    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54
    55
    56
    57
    58
    59
    60
    61
    62
    63
    64
    65
    66
    67
    68
    69
    70
    71
    72
    73
    74
    75
    76
    77
    78
    79
    80
    81
    82
    83
    84
    85
    86
    87
    88
    89
    90
    91
    92
    93
    94
    95
    96
    97
    98
    99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254

    //! 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);
        }
    }