//! Lane layout for the history graph (hub spec §6.7): each change takes a
//! lane, its first dependency continues that lane, other dependencies
//! take free lanes, so edges flow downward from a change to what it
//! depends on. Ported from the VS Code extension's webview.

use std::collections::HashMap;

/// A row of the graph: the change's lane and the edges to the rows below.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Row {
    pub lane: usize,
    /// `(from_lane, to_lane)` for each dependency drawn from this row.
    pub edges: Vec<(usize, usize)>,
    /// Lanes that pass through this row without touching it.
    pub through: Vec<usize>,
}

/// `changes` newest first as `(hash, deps)`. Dependencies outside the list
/// end their lane at the bottom.
pub fn layout(changes: &[(String, Vec<String>)]) -> Vec<Row> {
    let mut active: Vec<Option<String>> = Vec::new();
    let mut rows = Vec::with_capacity(changes.len());
    let known: std::collections::HashSet<&str> = changes.iter().map(|(h, _)| h.as_str()).collect();
    let mut lanes: HashMap<String, usize> = HashMap::new();
    for (hash, deps) in changes {
        let lane = match active.iter().position(|a| a.as_deref() == Some(hash)) {
            Some(l) => l,
            None => match active.iter().position(|a| a.is_none()) {
                Some(l) => {
                    active[l] = Some(hash.clone());
                    l
                }
                None => {
                    active.push(Some(hash.clone()));
                    active.len() - 1
                }
            },
        };
        lanes.insert(hash.clone(), lane);
        let mut edges = Vec::new();
        let deps: Vec<&String> = deps.iter().filter(|d| known.contains(d.as_str())).collect();
        active[lane] = None;
        for (i, d) in deps.iter().enumerate() {
            let target = match active.iter().position(|a| a.as_deref() == Some(d.as_str())) {
                Some(l) => l,
                None => {
                    let l = if i == 0 && active[lane].is_none() {
                        lane
                    } else {
                        match active.iter().position(|a| a.is_none()) {
                            Some(l) => l,
                            None => {
                                active.push(None);
                                active.len() - 1
                            }
                        }
                    };
                    active[l] = Some((*d).clone());
                    l
                }
            };
            edges.push((lane, target));
        }
        let through: Vec<usize> = active
            .iter()
            .enumerate()
            .filter(|(l, a)| a.is_some() && *l != lane && !edges.iter().any(|e| e.1 == *l))
            .map(|(l, _)| l)
            .collect();
        rows.push(Row { lane, edges, through });
        // Trim trailing free lanes so the graph stays narrow.
        while active.last().is_some_and(|a| a.is_none()) {
            active.pop();
        }
    }
    rows
}

pub fn width(rows: &[Row]) -> usize {
    rows.iter()
        .map(|r| r.lane.max(r.edges.iter().map(|e| e.1).max().unwrap_or(0)).max(r.through.iter().copied().max().unwrap_or(0)))
        .max()
        .map(|m| m + 1)
        .unwrap_or(1)
}

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

    fn c(h: &str, deps: &[&str]) -> (String, Vec<String>) {
        (h.into(), deps.iter().map(|d| d.to_string()).collect())
    }

    #[test]
    fn linear_history_is_one_lane() {
        let rows = layout(&[c("c", &["b"]), c("b", &["a"]), c("a", &[])]);
        assert!(rows.iter().all(|r| r.lane == 0), "{rows:?}");
        assert_eq!(rows[0].edges, vec![(0, 0)]);
        assert_eq!(width(&rows), 1);
    }

    #[test]
    fn independent_changes_take_lanes_and_rejoin() {
        // d depends on b and c, which both depend on a: a diamond.
        let rows = layout(&[c("d", &["b", "c"]), c("b", &["a"]), c("c", &["a"]), c("a", &[])]);
        assert_eq!(rows[0].lane, 0);
        assert_eq!(rows[0].edges, vec![(0, 0), (0, 1)]);
        assert_eq!(rows[1].lane, 0);
        assert_eq!(rows[2].lane, 1);
        assert_eq!(rows[2].edges, vec![(1, 0)], "c joins a's lane");
        assert_eq!(rows[3].lane, 0);
        assert_eq!(width(&rows), 2);
        // Two heads with no common change: two lanes throughout.
        let rows = layout(&[c("x", &[]), c("y", &[])]);
        assert_eq!((rows[0].lane, rows[1].lane), (0, 0), "a lane is reused once its change is drawn");
    }
}