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    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: http://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, Debug)]
    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 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() }
    }

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