Add an opt-in to store incoming edges in VecGraph + misc
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,99 +1,235 @@ | ||
| use crate::graph::{DirectedGraph, NumEdges, Predecessors, Successors}; | ||
| use rustc_index::{Idx, IndexVec}; | ||
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| #[cfg(test)] | ||
| mod tests; | ||
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| /// A directed graph, efficient for cases where node indices are pre-existing. | ||
| /// | ||
| /// If `BR` is true, the graph will store back-references, allowing you to get predecessors. | ||
| pub struct VecGraph<N: Idx, const BR: bool = false> { | ||
| // This is basically a `HashMap<N, (Vec<N>, If<BR, Vec<N>>)>` -- a map from a node index, to | ||
| // a list of targets of outgoing edges and (if enabled) a list of sources of incoming edges. | ||
| // | ||
| // However, it is condensed into two arrays as an optimization. | ||
| // | ||
| // `node_starts[n]` is the start of the list of targets of outgoing edges for node `n`. | ||
| // So you can get node's successors with `edge_targets[node_starts[n]..node_starts[n + 1]]`. | ||
| // | ||
| // If `BR` is true (back references are enabled), then `node_starts[n + edge_count]` is the | ||
| // start of the list of *sources* of incoming edges. You can get predecessors of a node | ||
| // similarly to its successors but offsetting by `edge_count`. `edge_count` is | ||
| // `edge_targets.len()/2` (again, in case BR is true) because half of the vec is back refs. | ||
| // | ||
| // All of this might be confusing, so here is an example graph and its representation: | ||
| // | ||
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| // n3 ----+ | ||
| // ^ | (if BR = true) | ||
| // | v outgoing edges incoming edges | ||
| // n0 -> n1 -> n2 ______________ __________________ | ||
| // / \ / \ | ||
| // node indices[1]: n0, n1, n2, n3, n0, n1, n2, n3, n/a | ||
| // vec indices: n0, n1, n2, n3, n4, n5, n6, n7, n8 | ||
| // node_starts: [0, 1, 3, 4 4, 4, 5, 7, 8] | ||
| // | | | | | | | | | | ||
| // | | +---+ +---+ | +---+ | | ||
| // | | | | | | | | ||
| // v v v v v v v | ||
| // edge_targets: [n1, n2, n3, n2 n0, n1, n3, n1] | ||
| // / \____/ | | \____/ \ | ||
| // n0->n1 / | | \ n3<-n1 | ||
| // / n3->n2 [2] n1<-n0 [2] \ | ||
| // n1->n2, n1->n3 n2<-n1, n2<-n3 | ||
| // | ||
| // The incoming edges are basically stored in the same way as outgoing edges, but offset and | ||
| // the graph they store is the inverse of the original. Last index in the `node_starts` array | ||
| // always points to one-past-the-end, so that we don't need to bound check `node_starts[n + 1]` | ||
| // | ||
| // [1]: "node indices" are the indices a user of `VecGraph` might use, | ||
| // note that they are different from "vec indices", | ||
| // which are the real indices you need to index `node_starts` | ||
| // | ||
| // [2]: Note that even though n2 also points to here, | ||
| // the next index also points here, so n2 has no | ||
| // successors (`edge_targets[3..3] = []`). | ||
| // Similarly with n0 and incoming edges | ||
| // | ||
| // If this is still confusing... then sorry :( | ||
| // | ||
| /// Indices into `edge_targets` that signify a start of list of edges. | ||
| node_starts: IndexVec<N, usize>, | ||
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| /// Targets (or sources for back refs) of edges | ||
| edge_targets: Vec<N>, | ||
| } | ||
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| impl<N: Idx + Ord, const BR: bool> VecGraph<N, BR> { | ||
| pub fn new(num_nodes: usize, mut edge_pairs: Vec<(N, N)>) -> Self { | ||
| let num_edges = edge_pairs.len(); | ||
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| let nodes_cap = match BR { | ||
| // +1 for special entry at the end, pointing one past the end of `edge_targets` | ||
| false => num_nodes + 1, | ||
| // *2 for back references | ||
| true => (num_nodes * 2) + 1, | ||
| }; | ||
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| let edges_cap = match BR { | ||
| false => num_edges, | ||
| // *2 for back references | ||
| true => num_edges * 2, | ||
| }; | ||
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| let mut node_starts = IndexVec::with_capacity(nodes_cap); | ||
| let mut edge_targets = Vec::with_capacity(edges_cap); | ||
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| // Sort the edges by the source -- this is important. | ||
| edge_pairs.sort(); | ||
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| // Fill forward references | ||
| create_index( | ||
| num_nodes, | ||
| &mut edge_pairs.iter().map(|&(src, _)| src), | ||
| &mut edge_pairs.iter().map(|&(_, tgt)| tgt), | ||
| &mut edge_targets, | ||
| &mut node_starts, | ||
| ); | ||
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| // Fill back references | ||
| if BR { | ||
| // Pop the special "last" entry, it will be replaced by first back ref | ||
| node_starts.pop(); | ||
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| // Re-sort the edges so that they are sorted by target | ||
| edge_pairs.sort_by_key(|&(src, tgt)| (tgt, src)); | ||
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| create_index( | ||
| // Back essentially double the number of nodes | ||
| num_nodes * 2, | ||
| // NB: the source/target are switched here too | ||
| // NB: we double the key index, so that we can later use *2 to get the back references | ||
| &mut edge_pairs.iter().map(|&(_, tgt)| N::new(tgt.index() + num_nodes)), | ||
| &mut edge_pairs.iter().map(|&(src, _)| src), | ||
| &mut edge_targets, | ||
| &mut node_starts, | ||
| ); | ||
| } | ||
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| Self { node_starts, edge_targets } | ||
| } | ||
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| /// Gets the successors for `source` as a slice. | ||
| pub fn successors(&self, source: N) -> &[N] { | ||
| assert!(source.index() < self.num_nodes()); | ||
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| let start_index = self.node_starts[source]; | ||
| let end_index = self.node_starts[source.plus(1)]; | ||
| &self.edge_targets[start_index..end_index] | ||
| } | ||
| } | ||
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| impl<N: Idx + Ord> VecGraph<N, true> { | ||
| /// Gets the predecessors for `target` as a slice. | ||
| pub fn predecessors(&self, target: N) -> &[N] { | ||
| assert!(target.index() < self.num_nodes()); | ||
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| let target = N::new(target.index() + self.num_nodes()); | ||
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| let start_index = self.node_starts[target]; | ||
| let end_index = self.node_starts[target.plus(1)]; | ||
| &self.edge_targets[start_index..end_index] | ||
| } | ||
| } | ||
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| /// Creates/initializes the index for the [`VecGraph`]. A helper for [`VecGraph::new`]. | ||
| /// | ||
| /// - `num_nodes` is the target number of nodes in the graph | ||
| /// - `sorted_edge_sources` are the edge sources, sorted | ||
| /// - `associated_edge_targets` are the edge *targets* in the same order as sources | ||
| /// - `edge_targets` is the vec of targets to be extended | ||
| /// - `node_starts` is the index to be filled | ||
| fn create_index<N: Idx + Ord>( | ||
| num_nodes: usize, | ||
| sorted_edge_sources: &mut dyn Iterator<Item = N>, | ||
| associated_edge_targets: &mut dyn Iterator<Item = N>, | ||
| edge_targets: &mut Vec<N>, | ||
| node_starts: &mut IndexVec<N, usize>, | ||
| ) { | ||
| let offset = edge_targets.len(); | ||
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| // Store the *target* of each edge into `edge_targets`. | ||
| edge_targets.extend(associated_edge_targets); | ||
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| // Create the *edge starts* array. We are iterating over the | ||
| // (sorted) edge pairs. We maintain the invariant that the | ||
| // length of the `node_starts` array is enough to store the | ||
| // current source node -- so when we see that the source node | ||
| // for an edge is greater than the current length, we grow the | ||
| // edge-starts array by just enough. | ||
| for (index, source) in sorted_edge_sources.enumerate() { | ||
| // If we have a list like `[(0, x), (2, y)]`: | ||
| // | ||
| // - Start out with `node_starts` of `[]` | ||
| // - Iterate to `(0, x)` at index 0: | ||
| // - Push one entry because `node_starts.len()` (0) is <= the source (0) | ||
| // - Leaving us with `node_starts` of `[0]` | ||
| // - Iterate to `(2, y)` at index 1: | ||
| // - Push one entry because `node_starts.len()` (1) is <= the source (2) | ||
| // - Push one entry because `node_starts.len()` (2) is <= the source (2) | ||
| // - Leaving us with `node_starts` of `[0, 1, 1]` | ||
| // - Loop terminates | ||
| while node_starts.len() <= source.index() { | ||
| node_starts.push(index + offset); | ||
| } | ||
| } | ||
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| // Pad out the `node_starts` array so that it has `num_nodes + | ||
| // 1` entries. Continuing our example above, if `num_nodes` is | ||
| // be `3`, we would push one more index: `[0, 1, 1, 2]`. | ||
| // | ||
| // Interpretation of that vector: | ||
| // | ||
| // [0, 1, 1, 2] | ||
| // ---- range for N=2 | ||
| // ---- range for N=1 | ||
| // ---- range for N=0 | ||
| while node_starts.len() <= num_nodes { | ||
| node_starts.push(edge_targets.len()); | ||
| } | ||
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| assert_eq!(node_starts.len(), num_nodes + 1); | ||
| } | ||
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| impl<N: Idx, const BR: bool> DirectedGraph for VecGraph<N, BR> { | ||
| type Node = N; | ||
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| fn num_nodes(&self) -> usize { | ||
| match BR { | ||
| false => self.node_starts.len() - 1, | ||
| // If back refs are enabled, half of the array is said back refs | ||
| true => (self.node_starts.len() - 1) / 2, | ||
| } | ||
| } | ||
| } | ||
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| impl<N: Idx, const BR: bool> NumEdges for VecGraph<N, BR> { | ||
| fn num_edges(&self) -> usize { | ||
| match BR { | ||
| false => self.edge_targets.len(), | ||
| // If back refs are enabled, half of the array is reversed edges for them | ||
| true => self.edge_targets.len() / 2, | ||
| } | ||
| } | ||
| } | ||
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| impl<N: Idx + Ord, const BR: bool> Successors for VecGraph<N, BR> { | ||
| fn successors(&self, node: N) -> impl Iterator<Item = Self::Node> { | ||
| self.successors(node).iter().cloned() | ||
| } | ||
| } | ||
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| impl<N: Idx + Ord> Predecessors for VecGraph<N, true> { | ||
| fn predecessors(&self, node: Self::Node) -> impl Iterator<Item = Self::Node> { | ||
| self.predecessors(node).iter().cloned() | ||
| } | ||
| } | ||
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nit: could you add a basic test or two for the predecessors functionality? (I think you also need to update some of the existing tests to compile with the extra generic parameter) 🙂
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I did not need to update existing tests, since the new generic parameter has a default.
I added a test for predecessors + added predecessor-aware mode for existing tests (so that we are more sure that enabling predecessors doesn't break other stuff).