//! Lane layout for drawing history as a graph: each change takes a lane,
//! its first dependency continues that lane, other dependencies take free
//! lanes, so edges flow from a change down to what it depends on. Shared
//! by `oxy log` and hosts, so every view draws the same picture.

use crate::hash::ChangeHash;
use std::collections::{HashMap, HashSet};

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

/// `changes` newest first as `(hash, deps)`. A dependency outside the
/// list ends its lane at the bottom.
pub fn layout(changes: &[(ChangeHash, Vec<ChangeHash>)]) -> Vec<Row> {
    let mut active: Vec<Option<ChangeHash>> = Vec::new();
    let mut rows = Vec::with_capacity(changes.len());
    let known: HashSet<&ChangeHash> = changes.iter().map(|(h, _)| h).collect();
    let mut lanes: HashMap<ChangeHash, usize> = HashMap::new();
    for (hash, deps) in changes {
        let lane = match active.iter().position(|a| a.as_ref() == Some(hash)) {
            Some(l) => l,
            None => match active.iter().position(|a| a.is_none()) {
                Some(l) => {
                    active[l] = Some(*hash);
                    l
                }
                None => {
                    active.push(Some(*hash));
                    active.len() - 1
                }
            },
        };
        lanes.insert(*hash, lane);
        let mut edges = Vec::new();
        let deps: Vec<&ChangeHash> = deps.iter().filter(|d| known.contains(d)).collect();
        active[lane] = None;
        for (i, d) in deps.iter().enumerate() {
            let target = match active.iter().position(|a| a.as_ref() == Some(*d)) {
                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);
                    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,
        });
        while active.last().is_some_and(|a| a.is_none()) {
            active.pop();
        }
    }
    rows
}

/// The number of lanes the rows use.
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)
}

/// One cell of a graph row's lower line, the line under the change.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Cell {
    /// Nothing in this column.
    Blank,
    /// A lane carries on down.
    Line,
    /// The change's lane ends at a dependency not drawn (too far down, or
    /// not in the list): `~`.
    Elided,
    /// The left end of a join: this lane carries on, and the lane to its
    /// right turns into it (`├`).
    JoinLeft,
    /// A join passes a column with no lane (`─`).
    JoinPass,
    /// A join crosses a lane that carries on (`┼`).
    JoinCross,
    /// The right end of a join: this lane turns left and ends (`╯`).
    JoinRight,
}

/// A row of the drawn graph: the change's column, which other columns
/// carry a lane past it, and the line under it.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct GraphRow {
    pub column: usize,
    /// Columns with a lane on the change's own line, besides its own.
    pub passing: Vec<usize>,
    /// The line under the change, one cell per column.
    pub below: Vec<Cell>,
}

/// Lay out `changes` (newest first, as `(hash, deps)`) for a terminal
/// graph. A change's dependencies form a wide graph, since a change builds
/// on whatever last touched the lines it edits, so only one edge is drawn
/// per change: to its newest dependency, the change it most directly
/// builds on. An edge longer than `max_edge` rows, or to a change not in
/// the list, is elided (`~`), as are the other dependencies, so the
/// picture keeps to a few lanes. Two lanes heading for the same change
/// join into the left one.
pub fn graph(changes: &[(ChangeHash, Vec<ChangeHash>)], max_edge: usize) -> Vec<GraphRow> {
    let index: HashMap<&ChangeHash, usize> = changes
        .iter()
        .enumerate()
        .map(|(i, (h, _))| (h, i))
        .collect();
    let mut active: Vec<Option<ChangeHash>> = Vec::new();
    let mut rows = Vec::with_capacity(changes.len());
    for (i, (hash, deps)) in changes.iter().enumerate() {
        let column = match active.iter().position(|a| a.as_ref() == Some(hash)) {
            Some(c) => c,
            None => match active.iter().position(Option::is_none) {
                Some(c) => {
                    active[c] = Some(*hash);
                    c
                }
                None => {
                    active.push(Some(*hash));
                    active.len() - 1
                }
            },
        };
        let passing: Vec<usize> = active
            .iter()
            .enumerate()
            .filter(|(c, a)| *c != column && a.is_some())
            .map(|(c, _)| c)
            .collect();
        // The newest dependency below this row (the list is newest first).
        let nearest = deps
            .iter()
            .filter_map(|d| index.get(d).copied())
            .filter(|&j| j > i)
            .min();
        let parent = nearest.filter(|&j| j - i <= max_edge).map(|j| changes[j].0);
        active[column] = None;
        let mut join: Option<(usize, usize)> = None;
        if let Some(p) = parent {
            match active.iter().position(|a| a.as_ref() == Some(&p)) {
                // Another lane already heads there: join the two, leftwards.
                Some(other) => {
                    let (left, right) = (other.min(column), other.max(column));
                    active[right] = None;
                    active[left] = Some(p);
                    join = Some((left, right));
                }
                None => active[column] = Some(p),
            }
        }
        let width = active.len().max(column + 1);
        let mut below = vec![Cell::Blank; width];
        for (c, a) in active.iter().enumerate() {
            if a.is_some() {
                below[c] = Cell::Line;
            }
        }
        if let Some((left, right)) = join {
            below[left] = Cell::JoinLeft;
            for cell in below.iter_mut().take(right).skip(left + 1) {
                *cell = if *cell == Cell::Line {
                    Cell::JoinCross
                } else {
                    Cell::JoinPass
                };
            }
            below[right] = Cell::JoinRight;
        } else if parent.is_none() && !deps.is_empty() {
            below[column] = Cell::Elided;
        }
        while below.len() > 1 && below.last() == Some(&Cell::Blank) {
            below.pop();
        }
        rows.push(GraphRow {
            column,
            passing,
            below,
        });
        while active.last().is_some_and(Option::is_none) {
            active.pop();
        }
    }
    rows
}

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

    fn h(n: u8) -> ChangeHash {
        ObjectRef([n; 32])
    }

    #[test]
    fn linear_history_is_one_lane() {
        let rows = layout(&[(h(3), vec![h(2)]), (h(2), vec![h(1)]), (h(1), vec![])]);
        assert!(rows.iter().all(|r| r.lane == 0), "{rows:?}");
        assert_eq!(rows[0].edges, vec![(0, 0)]);
        assert_eq!(width(&rows), 1);
    }

    #[test]
    fn a_diamond_takes_two_lanes_and_rejoins() {
        let rows = layout(&[
            (h(4), vec![h(2), h(3)]),
            (h(2), vec![h(1)]),
            (h(3), vec![h(1)]),
            (h(1), vec![]),
        ]);
        assert_eq!(rows[0].edges, vec![(0, 0), (0, 1)]);
        assert_eq!(rows[2].lane, 1);
        assert_eq!(
            rows[2].edges,
            vec![(1, 0)],
            "the second branch joins the first lane"
        );
        assert_eq!(width(&rows), 2);
    }

    #[test]
    fn the_graph_draws_the_newest_dependency_and_joins_lanes_left() {
        // 4 and 3 both build on 2; 2 on 1; 3 also depends on 1 (not drawn).
        let rows = graph(
            &[
                (h(4), vec![h(2)]),
                (h(3), vec![h(1), h(2)]),
                (h(2), vec![h(1)]),
                (h(1), vec![]),
            ],
            12,
        );
        assert_eq!(rows[0].column, 0);
        assert_eq!(rows[0].below, vec![Cell::Line]);
        assert_eq!(rows[1].column, 1, "a second head takes a new lane");
        assert_eq!(rows[1].passing, vec![0]);
        assert_eq!(rows[1].below, vec![Cell::JoinLeft, Cell::JoinRight]);
        assert_eq!(rows[2].column, 0);
        assert_eq!(rows[3].below, vec![Cell::Blank], "a root ends its lane");
    }

    #[test]
    fn a_long_edge_is_elided() {
        let mut changes = vec![(h(100), vec![h(1)])];
        for n in (2..=20).rev() {
            changes.push((h(n), vec![]));
        }
        changes.push((h(1), vec![]));
        let rows = graph(&changes, 5);
        assert_eq!(rows[0].below, vec![Cell::Elided]);
        assert!(
            rows.iter().all(|r| r.below.len() == 1),
            "one lane all the way"
        );
        let rows = graph(&changes, 100);
        assert_eq!(rows[0].below, vec![Cell::Line], "drawn when short enough");
    }
}