1 files changed, 84 insertions, 0 deletions
diff --git a/third_party/rust/regex/src/sparse.rs b/third_party/rust/regex/src/sparse.rs
new file mode 100644
index 0000000000..98b726613d
--- /dev/null
+++ b/third_party/rust/regex/src/sparse.rs
@@ -0,0 +1,84 @@
+use std::fmt;
+use std::ops::Deref;
+use std::slice;
+
+/// A sparse set used for representing ordered NFA states.
+///
+/// This supports constant time addition and membership testing. Clearing an
+/// entire set can also be done in constant time. Iteration yields elements
+/// in the order in which they were inserted.
+///
+/// The data structure is based on: https://research.swtch.com/sparse
+/// Note though that we don't actually use uninitialized memory. We generally
+/// reuse allocations, so the initial allocation cost is bareable. However,
+/// its other properties listed above are extremely useful.
+#[derive(Clone)]
+pub struct SparseSet {
+    /// Dense contains the instruction pointers in the order in which they
+    /// were inserted.
+    dense: Vec<usize>,
+    /// Sparse maps instruction pointers to their location in dense.
+    ///
+    /// An instruction pointer is in the set if and only if
+    /// sparse[ip] < dense.len() && ip == dense[sparse[ip]].
+    sparse: Box<[usize]>,
+}
+
+impl SparseSet {
+    pub fn new(size: usize) -> SparseSet {
+        SparseSet {
+            dense: Vec::with_capacity(size),
+            sparse: vec![0; size].into_boxed_slice(),
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.dense.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.dense.is_empty()
+    }
+
+    pub fn capacity(&self) -> usize {
+        self.dense.capacity()
+    }
+
+    pub fn insert(&mut self, value: usize) {
+        let i = self.len();
+        assert!(i < self.capacity());
+        self.dense.push(value);
+        self.sparse[value] = i;
+    }
+
+    pub fn contains(&self, value: usize) -> bool {
+        let i = self.sparse[value];
+        self.dense.get(i) == Some(&value)
+    }
+
+    pub fn clear(&mut self) {
+        self.dense.clear();
+    }
+}
+
+impl fmt::Debug for SparseSet {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "SparseSet({:?})", self.dense)
+    }
+}
+
+impl Deref for SparseSet {
+    type Target = [usize];
+
+    fn deref(&self) -> &Self::Target {
+        &self.dense
+    }
+}
+
+impl<'a> IntoIterator for &'a SparseSet {
+    type Item = &'a usize;
+    type IntoIter = slice::Iter<'a, usize>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}