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use frappe::{Sink, Stream};

pub trait Inverse {
    fn inverse(self) -> Self;
}

pub struct History<T: Inverse> {
    events: Vec<T>,
    events_inv: Vec<T>,
    event_sink: Sink<T>
}

impl<T: Inverse> History<T> {
    pub fn new() -> Self {
        Self {
            events: Vec::new(),
            events_inv: Vec::new(),
            event_sink: Sink::new()
        }
    }

    pub fn len(&self) -> usize {
        self.events.len()
    }

    pub fn len_redo(&self) -> usize {
        self.events_inv.len()
    }

    pub fn events(&self) -> Stream<T> {
        self.event_sink.stream()
    }

    pub fn record(&mut self, event: T) {
        self.event_sink.send_ref(&event);
        self.events.push(event);
    }

    pub fn undo(&mut self) -> bool {
        if let Some(event) = self.events.pop() {
            let event = event.inverse();
            self.event_sink.send_ref(&event);
            self.events_inv.push(event);
            true
        } else {
            false
        }
    }

    pub fn redo(&mut self) -> bool {
        if let Some(event) = self.events_inv.pop() {
            let event = event.inverse();
            self.event_sink.send_ref(&event);
            self.events.push(event);
            true
        } else {
            false
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Meaningless test event.
    #[derive(PartialEq, Clone, Debug)]
    enum Event {
        Forward,
        Backward
    }

    impl Inverse for Event {
        fn inverse(self) -> Self {
            match &self {
                &Event::Forward  => Event::Backward,
                &Event::Backward => Event::Forward
            }
        }
    }

    #[test]
    fn main() {
        let mut hist = History::new();
        let last_event = hist.events().hold(Event::Forward);

        assert!(!hist.undo());
        assert!(!hist.redo());

        hist.record(Event::Forward);
        assert_eq!(Event::Forward, last_event.sample());
        hist.record(Event::Backward);
        assert_eq!(Event::Backward, last_event.sample());

        assert!(hist.undo());
        assert_eq!(Event::Forward, last_event.sample());
        assert!(hist.undo());
        assert_eq!(Event::Backward, last_event.sample());
        assert!(!hist.undo());

        assert!(hist.redo());
        assert_eq!(Event::Forward, last_event.sample());
        assert!(hist.redo());
        assert_eq!(Event::Backward, last_event.sample());
        assert!(!hist.redo());
    }
}