//! An implementation of the Observer pattern.

use std::{
    cell::RefCell,
    rc::Rc,
};

/// A value that implements the Observer pattern.
///
/// Consumers can connect to receive callbacks when the value changes.
pub struct ObservableValue<T>
    where T: Clone
{
    value: T,
    subs: Vec<Subscription<T>>,
    new_id: usize,
}

/// The identifier for a subscription, used to disconnect it when no longer required.
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct SubscriptionId(usize);

struct Subscription<T> {
    id: SubscriptionId,
    callback: Box<dyn Fn(&T)>
}

impl<T> ObservableValue<T>
    where T: Clone
{
    /// Construct an `ObservableValue`.
    pub fn new(initial_value: T) -> ObservableValue<T> {
        ObservableValue {
            value: initial_value,
            new_id: 0,
            subs: Vec::with_capacity(0),
        }
    }

    /// Get the current value
    pub fn get(&self) -> &T {
        &self.value
    }

    /// Set a new value and notify all connected subscribers.
    pub fn set(&mut self, new_value: &T) {
        self.value = new_value.clone();
        self.call_subscribers();
    }

    fn call_subscribers(&self) {
        for sub in self.subs.iter() {
            (sub.callback)(&self.value)
        }
    }

    /// Connect a new subscriber that will receive callbacks when the
    /// value is set.
    ///
    /// Returns a SubscriptionId to disconnect the subscription when
    /// no longer required.
    pub fn connect<F>(&mut self, callback: F) -> SubscriptionId
        where F: (Fn(&T)) + 'static
    {
        let id = SubscriptionId(self.new_id);
        self.new_id = self.new_id.checked_add(1).expect("No overflow");

        self.subs.push(Subscription {
            id,
            callback: Box::new(callback),
        });
        self.subs.shrink_to_fit();
        id
    }

    /// Disconnect an existing subscription.
    pub fn disconnect(&mut self, sub_id: SubscriptionId) {
        self.subs.retain(|sub| sub.id != sub_id);
        self.subs.shrink_to_fit();
    }

    /// Divide this instance into a read half (can listen for updates, but cannot
    /// write new values) and a write half (can write new values).
    pub fn split(self) -> (ReadHalf<T>, WriteHalf<T>) {
        let inner = Rc::new(RefCell::new(self));
        (
            ReadHalf {
                inner: inner.clone(),
            },
            WriteHalf {
                inner: inner
            }
        )
    }
}

/// The read half of an `ObservableValue`, which can only listen for
/// updates and read the current value.
pub struct ReadHalf<T>
    where T: Clone
{
    inner: Rc<RefCell<ObservableValue<T>>>,
}

/// The write half of an `ObservableValue`, which can write new values.
pub struct WriteHalf<T>
    where T: Clone
{
    inner: Rc<RefCell<ObservableValue<T>>>,
}

impl<T> ReadHalf<T>
    where T: Clone
{
    /// Get the current value
    pub fn get(&self) -> T {
        self.inner.borrow().get().clone()
    }

    /// Connect a new subscriber that will receive callbacks when the
    /// value is set.
    ///
    /// Returns a SubscriptionId to disconnect the subscription when
    /// no longer required.
    pub fn connect<F>(&mut self, callback: F) -> SubscriptionId
        where F: (Fn(&T)) + 'static
    {
        self.inner.borrow_mut().connect(callback)
    }

    /// Disconnect an existing subscription.
    pub fn disconnect(&mut self, sub_id: SubscriptionId) {
        self.inner.borrow_mut().disconnect(sub_id)
    }
}

impl<T> WriteHalf<T>
    where T: Clone
{
    /// Set a new value and notify all connected subscribers.
    pub fn set(&mut self, new_value: &T) {
        self.inner.borrow_mut().set(new_value)
    }
}

#[cfg(test)]
mod test {
    use std::{
        cell::Cell,
        rc::Rc,
    };
    use super::ObservableValue;

    #[test]
    fn new_get_set() {
        let mut ov = ObservableValue::new(17);
        assert_eq!(*ov.get(), 17);

        ov.set(&18);
        assert_eq!(*ov.get(), 18);
    }

    #[test]
    fn connect_set() {
        let mut ov = ObservableValue::<u32>::new(17);
        let mirror: Rc<Cell<u32>> = Rc::new(Cell::new(0));

        let mc = mirror.clone();
        ov.connect(move |val| {
            mc.set(*val);
        });

        // Check callback not yet called.
        assert_eq!(mirror.get(), 0);

        ov.set(&18);

        // Check the callback was called with the correct value.
        assert_eq!(mirror.get(), 18);
    }

    #[test]
    fn disconnect() {
        let mut ov = ObservableValue::<u32>::new(17);
        let mirror_1: Rc<Cell<u32>> = Rc::new(Cell::new(0));
        let mirror_2: Rc<Cell<u32>> = Rc::new(Cell::new(0));

        let mc1 = mirror_1.clone();
        let sub_id_1 = ov.connect(move |val| {
            mc1.set(*val);
        });

        let mc2 = mirror_2.clone();
        let _sub_id_2 = ov.connect(move |val| {
            mc2.set(*val);
        });

        // Both mirrors are connected with callbacks, set() updates both mirror values.
        ov.set(&18);
        assert_eq!(mirror_1.get(), 18);
        assert_eq!(mirror_2.get(), 18);

        ov.disconnect(sub_id_1);

        // Only sub_id_2 is still connected, set() only updates one mirror value.
        ov.set(&19);
        assert_eq!(mirror_1.get(), 18);
        assert_eq!(mirror_2.get(), 19);
    }

    #[test]
    fn split() {
        let ov = ObservableValue::<u32>::new(17);
        let (mut r, mut w) = ov.split();

        let mirror: Rc<Cell<u32>> = Rc::new(Cell::new(0));

        let mc = mirror.clone();
        r.connect(move |val| {
            mc.set(*val);
        });

        // Check callback not yet called.
        assert_eq!(mirror.get(), 0);

        w.set(&18);

        // Check the callback was called with the correct value.
        assert_eq!(mirror.get(), 18);
    }
}