pub const VIEW_ROWS: usize = 21;

pub struct ScrollbackView {
    start: usize,
    maximum_start: usize,
    total: usize,
}

impl ScrollbackView {
    pub const fn new(total: usize) -> Self {
        let maximum_start = total.saturating_sub(VIEW_ROWS);
        Self {
            start: maximum_start,
            maximum_start,
            total,
        }
    }

    pub const fn start(&self) -> usize {
        self.start
    }

    pub const fn total(&self) -> usize {
        self.total
    }

    pub fn end(&self) -> usize {
        (self.start + VIEW_ROWS).min(self.total)
    }

    pub fn older_line(&mut self) -> bool {
        if self.start == 0 {
            return false;
        }
        self.start -= 1;
        true
    }

    pub fn newer_line(&mut self) -> bool {
        if self.start == self.maximum_start {
            return false;
        }
        self.start += 1;
        true
    }

    pub fn older_page(&mut self) -> bool {
        let next = self.start.saturating_sub(VIEW_ROWS);
        if next == self.start {
            return false;
        }
        self.start = next;
        true
    }

    pub fn newer_page(&mut self) -> bool {
        let next = (self.start + VIEW_ROWS).min(self.maximum_start);
        if next == self.start {
            return false;
        }
        self.start = next;
        true
    }
}

#[cfg(test)]
mod tests {
    use super::{ScrollbackView, VIEW_ROWS};

    #[test]
    fn starts_at_latest_page() {
        let view = ScrollbackView::new(40);
        assert_eq!(view.start(), 40 - VIEW_ROWS);
        assert_eq!(view.end(), 40);
    }

    #[test]
    fn clamps_line_navigation() {
        let mut view = ScrollbackView::new(VIEW_ROWS + 2);
        assert!(view.older_line());
        assert!(view.older_line());
        assert!(!view.older_line());
        assert!(view.newer_line());
        assert!(view.newer_line());
        assert!(!view.newer_line());
    }

    #[test]
    fn pages_include_partial_oldest_page() {
        let mut view = ScrollbackView::new(VIEW_ROWS * 2 + 10);
        assert_eq!(view.start(), VIEW_ROWS + 10);
        assert!(view.older_page());
        assert_eq!(view.start(), 10);
        assert!(view.older_page());
        assert_eq!(view.start(), 0);
        assert!(view.newer_page());
        assert_eq!(view.start(), VIEW_ROWS);
        assert!(view.newer_page());
        assert_eq!(view.start(), VIEW_ROWS + 10);
    }

    #[test]
    fn short_transcript_does_not_scroll() {
        let mut view = ScrollbackView::new(3);
        assert_eq!(view.start(), 0);
        assert_eq!(view.end(), 3);
        assert!(!view.older_line());
        assert!(!view.newer_page());
    }
}