cargo/util/

dependency_queue.rs

//! A graph-like structure used to represent a set of dependencies and in what
//! order they should be built.
//!
//! This structure is used to store the dependency graph and dynamically update
//! it to figure out when a dependency should be built.
//!
//! Dependencies in this queue are represented as a (node, edge) pair. This is
//! used to model nodes which produce multiple outputs at different times but
//! some nodes may only require one of the outputs and can start before the
//! whole node is finished.

use std::collections::{HashMap, HashSet};
use std::hash::Hash;

#[derive(Debug)]
pub struct DependencyQueue<N: Hash + Eq, E: Hash + Eq, V> {
    /// A list of all known keys to build.
    ///
    /// The value of the hash map is list of dependencies which still need to be
    /// built before the package can be built. Note that the set is dynamically
    /// updated as more dependencies are built.
    dep_map: HashMap<N, (HashSet<(N, E)>, V)>,

    /// A reverse mapping of a package to all packages that depend on that
    /// package.
    ///
    /// This map is statically known and does not get updated throughout the
    /// lifecycle of the `DependencyQueue`.
    ///
    /// This is sort of like a `HashMap<(N, E), HashSet<N>>` map, but more
    /// easily indexable with just an `N`
    reverse_dep_map: HashMap<N, HashMap<E, HashSet<N>>>,

    /// The relative priority of this package. Higher values should be scheduled sooner.
    priority: HashMap<N, usize>,

    /// An expected cost for building this package. Used to determine priority.
    cost: HashMap<N, usize>,
}

impl<N: Hash + Eq, E: Hash + Eq, V> Default for DependencyQueue<N, E, V> {
    fn default() -> DependencyQueue<N, E, V> {
        DependencyQueue::new()
    }
}

impl<N: Hash + Eq, E: Hash + Eq, V> DependencyQueue<N, E, V> {
    /// Creates a new dependency queue with 0 packages.
    pub fn new() -> DependencyQueue<N, E, V> {
        DependencyQueue {
            dep_map: HashMap::new(),
            reverse_dep_map: HashMap::new(),
            priority: HashMap::new(),
            cost: HashMap::new(),
        }
    }
}

impl<N: Hash + Eq + Clone, E: Eq + Hash + Clone, V> DependencyQueue<N, E, V> {
    /// Adds a new node and its dependencies to this queue.
    ///
    /// The `key` specified is a new node in the dependency graph, and the node
    /// depend on all the dependencies iterated by `dependencies`. Each
    /// dependency is a node/edge pair, where edges can be thought of as
    /// productions from nodes (aka if it's just `()` it's just waiting for the
    /// node to finish).
    ///
    /// An optional `value` can also be associated with `key` which is reclaimed
    /// when the node is ready to go.
    ///
    /// The cost parameter can be used to hint at the relative cost of building
    /// this node. This implementation does not care about the units of this value, so
    /// the calling code is free to use whatever they'd like. In general, higher cost
    /// nodes are expected to take longer to build.
    pub fn queue(
        &mut self,
        key: N,
        value: V,
        dependencies: impl IntoIterator<Item = (N, E)>,
        cost: usize,
    ) {
        assert!(!self.dep_map.contains_key(&key));

        let mut my_dependencies = HashSet::new();
        for (dep, edge) in dependencies {
            my_dependencies.insert((dep.clone(), edge.clone()));
            self.reverse_dep_map
                .entry(dep)
                .or_insert_with(HashMap::new)
                .entry(edge)
                .or_insert_with(HashSet::new)
                .insert(key.clone());
        }
        self.dep_map.insert(key.clone(), (my_dependencies, value));
        self.cost.insert(key, cost);
    }

    /// All nodes have been added, calculate some internal metadata and prepare
    /// for `dequeue`.
    pub fn queue_finished(&mut self) {
        let mut out = HashMap::new();
        for key in self.dep_map.keys() {
            depth(key, &self.reverse_dep_map, &mut out);
        }
        self.priority = out
            .into_iter()
            .map(|(n, set)| {
                let total_cost =
                    self.cost[&n] + set.iter().map(|key| self.cost[key]).sum::<usize>();
                (n, total_cost)
            })
            .collect();

        /// Creates a flattened reverse dependency list. For a given key, finds the
        /// set of nodes which depend on it, including transitively. This is different
        /// from `self.reverse_dep_map` because `self.reverse_dep_map` only maps one level
        /// of reverse dependencies.
        fn depth<'a, N: Hash + Eq + Clone, E: Hash + Eq + Clone>(
            key: &N,
            map: &HashMap<N, HashMap<E, HashSet<N>>>,
            results: &'a mut HashMap<N, HashSet<N>>,
        ) -> &'a HashSet<N> {
            if results.contains_key(key) {
                let depth = &results[key];
                assert!(!depth.is_empty(), "cycle in DependencyQueue");
                return depth;
            }
            results.insert(key.clone(), HashSet::new());

            let mut set = HashSet::new();
            set.insert(key.clone());

            for dep in map
                .get(key)
                .into_iter()
                .flat_map(|it| it.values())
                .flatten()
            {
                set.extend(depth(dep, map, results).iter().cloned())
            }

            let slot = results.get_mut(key).unwrap();
            *slot = set;
            &*slot
        }
    }

    /// Dequeues a package that is ready to be built.
    ///
    /// A package is ready to be built when it has 0 un-built dependencies. If
    /// `None` is returned then no packages are ready to be built.
    pub fn dequeue(&mut self) -> Option<(N, V, usize)> {
        let (key, priority) = self
            .dep_map
            .iter()
            .filter(|(_, (deps, _))| deps.is_empty())
            .map(|(key, _)| (key.clone(), self.priority[key]))
            .max_by_key(|(_, priority)| *priority)?;
        let (_, data) = self.dep_map.remove(&key).unwrap();
        Some((key, data, priority))
    }

    /// Returns `true` if there are remaining packages to be built.
    pub fn is_empty(&self) -> bool {
        self.dep_map.is_empty()
    }

    /// Returns the number of remaining packages to be built.
    pub fn len(&self) -> usize {
        self.dep_map.len()
    }

    /// Indicate that something has finished.
    ///
    /// Calling this function indicates that the `node` has produced `edge`. All
    /// remaining work items which only depend on this node/edge pair are now
    /// candidates to start their job.
    ///
    /// Returns the nodes that are now allowed to be dequeued as a result of
    /// finishing this node.
    pub fn finish(&mut self, node: &N, edge: &E) -> Vec<&N> {
        // hashset<Node>
        let reverse_deps = self.reverse_dep_map.get(node).and_then(|map| map.get(edge));
        let Some(reverse_deps) = reverse_deps else {
            return Vec::new();
        };
        let key = (node.clone(), edge.clone());
        let mut result = Vec::new();
        for dep in reverse_deps.iter() {
            let edges = &mut self.dep_map.get_mut(dep).unwrap().0;
            assert!(edges.remove(&key));
            if edges.is_empty() {
                result.push(dep);
            }
        }
        result
    }
}

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

    #[test]
    fn deep_first_equal_cost() {
        let mut q = DependencyQueue::new();

        q.queue(1, (), vec![], 1);
        q.queue(2, (), vec![(1, ())], 1);
        q.queue(3, (), vec![], 1);
        q.queue(4, (), vec![(2, ()), (3, ())], 1);
        q.queue(5, (), vec![(4, ()), (3, ())], 1);
        q.queue_finished();

        assert_eq!(q.dequeue(), Some((1, (), 5)));
        assert_eq!(q.dequeue(), Some((3, (), 4)));
        assert_eq!(q.dequeue(), None);
        q.finish(&3, &());
        assert_eq!(q.dequeue(), None);
        q.finish(&1, &());
        assert_eq!(q.dequeue(), Some((2, (), 4)));
        assert_eq!(q.dequeue(), None);
        q.finish(&2, &());
        assert_eq!(q.dequeue(), Some((4, (), 3)));
        assert_eq!(q.dequeue(), None);
        q.finish(&4, &());
        assert_eq!(q.dequeue(), Some((5, (), 2)));
    }

    #[test]
    fn sort_by_highest_cost() {
        let mut q = DependencyQueue::new();

        q.queue(1, (), vec![], 1);
        q.queue(2, (), vec![(1, ())], 1);
        q.queue(3, (), vec![], 4);
        q.queue(4, (), vec![(2, ()), (3, ())], 1);
        q.queue_finished();

        assert_eq!(q.dequeue(), Some((3, (), 9)));
        assert_eq!(q.dequeue(), Some((1, (), 4)));
        assert_eq!(q.dequeue(), None);
        q.finish(&3, &());
        assert_eq!(q.dequeue(), None);
        q.finish(&1, &());
        assert_eq!(q.dequeue(), Some((2, (), 3)));
        assert_eq!(q.dequeue(), None);
        q.finish(&2, &());
        assert_eq!(q.dequeue(), Some((4, (), 2)));
        assert_eq!(q.dequeue(), None);
        q.finish(&4, &());
        assert_eq!(q.dequeue(), None);
    }
}