crates/oxy-core/src/graph.rs at 57f8f0b3a4d0 · andymccall/oxygen

# 57f8f0b3a4d0 oxygen / crates / oxy-core / src / graph.rs
26.9 KiB · Rust · last changed Checkpoints: serialized states per bookmark, automatic every core.checkpoint-interval changes, oxy checkpoint, checkpoint-accelerated load_state; states keep change metadata only 1 month ago source blame history raw
//! The per-file atom graph (spec §7.3), its materialization, and text conflict
//! detection (spec §9.1, §9.5).
//!
//! The graph is a pure function of the set of changes applied to it: inserts
//! add vertices and edges, deletes set a flag, and nothing is ever removed.

use crate::hash::ChangeHash;
use crate::objects::AtomRange;
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};

/// `(introducing change, index within that change for this node)`.
pub type AtomKey = (ChangeHash, u32);

#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Debug)]
pub enum Vertex {
    Start,
    Atom(AtomKey),
    End,
}

/// Transitive dependency test between changes; `depends_on(a, a)` is true.
pub trait DepOracle {
    fn depends_on(&self, a: &ChangeHash, b: &ChangeHash) -> bool;
}

/// No change depends on any other: every pair commutes.
pub struct NoDeps;
impl DepOracle for NoDeps {
    fn depends_on(&self, a: &ChangeHash, b: &ChangeHash) -> bool {
        a == b
    }
}

#[derive(Default, Clone, Debug)]
struct AtomInfo {
    /// `None` until the introducing change has been applied.
    content: Option<Vec<u8>>,
    deleted_by: BTreeSet<ChangeHash>,
}

#[derive(Clone, Debug)]
pub struct FileGraph {
    atoms: HashMap<AtomKey, AtomInfo>,
    succ: HashMap<Vertex, BTreeSet<Vertex>>,
    pred: HashMap<Vertex, BTreeSet<Vertex>>,
}

impl Default for FileGraph {
    fn default() -> Self {
        Self::new()
    }
}

/// One piece of a materialized file.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum Segment {
    Lines(Vec<AtomKey>),
    /// Order conflict: alternatives that cannot be ordered against each other.
    Conflict(Vec<Vec<AtomKey>>),
    /// Delete/insert conflict: `inserted` was anchored to `deleted` by a change
    /// that commutes with the deletion.
    Zombie { deleted: Vec<AtomKey>, inserted: Vec<AtomKey> },
}

#[derive(Clone, Debug, PartialEq, Eq, Default)]
pub struct Materialized {
    pub segments: Vec<Segment>,
}

impl Materialized {
    pub fn is_conflicted(&self) -> bool {
        self.segments.iter().any(|s| !matches!(s, Segment::Lines(_)))
    }
    pub fn conflict_count(&self) -> usize {
        self.segments.iter().filter(|s| !matches!(s, Segment::Lines(_))).count()
    }
    /// Every live atom in materialized order, conflict sides concatenated.
    pub fn live_atoms(&self) -> Vec<AtomKey> {
        let mut out = Vec::new();
        for s in &self.segments {
            match s {
                Segment::Lines(a) => out.extend(a),
                Segment::Conflict(sides) => sides.iter().for_each(|x| out.extend(x)),
                Segment::Zombie { inserted, .. } => out.extend(inserted),
            }
        }
        out
    }
}

impl FileGraph {
    pub fn new() -> Self {
        let mut g = FileGraph { atoms: HashMap::new(), succ: HashMap::new(), pred: HashMap::new() };
        g.edge(Vertex::Start, Vertex::End);
        g
    }

    fn edge(&mut self, a: Vertex, b: Vertex) {
        self.succ.entry(a).or_default().insert(b);
        self.pred.entry(b).or_default().insert(a);
        self.succ.entry(b).or_default();
        self.pred.entry(a).or_default();
    }

    /// Apply an `Insert` by `change` whose first new atom has index `first_index`.
    pub fn insert(&mut self, change: ChangeHash, first_index: u32, after: Vertex, before: Vertex, lines: &[Vec<u8>]) {
        let mut prev = after;
        for (i, line) in lines.iter().enumerate() {
            let key = (change, first_index + i as u32);
            self.atoms.entry(key).or_default().content = Some(line.clone());
            let v = Vertex::Atom(key);
            self.edge(prev, v);
            prev = v;
        }
        self.edge(prev, before);
    }

    /// Apply a `Delete` of `range` by change `by`.
    pub fn delete(&mut self, by: ChangeHash, range: &AtomRange) {
        for i in range.from..range.to {
            self.atoms.entry((range.change, i)).or_default().deleted_by.insert(by);
        }
    }

    pub fn is_live(&self, k: &AtomKey) -> bool {
        self.atoms.get(k).is_some_and(|a| a.content.is_some() && a.deleted_by.is_empty())
    }

    pub fn content(&self, k: &AtomKey) -> Option<&[u8]> {
        self.atoms.get(k).and_then(|a| a.content.as_deref())
    }

    pub fn deleters(&self, k: &AtomKey) -> impl Iterator<Item = &ChangeHash> {
        self.atoms.get(k).into_iter().flat_map(|a| a.deleted_by.iter())
    }

    /// Number of atoms introduced by `change` in this graph (for index bookkeeping).
    pub fn atom_count(&self) -> usize {
        self.atoms.len()
    }

    fn is_ghost(&self, k: &AtomKey, info: &AtomInfo, deps: &dyn DepOracle) -> bool {
        // A deleted atom is a ghost when a live direct successor was inserted by
        // a change that no deleter depends on (spec §9.1, delete/insert).
        self.succ.get(&Vertex::Atom(*k)).is_some_and(|ss| {
            ss.iter().any(|s| match s {
                Vertex::Atom(y) if self.is_live(y) => !info.deleted_by.iter().any(|d| deps.depends_on(d, &y.0)),
                _ => false,
            })
        })
    }

    /// Successors of `v` in the graph with non-kept vertices contracted away.
    fn reduced_succ(&self, v: Vertex, kept: &HashSet<Vertex>, memo: &mut HashMap<Vertex, BTreeSet<Vertex>>) -> BTreeSet<Vertex> {
        let mut out = BTreeSet::new();
        let mut stack: Vec<Vertex> = self.succ.get(&v).map(|s| s.iter().copied().collect()).unwrap_or_default();
        let mut seen = HashSet::new();
        while let Some(s) = stack.pop() {
            if kept.contains(&s) {
                out.insert(s);
            } else if seen.insert(s) {
                if let Some(m) = memo.get(&s) {
                    out.extend(m.iter().copied());
                } else if let Some(next) = self.succ.get(&s) {
                    stack.extend(next.iter().copied());
                }
            }
        }
        out
    }

    /// Order the live atoms and locate conflicts.
    pub fn materialize(&self, deps: &dyn DepOracle) -> Materialized {
        // 1. Vertices that survive contraction: live atoms, ghosts, Start, End.
        let mut kept: HashSet<Vertex> = HashSet::new();
        let mut ghosts: HashSet<AtomKey> = HashSet::new();
        kept.insert(Vertex::Start);
        kept.insert(Vertex::End);
        for (k, info) in &self.atoms {
            if info.content.is_none() {
                continue;
            }
            if info.deleted_by.is_empty() {
                kept.insert(Vertex::Atom(*k));
            } else if self.is_ghost(k, info, deps) {
                kept.insert(Vertex::Atom(*k));
                ghosts.insert(*k);
            }
        }

        // 2. Reduced graph over kept vertices.
        let mut memo = HashMap::new();
        let mut rsucc: HashMap<Vertex, BTreeSet<Vertex>> = HashMap::new();
        let mut indeg: HashMap<Vertex, usize> = HashMap::new();
        for &u in &kept {
            indeg.entry(u).or_insert(0);
            let rs = self.reduced_succ(u, &kept, &mut memo);
            for &w in &rs {
                *indeg.entry(w).or_insert(0) += 1;
            }
            rsucc.insert(u, rs);
        }

        // 3. Kahn's algorithm with side labelling for conflict regions.
        let mut ready: BTreeSet<Vertex> = indeg.iter().filter(|(_, &d)| d == 0).map(|(v, _)| *v).collect();
        let mut side: HashMap<Vertex, u32> = HashMap::new();
        let mut joins: HashSet<Vertex> = HashSet::new();
        let mut next_side = 0u32;
        let mut plain: Vec<Vertex> = Vec::new();
        let mut conflict: Option<BTreeMap<u32, Vec<Vertex>>> = None;
        let mut segments: Vec<Segment> = Vec::new();

        let flush_plain = |plain: &mut Vec<Vertex>, segments: &mut Vec<Segment>| {
            let atoms: Vec<AtomKey> = plain.iter().filter_map(|v| if let Vertex::Atom(k) = v { Some(*k) } else { None }).collect();
            plain.clear();
            if !atoms.is_empty() {
                segments.extend(split_zombies(atoms, &ghosts));
            }
        };
        let finish_conflict = |c: BTreeMap<u32, Vec<Vertex>>, segments: &mut Vec<Segment>| {
            let sides: Vec<Vec<AtomKey>> = c
                .into_values()
                .map(|vs| {
                    vs.into_iter()
                        .filter_map(|v| match v {
                            Vertex::Atom(k) if !ghosts.contains(&k) => Some(k),
                            _ => None,
                        })
                        .collect::<Vec<_>>()
                })
                .filter(|s| !s.is_empty())
                .collect();
            match sides.len() {
                0 => {}
                1 => segments.push(Segment::Lines(sides.into_iter().next().unwrap())),
                _ => segments.push(Segment::Conflict(sides)),
            }
        };

        while !ready.is_empty() {
            if ready.len() >= 2 {
                if conflict.is_none() {
                    flush_plain(&mut plain, &mut segments);
                    conflict = Some(BTreeMap::new());
                }
                for r in &ready {
                    if !side.contains_key(r) {
                        side.insert(*r, next_side);
                        next_side += 1;
                    }
                }
            }
            let v = ready.pop_first().unwrap();
            let my_side = side.get(&v).copied();
            match (&mut conflict, my_side) {
                (Some(c), Some(s)) => c.entry(s).or_default().push(v),
                (Some(_), None) => {
                    let c = conflict.take().unwrap();
                    finish_conflict(c, &mut segments);
                    plain.push(v);
                }
                (None, _) => plain.push(v),
            }
            for &w in &rsucc[&v] {
                let d = indeg.get_mut(&w).unwrap();
                *d -= 1;
                if conflict.is_some() && !joins.contains(&w) {
                    match (my_side, side.get(&w).copied()) {
                        (Some(s), None) => {
                            side.insert(w, s);
                        }
                        (Some(s), Some(t)) if s != t => {
                            side.remove(&w);
                            joins.insert(w);
                        }
                        (None, Some(_)) => {
                            side.remove(&w);
                            joins.insert(w);
                        }
                        _ => {}
                    }
                }
                if *d == 0 {
                    ready.insert(w);
                }
            }
        }
        if let Some(c) = conflict.take() {
            finish_conflict(c, &mut segments);
        }
        flush_plain(&mut plain, &mut segments);
        Materialized { segments }
    }

    /// Render a materialization to bytes with conflict markers (spec §9.5).
    /// `label` gives the short description of a change for marker headers.
    pub fn render(&self, m: &Materialized, label: &dyn Fn(&ChangeHash) -> String) -> Vec<u8> {
        let total = m.conflict_count();
        let mut n = 0;
        let mut out: Vec<u8> = Vec::new();
        let push_atoms = |out: &mut Vec<u8>, atoms: &[AtomKey]| {
            for a in atoms {
                if let Some(c) = self.content(a) {
                    out.extend_from_slice(c);
                }
            }
        };
        let newline_if_needed = |out: &mut Vec<u8>| {
            if !out.is_empty() && *out.last().unwrap() != b'\n' {
                out.push(b'\n');
            }
        };
        for seg in &m.segments {
            match seg {
                Segment::Lines(a) => push_atoms(&mut out, a),
                Segment::Conflict(sides) => {
                    n += 1;
                    let k = sides.len();
                    for (i, s) in sides.iter().enumerate() {
                        newline_if_needed(&mut out);
                        let who = s.first().map(|a| label(&a.0)).unwrap_or_default();
                        let marker = if i == 0 { "<<<<<<<" } else { "=======" };
                        out.extend_from_slice(
                            format!("{marker} oxy conflict {n} of {total}, side {} of {k}: change {who}\n", i + 1).as_bytes(),
                        );
                        push_atoms(&mut out, s);
                    }
                    newline_if_needed(&mut out);
                    out.extend_from_slice(format!(">>>>>>> oxy conflict {n} of {total} end\n").as_bytes());
                }
                Segment::Zombie { deleted, inserted } => {
                    n += 1;
                    newline_if_needed(&mut out);
                    let deleter = deleted
                        .first()
                        .and_then(|a| self.deleters(a).next().copied())
                        .map(|d| label(&d))
                        .unwrap_or_default();
                    let inserter = inserted.first().map(|a| label(&a.0)).unwrap_or_default();
                    out.extend_from_slice(
                        format!("<<<<<<< oxy conflict {n} of {total}, side 1 of 2: deleted by change {deleter}\n-------\n").as_bytes(),
                    );
                    push_atoms(&mut out, deleted);
                    newline_if_needed(&mut out);
                    out.extend_from_slice(format!("======= oxy conflict {n} of {total}, side 2 of 2: change {inserter}\n").as_bytes());
                    push_atoms(&mut out, inserted);
                    newline_if_needed(&mut out);
                    out.extend_from_slice(format!(">>>>>>> oxy conflict {n} of {total} end\n").as_bytes());
                }
            }
        }
        out
    }
}

fn vertex_value(v: &Vertex) -> crate::cbor::Value {
    use crate::cbor::Value;
    match v {
        Vertex::Start => Value::Uint(0),
        Vertex::End => Value::Uint(1),
        Vertex::Atom((c, i)) => Value::Array(vec![Value::Bytes(c.0.to_vec()), Value::Uint(*i as u64)]),
    }
}

fn vertex_from(v: &crate::cbor::Value) -> Result<Vertex, crate::objects::ObjectError> {
    use crate::cbor::Value;
    use crate::objects::ObjectError;
    match v {
        Value::Uint(0) => Ok(Vertex::Start),
        Value::Uint(1) => Ok(Vertex::End),
        Value::Array(a) if a.len() == 2 => Ok(Vertex::Atom((crate::objects::oref(&a[0], "vertex")?, crate::objects::u32_(&a[1], "vertex")?))),
        _ => Err(ObjectError::Schema("vertex: expected 0, 1, or [hash, index]".into())),
    }
}

impl FileGraph {
    /// Serialize for a checkpoint: `{0: atoms [[hash, index, content|null, [deleters]]], 1: edges [[from, to]]}`.
    pub fn to_value(&self) -> crate::cbor::Value {
        use crate::cbor::Value;
        let mut atoms: Vec<(&AtomKey, &AtomInfo)> = self.atoms.iter().collect();
        atoms.sort_by_key(|(k, _)| **k);
        let mut edges: Vec<(Vertex, Vertex)> = self.succ.iter().flat_map(|(a, bs)| bs.iter().map(move |b| (*a, *b))).collect();
        edges.sort();
        Value::map([
            (
                0,
                Value::Array(
                    atoms
                        .into_iter()
                        .map(|((c, i), info)| {
                            Value::Array(vec![
                                Value::Bytes(c.0.to_vec()),
                                Value::Uint(*i as u64),
                                info.content.as_ref().map(|b| Value::Bytes(b.clone())).unwrap_or(Value::Null),
                                Value::Array(info.deleted_by.iter().map(|d| Value::Bytes(d.0.to_vec())).collect()),
                            ])
                        })
                        .collect(),
                ),
            ),
            (1, Value::Array(edges.iter().map(|(a, b)| Value::Array(vec![vertex_value(a), vertex_value(b)])).collect())),
        ])
    }

    pub fn from_value(v: &crate::cbor::Value) -> Result<FileGraph, crate::objects::ObjectError> {
        use crate::objects::{array, bytes, field, map, oref, u32_, ObjectError};
        let m = map(v, "file graph")?;
        let mut g = FileGraph { atoms: HashMap::new(), succ: HashMap::new(), pred: HashMap::new() };
        for a in array(field(m, 0, "atoms")?, "atoms")? {
            let e = array(a, "atom")?;
            if e.len() != 4 {
                return Err(ObjectError::Schema("atom: expected 4 fields".into()));
            }
            let key = (oref(&e[0], "atom change")?, u32_(&e[1], "atom index")?);
            let content = if e[2].is_null() { None } else { Some(bytes(&e[2], "atom content")?.to_vec()) };
            let deleted_by = array(&e[3], "deleters")?.iter().map(|d| oref(d, "deleter")).collect::<Result<BTreeSet<_>, _>>()?;
            g.atoms.insert(key, AtomInfo { content, deleted_by });
        }
        for e in array(field(m, 1, "edges")?, "edges")? {
            let pair = array(e, "edge")?;
            if pair.len() != 2 {
                return Err(ObjectError::Schema("edge: expected [from, to]".into()));
            }
            g.edge(vertex_from(&pair[0])?, vertex_from(&pair[1])?);
        }
        Ok(g)
    }
}

/// Split a plain run into `Lines` and `Zombie` segments around ghost atoms.
fn split_zombies(atoms: Vec<AtomKey>, ghosts: &HashSet<AtomKey>) -> Vec<Segment> {
    let mut out = Vec::new();
    let mut lines: Vec<AtomKey> = Vec::new();
    let mut i = 0;
    while i < atoms.len() {
        if !ghosts.contains(&atoms[i]) {
            lines.push(atoms[i]);
            i += 1;
            continue;
        }
        if !lines.is_empty() {
            out.push(Segment::Lines(std::mem::take(&mut lines)));
        }
        let mut deleted = Vec::new();
        while i < atoms.len() && ghosts.contains(&atoms[i]) {
            deleted.push(atoms[i]);
            i += 1;
        }
        let mut inserted = Vec::new();
        if i < atoms.len() {
            let ch = atoms[i].0;
            while i < atoms.len() && atoms[i].0 == ch && !ghosts.contains(&atoms[i]) {
                inserted.push(atoms[i]);
                i += 1;
            }
        }
        out.push(Segment::Zombie { deleted, inserted });
    }
    if !lines.is_empty() {
        out.push(Segment::Lines(lines));
    }
    out
}

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

    fn h(n: u8) -> ChangeHash {
        ObjectRef([n; 32])
    }
    fn lines(v: &[&str]) -> Vec<Vec<u8>> {
        v.iter().map(|s| s.as_bytes().to_vec()).collect()
    }
    fn text(g: &FileGraph, m: &Materialized) -> String {
        String::from_utf8(g.render(m, &|c| c.short(4))).unwrap()
    }

    struct Deps(HashMap<ChangeHash, Vec<ChangeHash>>);
    impl DepOracle for Deps {
        fn depends_on(&self, a: &ChangeHash, b: &ChangeHash) -> bool {
            if a == b {
                return true;
            }
            let mut stack = vec![*a];
            let mut seen = HashSet::new();
            while let Some(x) = stack.pop() {
                if let Some(ds) = self.0.get(&x) {
                    for d in ds {
                        if d == b {
                            return true;
                        }
                        if seen.insert(*d) {
                            stack.push(*d);
                        }
                    }
                }
            }
            false
        }
    }

    #[test]
    fn sequential_inserts_and_deletes() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n", "c\n"]));
        let m = g.materialize(&NoDeps);
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "a\nb\nc\n");

        // Change 2 depends on 1, inserts between b and c and deletes a.
        g.insert(h(2), 0, Vertex::Atom((h(1), 1)), Vertex::Atom((h(1), 2)), &lines(&["x\n"]));
        g.delete(h(2), &AtomRange { change: h(1), from: 0, to: 1 });
        let deps = Deps([(h(2), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "b\nx\nc\n");
        assert_eq!(m.live_atoms(), vec![(h(1), 1), (h(2), 0), (h(1), 2)]);
    }

    #[test]
    fn concurrent_inserts_at_same_anchor_conflict() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n"]));
        g.insert(h(2), 0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), &lines(&["from-2\n"]));
        g.insert(h(3), 0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), &lines(&["from-3\n", "more-3\n"]));
        let deps = Deps([(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(m.is_conflicted());
        assert_eq!(m.conflict_count(), 1);
        assert_eq!(
            m.segments,
            vec![
                Segment::Lines(vec![(h(1), 0)]),
                Segment::Conflict(vec![vec![(h(2), 0)], vec![(h(3), 0), (h(3), 1)]]),
                Segment::Lines(vec![(h(1), 1)]),
            ]
        );
        let t = text(&g, &m);
        assert!(t.starts_with("a\n<<<<<<< oxy conflict 1 of 1, side 1 of 2: change 0202\nfrom-2\n======= oxy conflict 1 of 1, side 2 of 2: change 0303\nfrom-3\nmore-3\n>>>>>>> oxy conflict 1 of 1 end\nb\n"), "{t}");
    }

    #[test]
    fn concurrent_inserts_at_different_anchors_do_not_conflict() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n", "c\n"]));
        g.insert(h(2), 0, Vertex::Start, Vertex::Atom((h(1), 0)), &lines(&["top\n"]));
        g.insert(h(3), 0, Vertex::Atom((h(1), 2)), Vertex::End, &lines(&["bottom\n"]));
        let deps = Deps([(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "top\na\nb\nc\nbottom\n");
    }

    #[test]
    fn concurrent_deletes_of_the_same_line_agree() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n"]));
        g.delete(h(2), &AtomRange { change: h(1), from: 0, to: 1 });
        g.delete(h(3), &AtomRange { change: h(1), from: 0, to: 1 });
        let deps = Deps([(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "b\n");
    }

    #[test]
    fn delete_insert_conflict_is_a_zombie() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n"]));
        // 2 deletes a; 3 (concurrent) inserts after a.
        g.delete(h(2), &AtomRange { change: h(1), from: 0, to: 1 });
        g.insert(h(3), 0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), &lines(&["after-a\n"]));
        let deps = Deps([(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(m.is_conflicted());
        assert_eq!(
            m.segments,
            vec![
                Segment::Zombie { deleted: vec![(h(1), 0)], inserted: vec![(h(3), 0)] },
                Segment::Lines(vec![(h(1), 1)]),
            ]
        );
        let t = text(&g, &m);
        assert!(t.contains("-------\na\n======="), "{t}");
        assert!(t.contains("after-a\n>>>>>>>"), "{t}");

        // If the deleter saw the insert (depends on 3), there is no conflict.
        let deps = Deps([(h(2), vec![h(1), h(3)]), (h(3), vec![h(1)])].into_iter().collect());
        let m = g.materialize(&deps);
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "after-a\nb\n");
    }

    #[test]
    fn resolution_collapses_a_conflict() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n"]));
        g.insert(h(2), 0, Vertex::Start, Vertex::Atom((h(1), 0)), &lines(&["x\n"]));
        g.insert(h(3), 0, Vertex::Start, Vertex::Atom((h(1), 0)), &lines(&["y\n"]));
        let mut deps: HashMap<_, _> = [(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect();
        assert!(g.materialize(&Deps(deps.clone())).is_conflicted());
        // Resolution keeps y, drops x.
        g.delete(h(4), &AtomRange { change: h(2), from: 0, to: 1 });
        deps.insert(h(4), vec![h(2), h(3)]);
        let m = g.materialize(&Deps(deps));
        assert!(!m.is_conflicted());
        assert_eq!(text(&g, &m), "y\na\n");
    }

    #[test]
    fn empty_and_unterminated_files() {
        let g = FileGraph::new();
        let m = g.materialize(&NoDeps);
        assert_eq!(m.segments, vec![]);
        assert_eq!(text(&g, &m), "");
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["no newline"]));
        assert_eq!(text(&g, &g.materialize(&NoDeps)), "no newline");
    }

    #[test]
    fn serialization_round_trip() {
        let mut g = FileGraph::new();
        g.insert(h(1), 0, Vertex::Start, Vertex::End, &lines(&["a\n", "b\n"]));
        g.insert(h(2), 0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), &lines(&["x\n"]));
        g.delete(h(3), &AtomRange { change: h(1), from: 1, to: 2 });
        let v = g.to_value();
        let bytes = crate::cbor::encode(&v);
        let back = FileGraph::from_value(&crate::cbor::decode(&bytes).unwrap()).unwrap();
        let deps = Deps([(h(2), vec![h(1)]), (h(3), vec![h(1)])].into_iter().collect());
        assert_eq!(back.materialize(&deps), g.materialize(&deps));
        assert_eq!(text(&back, &back.materialize(&deps)), "a\nx\n");
        assert_eq!(crate::cbor::encode(&back.to_value()), bytes, "deterministic");
    }

    /// Spec §7.3: the graph is a function of the set of changes, not their order.
    #[test]
    fn application_order_is_irrelevant() {
        use rand::seq::SliceRandom;
        type Op = (ChangeHash, Vec<(u32, Vertex, Vertex, Vec<Vec<u8>>)>, Vec<AtomRange>);
        let ops: Vec<Op> = vec![
            (h(1), vec![(0, Vertex::Start, Vertex::End, lines(&["a\n", "b\n", "c\n"]))], vec![]),
            (h(2), vec![(0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), lines(&["p\n"]))], vec![]),
            (h(3), vec![(0, Vertex::Atom((h(1), 0)), Vertex::Atom((h(1), 1)), lines(&["q\n", "r\n"]))], vec![AtomRange { change: h(1), from: 2, to: 3 }]),
            (h(4), vec![(0, Vertex::Atom((h(3), 1)), Vertex::Atom((h(1), 1)), lines(&["s\n"]))], vec![AtomRange { change: h(1), from: 1, to: 2 }]),
            (h(5), vec![(0, Vertex::Atom((h(1), 2)), Vertex::End, lines(&["z\n"]))], vec![]),
        ];
        let deps = Deps(
            [(h(2), vec![h(1)]), (h(3), vec![h(1)]), (h(4), vec![h(3), h(1)]), (h(5), vec![h(1)])].into_iter().collect(),
        );
        let build = |order: &[usize]| {
            let mut g = FileGraph::new();
            for &i in order {
                let (c, ins, dels) = &ops[i];
                for (idx, a, b, l) in ins {
                    g.insert(*c, *idx, *a, *b, l);
                }
                for d in dels {
                    g.delete(*c, d);
                }
            }
            let m = g.materialize(&deps);
            (m.clone(), text(&g, &m))
        };
        let reference = build(&[0, 1, 2, 3, 4]);
        assert!(reference.0.is_conflicted());
        let mut rng = rand::thread_rng();
        let mut order: Vec<usize> = (0..ops.len()).collect();
        for _ in 0..50 {
            order.shuffle(&mut rng);
            assert_eq!(build(&order), reference, "order {order:?}");
        }
    }
}
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