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    use std::char;
    use std::cmp;
    use std::fmt::Debug;
    use std::slice;
    use std::u8;

    // This module contains an *internal* implementation of interval sets.
    //
    // The primary invariant that interval sets guards is canonical ordering. That
    // is, every interval set contains an ordered sequence of intervals where
    // no two intervals are overlapping or adjacent. While this invariant is
    // occasionally broken within the implementation, it should be impossible for
    // callers to observe it.
    //
    // Since case folding (as implemented below) breaks that invariant, we roll
    // that into this API even though it is a little out of place in an otherwise
    // generic interval set.
    //
    // Some of the implementation complexity here is a result of me wanting to
    // preserve the sequential representation without using additional memory.
    // In many cases, we do use linear extra memory, but it is at most 2x and it
    // is amortized. If we relaxed the memory requirements, this implementation
    // could become much simpler. The extra memory is honestly probably OK, but
    // character classes (especially of the Unicode variety) can become quite
    // large, and it would be nice to keep regex compilation snappy even in debug
    // builds. (In the past, I have been careless with this area of code and it has
    // caused slow regex compilations in debug mode, so this isn't entirely
    // unwarranted.)
    //
    // Tests on this are relegated to the public API of HIR in src/hir.rs.

    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct IntervalSet<I> {
        ranges: Vec<I>,
    }

    impl<I: Interval> IntervalSet<I> {
        /// Create a new set from a sequence of intervals. Each interval is
        /// specified as a pair of bounds, where both bounds are inclusive.
        ///
        /// The given ranges do not need to be in any specific order, and ranges
        /// may overlap.
        pub fn new<T: IntoIterator<Item=I>>(intervals: T) -> IntervalSet<I> {
            let mut set = IntervalSet { ranges: intervals.into_iter().collect() };
            set.canonicalize();
            set
        }

        /// Add a new interval to this set.
        pub fn push(&mut self, interval: I) {
            // TODO: This could be faster. e.g., Push the interval such that
            // it preserves canonicalization.
            self.ranges.push(interval);
            self.canonicalize();
        }

        /// Return an iterator over all intervals in this set.
        ///
        /// The iterator yields intervals in ascending order.
        pub fn iter(&self) -> IntervalSetIter<I> {
            IntervalSetIter(self.ranges.iter())
        }

        /// Return an immutable slice of intervals in this set.
        ///
        /// The sequence returned is in canonical ordering.
        pub fn intervals(&self) -> &[I] {
            &self.ranges
        }

        /// Expand this interval set such that it contains all case folded
        /// characters. For example, if this class consists of the range `a-z`,
        /// then applying case folding will result in the class containing both the
        /// ranges `a-z` and `A-Z`.
        pub fn case_fold_simple(&mut self) {
            let len = self.ranges.len();
            for i in 0..len {
                let range = self.ranges[i];
                range.case_fold_simple(&mut self.ranges);
            }
            self.canonicalize();
        }

        /// Union this set with the given set, in place.
        pub fn union(&mut self, other: &IntervalSet<I>) {
            // This could almost certainly be done more efficiently.
            self.ranges.extend(&other.ranges);
            self.canonicalize();
        }

        /// Intersect this set with the given set, in place.
        pub fn intersect(&mut self, other: &IntervalSet<I>) {
            if self.ranges.is_empty() {
                return;
            }
            if other.ranges.is_empty() {
                self.ranges.clear();
                return;
            }

            // There should be a way to do this in-place with constant memory,
            // but I couldn't figure out a simple way to do it. So just append
            // the intersection to the end of this range, and then drain it before
            // we're done.
            let drain_end = self.ranges.len();

            let mut ita = (0..drain_end).into_iter();
            let mut itb = (0..other.ranges.len()).into_iter();
            let mut a = ita.next().unwrap();
            let mut b = itb.next().unwrap();
            loop {
                if let Some(ab) = self.ranges[a].intersect(&other.ranges[b]) {
                    self.ranges.push(ab);
                }
                let (it, aorb) =
                    if self.ranges[a].upper() < other.ranges[b].upper() {
                        (&mut ita, &mut a)
                    } else {
                        (&mut itb, &mut b)
                    };
                match it.next() {
                    Some(v) => *aorb = v,
                    None => break,
                }
            }
            self.ranges.drain(..drain_end);
        }

        /// Subtract the given set from this set, in place.
        pub fn difference(&mut self, other: &IntervalSet<I>) {
            if self.ranges.is_empty() || other.ranges.is_empty() {
                return;
            }

            // This algorithm is (to me) surprisingly complex. A search of the
            // interwebs indicate that this is a potentially interesting problem.
            // Folks seem to suggest interval or segment trees, but I'd like to
            // avoid the overhead (both runtime and conceptual) of that.
            //
            // The following is basically my Shitty First Draft. Therefore, in
            // order to grok it, you probably need to read each line carefully.
            // Simplifications are most welcome!
            //
            // Remember, we can assume the canonical format invariant here, which
            // says that all ranges are sorted, not overlapping and not adjacent in
            // each class.
            let drain_end = self.ranges.len();
            let (mut a, mut b) = (0, 0);
        'LOOP:
            while a < drain_end && b < other.ranges.len() {
                // Basically, the easy cases are when neither range overlaps with
                // each other. If the `b` range is less than our current `a`
                // range, then we can skip it and move on.
                if other.ranges[b].upper() < self.ranges[a].lower() {
                    b += 1;
                    continue;
                }
                // ... similarly for the `a` range. If it's less than the smallest
                // `b` range, then we can add it as-is.
                if self.ranges[a].upper() < other.ranges[b].lower() {
                    let range = self.ranges[a];
                    self.ranges.push(range);
                    a += 1;
                    continue;
                }
                // Otherwise, we have overlapping ranges.
                assert!(!self.ranges[a].is_intersection_empty(&other.ranges[b]));

                // This part is tricky and was non-obvious to me without looking
                // at explicit examples (see the tests). The trickiness stems from
                // two things: 1) subtracting a range from another range could
                // yield two ranges and 2) after subtracting a range, it's possible
                // that future ranges can have an impact. The loop below advances
                // the `b` ranges until they can't possible impact the current
                // range.
                //
                // For example, if our `a` range is `a-t` and our next three `b`
                // ranges are `a-c`, `g-i`, `r-t` and `x-z`, then we need to apply
                // subtraction three times before moving on to the next `a` range.
                let mut range = self.ranges[a];
                while b < other.ranges.len()
                    && !range.is_intersection_empty(&other.ranges[b])
                {
                    let old_range = range;
                    range = match range.difference(&other.ranges[b]) {
                        (None, None) => {
                            // We lost the entire range, so move on to the next
                            // without adding this one.
                            a += 1;
                            continue 'LOOP;
                        }
                        (Some(range1), None) | (None, Some(range1)) => range1,
                        (Some(range1), Some(range2)) => {
                            self.ranges.push(range1);
                            range2
                        }
                    };
                    // It's possible that the `b` range has more to contribute
                    // here. In particular, if it is greater than the original
                    // range, then it might impact the next `a` range *and* it
                    // has impacted the current `a` range as much as possible,
                    // so we can quit. We don't bump `b` so that the next `a`
                    // range can apply it.
                    if other.ranges[b].upper() > old_range.upper() {
                        break;
                    }
                    // Otherwise, the next `b` range might apply to the current
                    // `a` range.
                    b += 1;
                }
                self.ranges.push(range);
                a += 1;
            }
            while a < drain_end {
                let range = self.ranges[a];
                self.ranges.push(range);
                a += 1;
            }
            self.ranges.drain(..drain_end);
        }

        /// Compute the symmetric difference of the two sets, in place.
        ///
        /// This computes the symmetric difference of two interval sets. This
        /// removes all elements in this set that are also in the given set,
        /// but also adds all elements from the given set that aren't in this
        /// set. That is, the set will contain all elements in either set,
        /// but will not contain any elements that are in both sets.
        pub fn symmetric_difference(&mut self, other: &IntervalSet<I>) {
            // TODO(burntsushi): Fix this so that it amortizes allocation.
            let mut intersection = self.clone();
            intersection.intersect(other);
            self.union(other);
            self.difference(&intersection);
        }

        /// Negate this interval set.
        ///
        /// For all `x` where `x` is any element, if `x` was in this set, then it
        /// will not be in this set after negation.
        pub fn negate(&mut self) {
            if self.ranges.is_empty() {
                let (min, max) = (I::Bound::min_value(), I::Bound::max_value());
                self.ranges.push(I::create(min, max));
                return;
            }

            // There should be a way to do this in-place with constant memory,
            // but I couldn't figure out a simple way to do it. So just append
            // the negation to the end of this range, and then drain it before
            // we're done.
            let drain_end = self.ranges.len();

            // We do checked arithmetic below because of the canonical ordering
            // invariant.
            if self.ranges[0].lower() > I::Bound::min_value() {
                let upper = self.ranges[0].lower().decrement();
                self.ranges.push(I::create(I::Bound::min_value(), upper));
            }
            for i in 1..drain_end {
                let lower = self.ranges[i - 1].upper().increment();
                let upper = self.ranges[i].lower().decrement();
                self.ranges.push(I::create(lower, upper));
            }
            if self.ranges[drain_end - 1].upper() < I::Bound::max_value() {
                let lower = self.ranges[drain_end - 1].upper().increment();
                self.ranges.push(I::create(lower, I::Bound::max_value()));
            }
            self.ranges.drain(..drain_end);
        }

        /// Converts this set into a canonical ordering.
        fn canonicalize(&mut self) {
            if self.is_canonical() {
                return;
            }
            self.ranges.sort();
            assert!(!self.ranges.is_empty());

            // Is there a way to do this in-place with constant memory? I couldn't
            // figure out a way to do it. So just append the canonicalization to
            // the end of this range, and then drain it before we're done.
            let drain_end = self.ranges.len();
            for oldi in 0..drain_end {
                // If we've added at least one new range, then check if we can
                // merge this range in the previously added range.
                if self.ranges.len() > drain_end {
                    let (last, rest) = self.ranges.split_last_mut().unwrap();
                    if let Some(union) = last.union(&rest[oldi]) {
                        *last = union;
                        continue;
                    }
                }
                let range = self.ranges[oldi];
                self.ranges.push(range);
            }
            self.ranges.drain(..drain_end);
        }

        /// Returns true if and only if this class is in a canonical ordering.
        fn is_canonical(&self) -> bool {
            for pair in self.ranges.windows(2) {
                if pair[0] >= pair[1] {
                    return false;
                }
                if pair[0].is_contiguous(&pair[1]) {
                    return false;
                }
            }
            true
        }
    }

    /// An iterator over intervals.
    #[derive(Debug)]
    pub struct IntervalSetIter<'a, I: 'a>(slice::Iter<'a, I>);

    impl<'a, I> Iterator for IntervalSetIter<'a, I> {
        type Item = &'a I;

        fn next(&mut self) -> Option<&'a I> {
            self.0.next()
        }
    }

    pub trait Interval:
        Clone + Copy + Debug + Default + Eq + PartialEq + PartialOrd + Ord
    {
        type Bound: Bound;

        fn lower(&self) -> Self::Bound;
        fn upper(&self) -> Self::Bound;
        fn set_lower(&mut self, bound: Self::Bound);
        fn set_upper(&mut self, bound: Self::Bound);
        fn case_fold_simple(&self, intervals: &mut Vec<Self>);

        /// Create a new interval.
        fn create(lower: Self::Bound, upper: Self::Bound) -> Self {
            let mut int = Self::default();
            if lower <= upper {
                int.set_lower(lower);
                int.set_upper(upper);
            } else {
                int.set_lower(upper);
                int.set_upper(lower);
            }
            int
        }

        /// Union the given overlapping range into this range.
        ///
        /// If the two ranges aren't contiguous, then this returns `None`.
        fn union(&self, other: &Self) -> Option<Self> {
            if !self.is_contiguous(other) {
                return None;
            }
            let lower = cmp::min(self.lower(), other.lower());
            let upper = cmp::max(self.upper(), other.upper());
            Some(Self::create(lower, upper))
        }

        /// Intersect this range with the given range and return the result.
        ///
        /// If the intersection is empty, then this returns `None`.
        fn intersect(&self, other: &Self) -> Option<Self> {
            let lower = cmp::max(self.lower(), other.lower());
            let upper = cmp::min(self.upper(), other.upper());
            if lower <= upper {
                Some(Self::create(lower, upper))
            } else {
                None
            }
        }

        /// Subtract the given range from this range and return the resulting
        /// ranges.
        ///
        /// If subtraction would result in an empty range, then no ranges are
        /// returned.
        fn difference(&self, other: &Self) -> (Option<Self>, Option<Self>) {
            if self.is_subset(other) {
                return (None, None);
            }
            if self.is_intersection_empty(other) {
                return (Some(self.clone()), None);
            }
            let add_lower = other.lower() > self.lower();
            let add_upper = other.upper() < self.upper();
            // We know this because !self.is_subset(other) and the ranges have
            // a non-empty intersection.
            assert!(add_lower || add_upper);
            let mut ret = (None, None);
            if add_lower {
                let upper = other.lower().decrement();
                ret.0 = Some(Self::create(self.lower(), upper));
            }
            if add_upper {
                let lower = other.upper().increment();
                let range = Self::create(lower, self.upper());
                if ret.0.is_none() {
                    ret.0 = Some(range);
                } else {
                    ret.1 = Some(range);
                }
            }
            ret
        }

        /// Compute the symmetric difference the given range from this range. This
        /// returns the union of the two ranges minus its intersection.
        fn symmetric_difference(
            &self,
            other: &Self,
        ) -> (Option<Self>, Option<Self>) {
            let union = match self.union(other) {
                None => return (Some(self.clone()), Some(other.clone())),
                Some(union) => union,
            };
            let intersection = match self.intersect(other) {
                None => return (Some(self.clone()), Some(other.clone())),
                Some(intersection) => intersection,
            };
            union.difference(&intersection)
        }

        /// Returns true if and only if the two ranges are contiguous. Two ranges
        /// are contiguous if and only if the ranges are either overlapping or
        /// adjacent.
        fn is_contiguous(&self, other: &Self) -> bool {
            let lower1 = self.lower().as_u32();
            let upper1 = self.upper().as_u32();
            let lower2 = other.lower().as_u32();
            let upper2 = other.upper().as_u32();
            cmp::max(lower1, lower2) <= cmp::min(upper1, upper2).saturating_add(1)
        }

        /// Returns true if and only if the intersection of this range and the
        /// other range is empty.
        fn is_intersection_empty(&self, other: &Self) -> bool {
            let (lower1, upper1) = (self.lower(), self.upper());
            let (lower2, upper2) = (other.lower(), other.upper());
            cmp::max(lower1, lower2) > cmp::min(upper1, upper2)
        }

        /// Returns true if and only if this range is a subset of the other range.
        fn is_subset(&self, other: &Self) -> bool {
            let (lower1, upper1) = (self.lower(), self.upper());
            let (lower2, upper2) = (other.lower(), other.upper());
            (lower2 <= lower1 && lower1 <= upper2)
            && (lower2 <= upper1 && upper1 <= upper2)
        }
    }

    pub trait Bound: Copy + Clone + Debug + Eq + PartialEq + PartialOrd + Ord {
        fn min_value() -> Self;
        fn max_value() -> Self;
        fn as_u32(self) -> u32;
        fn increment(self) -> Self;
        fn decrement(self) -> Self;
    }

    impl Bound for u8 {
        fn min_value() -> Self { u8::MIN }
        fn max_value() -> Self { u8::MAX }
        fn as_u32(self) -> u32 { self as u32 }
        fn increment(self) -> Self { self.checked_add(1).unwrap() }
        fn decrement(self) -> Self { self.checked_sub(1).unwrap() }
    }

    impl Bound for char {
        fn min_value() -> Self { '\x00' }
        fn max_value() -> Self { '\u{10FFFF}' }
        fn as_u32(self) -> u32 { self as u32 }

        fn increment(self) -> Self {
            match self {
                '\u{D7FF}' => '\u{E000}',
                c => char::from_u32((c as u32).checked_add(1).unwrap()).unwrap(),
            }
        }

        fn decrement(self) -> Self {
            match self {
                '\u{E000}' => '\u{D7FF}',
                c => char::from_u32((c as u32).checked_sub(1).unwrap()).unwrap(),
            }
        }
    }

    // Tests for interval sets are written in src/hir.rs against the public API.

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