push/{frag:?}")
            }
        }),

        _ => return Ok(false)

    };
    Ok(true)
}