1 files changed, 109 insertions, 0 deletions
diff --git a/compiler/rustc_data_structures/src/graph/vec_graph/mod.rs b/compiler/rustc_data_structures/src/graph/vec_graph/mod.rs
new file mode 100644
index 000000000..3d91bcade
--- /dev/null
+++ b/compiler/rustc_data_structures/src/graph/vec_graph/mod.rs
@@ -0,0 +1,109 @@
+use std::cmp::Ord;
+
+use crate::graph::{DirectedGraph, GraphSuccessors, WithNumEdges, WithNumNodes, WithSuccessors};
+use rustc_index::vec::{Idx, IndexVec};
+
+#[cfg(test)]
+mod tests;
+
+pub struct VecGraph<N: Idx> {
+    /// Maps from a given node to an index where the set of successors
+    /// for that node starts. The index indexes into the `edges`
+    /// vector. To find the range for a given node, we look up the
+    /// start for that node and then the start for the next node
+    /// (i.e., with an index 1 higher) and get the range between the
+    /// two. This vector always has an extra entry so that this works
+    /// even for the max element.
+    node_starts: IndexVec<N, usize>,
+
+    edge_targets: Vec<N>,
+}
+
+impl<N: Idx + Ord> VecGraph<N> {
+    pub fn new(num_nodes: usize, mut edge_pairs: Vec<(N, N)>) -> Self {
+        // Sort the edges by the source -- this is important.
+        edge_pairs.sort();
+
+        let num_edges = edge_pairs.len();
+
+        // Store the *target* of each edge into `edge_targets`.
+        let edge_targets: Vec<N> = edge_pairs.iter().map(|&(_, target)| target).collect();
+
+        // Create the *edge starts* array. We are iterating over 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.
+        let mut node_starts = IndexVec::with_capacity(num_edges);
+        for (index, &(source, _)) in edge_pairs.iter().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);
+            }
+        }
+
+        // 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());
+        }
+
+        assert_eq!(node_starts.len(), num_nodes + 1);
+
+        Self { node_starts, edge_targets }
+    }
+
+    /// Gets the successors for `source` as a slice.
+    pub fn successors(&self, source: N) -> &[N] {
+        let start_index = self.node_starts[source];
+        let end_index = self.node_starts[source.plus(1)];
+        &self.edge_targets[start_index..end_index]
+    }
+}
+
+impl<N: Idx> DirectedGraph for VecGraph<N> {
+    type Node = N;
+}
+
+impl<N: Idx> WithNumNodes for VecGraph<N> {
+    fn num_nodes(&self) -> usize {
+        self.node_starts.len() - 1
+    }
+}
+
+impl<N: Idx> WithNumEdges for VecGraph<N> {
+    fn num_edges(&self) -> usize {
+        self.edge_targets.len()
+    }
+}
+
+impl<'graph, N: Idx> GraphSuccessors<'graph> for VecGraph<N> {
+    type Item = N;
+
+    type Iter = std::iter::Cloned<std::slice::Iter<'graph, N>>;
+}
+
+impl<N: Idx + Ord> WithSuccessors for VecGraph<N> {
+    fn successors(&self, node: N) -> <Self as GraphSuccessors<'_>>::Iter {
+        self.successors(node).iter().cloned()
+    }
+}