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use std::fmt;
use std::fmt::Debug;
use std::rc::Rc;

use crate::goals::Goal;
use crate::{
    constraints::{resolve_2, Constraint, ResolveFn, VarWatch},
    core::{State, Unify, Value},
};

/** A [projection goal](super) that allows creating a new goal based on
the resolved values. Create with [`project_2`].
*/
#[allow(clippy::type_complexity)]
pub struct Project2<A: Unify, B: Unify> {
    a: Value<A>,
    b: Value<B>,
    f: Rc<dyn Fn(Rc<A>, Rc<B>) -> Box<dyn Goal>>,
}

/** Create a [projection goal](super) that allows creating a new goal based on
the resolved values.

```
use canrun::{LVar, Query};
use canrun::goals::{project_2, all, both, unify, Succeed, Fail};

let (x, y) = (LVar::new(), LVar::new());
let goal = all![
    unify(1, x),
    unify(2, y),
    project_2(x, y, |x, y| if x < y { Box::new(Succeed) } else { Box::new(Fail) }),
];
let result: Vec<_> = goal.query((x, y)).collect();
assert_eq!(result, vec![(1, 2)])
```
*/
pub fn project_2<A, IA, B, IB, F>(a: IA, b: IB, func: F) -> Project2<A, B>
where
    A: Unify,
    IA: Into<Value<A>>,
    B: Unify,
    IB: Into<Value<B>>,
    F: Fn(Rc<A>, Rc<B>) -> Box<dyn Goal> + 'static,
{
    Project2 {
        a: a.into(),
        b: b.into(),
        f: Rc::new(func),
    }
}

impl<A: Unify, B: Unify> Clone for Project2<A, B> {
    fn clone(&self) -> Self {
        Self {
            a: self.a.clone(),
            b: self.b.clone(),
            f: self.f.clone(),
        }
    }
}

impl<A: Unify, B: Unify> Goal for Project2<A, B> {
    fn apply(&self, state: State) -> Option<State> {
        state.constrain(Rc::new(self.clone()))
    }
}

impl<A: Unify, B: Unify> Constraint for Project2<A, B> {
    fn attempt(&self, state: &State) -> Result<ResolveFn, VarWatch> {
        let (a, b) = resolve_2(&self.a, &self.b, state)?;
        let goal = (self.f)(a, b);
        Ok(Box::new(move |state| goal.apply(state)))
    }
}

impl<A: Unify + Debug, B: Unify + Debug> Debug for Project2<A, B> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Project2 {:?} {:?}", self.a, self.b)
    }
}

#[cfg(test)]
mod tests {
    use crate::{
        core::{LVar, Query},
        goals::{both::both, fail::Fail, project::project_2::project_2, succeed::Succeed, unify},
    };

    #[test]
    fn succeeds() {
        let x = LVar::new();
        let y = LVar::new();
        let goal = both(
            both(unify(1, x), unify(2, y)),
            project_2(x, y, |x, y| {
                if x < y {
                    Box::new(Succeed)
                } else {
                    Box::new(Fail)
                }
            }),
        );
        assert_eq!(goal.query(x).collect::<Vec<_>>(), vec![1]);
    }

    #[test]
    fn fails() {
        let x = LVar::new();
        let y = LVar::new();
        let goal = both(
            both(unify(1, x), unify(2, y)),
            project_2(x, y, |x, y| {
                if x > y {
                    Box::new(Succeed)
                } else {
                    Box::new(Fail)
                }
            }),
        );
        assert_eq!(goal.query(x).collect::<Vec<_>>(), vec![]);
    }
}