//use crate::*;
use std::sync::{Arc, RwLock};

#[derive(Debug, Default)] pub struct Memo<T, U> {
    pub value: T,
    pub view: Arc<RwLock<U>>
}

impl<T: PartialEq, U> Memo<T, U> {
    pub fn new (value: T, view: U) -> Self {
        Self { value, view: Arc::new(view.into()) }
    }
    pub fn update <R> (
        &mut self,
        newval: T,
        render: impl Fn(&mut U, &T, &T)->R
    ) -> Option<R> {
        if newval != self.value {
            let result = render(&mut*self.view.write().unwrap(), &newval, &self.value);
            self.value = newval;
            return Some(result);
        }
        None
    }
}

/// Clear a pre-allocated buffer, then write into it.
#[macro_export] macro_rules! rewrite {
    ($buf:ident, $($rest:tt)*) => { |$buf,_,_|{ $buf.clear(); write!($buf, $($rest)*) } }
}