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//! 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 total number of packages that are transitively waiting on this package priority: 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(), } } } impl<N: Hash + Eq + Clone, E: Eq + Hash + Clone, V> DependencyQueue<N, E, V> { /// Adds a new ndoe 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. pub fn queue(&mut self, key: N, value: V, dependencies: impl IntoIterator<Item = (N, E)>) { 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, (my_dependencies, value)); } /// 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)| (n, set.len())).collect(); 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)> { // Look at all our crates and find everything that's ready to build (no // deps). After we've got that candidate set select the one which has // the maximum priority in the dependency graph. This way we should // hopefully keep CPUs hottest the longest by ensuring that long // dependency chains are scheduled early on in the build process and the // leafs higher in the tree can fill in the cracks later. // // TODO: it'd be best here to throw in a heuristic of crate size as // well. For example how long did this crate historically take to // compile? How large is its source code? etc. let next = self .dep_map .iter() .filter(|(_, (deps, _))| deps.is_empty()) .map(|(key, _)| key.clone()) .max_by_key(|k| self.priority[k]); let key = match next { Some(key) => key, None => return None, }; let (_, data) = self.dep_map.remove(&key).unwrap(); Some((key, data)) } /// 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 reverse_deps = match reverse_deps { Some(deps) => deps, None => 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() { let mut q = DependencyQueue::new(); q.queue(1, (), vec![]); q.queue(2, (), vec![(1, ())]); q.queue(3, (), vec![]); q.queue(4, (), vec![(2, ()), (3, ())]); q.queue(5, (), vec![(4, ()), (3, ())]); q.queue_finished(); assert_eq!(q.dequeue(), Some((1, ()))); assert_eq!(q.dequeue(), Some((3, ()))); assert_eq!(q.dequeue(), None); q.finish(&3, &()); assert_eq!(q.dequeue(), None); q.finish(&1, &()); assert_eq!(q.dequeue(), Some((2, ()))); assert_eq!(q.dequeue(), None); q.finish(&2, &()); assert_eq!(q.dequeue(), Some((4, ()))); assert_eq!(q.dequeue(), None); q.finish(&4, &()); assert_eq!(q.dequeue(), Some((5, ()))); } }