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    use std::fmt;

    /// A representation of byte oriented equivalence classes.
    ///
    /// This is used in an FSM to reduce the size of the transition table. This can
    /// have a particularly large impact not only on the total size of an FSM, but
    /// also on compile times.
    #[derive(Clone, Copy)]
    pub struct ByteClasses([u8; 256]);

    impl ByteClasses {
        /// Creates a new set of equivalence classes where all bytes are mapped to
        /// the same class.
        pub fn empty() -> ByteClasses {
            ByteClasses([0; 256])
        }

        /// Creates a new set of equivalence classes where each byte belongs to
        /// its own equivalence class.
        pub fn singletons() -> ByteClasses {
            let mut classes = ByteClasses::empty();
            for i in 0..256 {
                classes.set(i as u8, i as u8);
            }
            classes
        }

        /// Set the equivalence class for the given byte.
        #[inline]
        pub fn set(&mut self, byte: u8, class: u8) {
            self.0[byte as usize] = class;
        }

        /// Get the equivalence class for the given byte.
        #[inline]
        pub fn get(&self, byte: u8) -> u8 {
            self.0[byte as usize]
        }

        /// Get the equivalence class for the given byte while forcefully
        /// eliding bounds checks.
        #[inline]
        pub unsafe fn get_unchecked(&self, byte: u8) -> u8 {
            *self.0.get_unchecked(byte as usize)
        }

        /// Return the total number of elements in the alphabet represented by
        /// these equivalence classes. Equivalently, this returns the total number
        /// of equivalence classes.
        #[inline]
        pub fn alphabet_len(&self) -> usize {
            self.0[255] as usize + 1
        }

        /// Returns true if and only if every byte in this class maps to its own
        /// equivalence class. Equivalently, there are 256 equivalence classes
        /// and each class contains exactly one byte.
        #[inline]
        pub fn is_singleton(&self) -> bool {
            self.alphabet_len() == 256
        }

        /// Returns an iterator over a sequence of representative bytes from each
        /// equivalence class. Namely, this yields exactly N items, where N is
        /// equivalent to the number of equivalence classes. Each item is an
        /// arbitrary byte drawn from each equivalence class.
        ///
        /// This is useful when one is determinizing an NFA and the NFA's alphabet
        /// hasn't been converted to equivalence classes yet. Picking an arbitrary
        /// byte from each equivalence class then permits a full exploration of
        /// the NFA instead of using every possible byte value.
        pub fn representatives(&self) -> ByteClassRepresentatives {
            ByteClassRepresentatives { classes: self, byte: 0, last_class: None }
        }

        /// Returns all of the bytes in the given equivalence class.
        ///
        /// The second element in the tuple indicates the number of elements in
        /// the array.
        fn elements(&self, equiv: u8) -> ([u8; 256], usize) {
            let (mut array, mut len) = ([0; 256], 0);
            for b in 0..256 {
                if self.get(b as u8) == equiv {
                    array[len] = b as u8;
                    len += 1;
                }
            }
            (array, len)
        }
    }

    impl fmt::Debug for ByteClasses {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            if self.is_singleton() {
                write!(f, "ByteClasses({{singletons}})")
            } else {
                write!(f, "ByteClasses(")?;
                for equiv in 0..self.alphabet_len() {
                    let (members, len) = self.elements(equiv as u8);
                    write!(f, " {} => {:?}", equiv, &members[..len])?;
                }
                write!(f, ")")
            }
        }
    }

    /// An iterator over representative bytes from each equivalence class.
    #[derive(Debug)]
    pub struct ByteClassRepresentatives<'a> {
        classes: &'a ByteClasses,
        byte: usize,
        last_class: Option<u8>,
    }

    impl<'a> Iterator for ByteClassRepresentatives<'a> {
        type Item = u8;

        fn next(&mut self) -> Option<u8> {
            while self.byte < 256 {
                let byte = self.byte as u8;
                let class = self.classes.get(byte);
                self.byte += 1;

                if self.last_class != Some(class) {
                    self.last_class = Some(class);
                    return Some(byte);
                }
            }
            None
        }
    }

    /// A byte class builder keeps track of an *approximation* of equivalence
    /// classes of bytes during NFA construction. That is, every byte in an
    /// equivalence class cannot discriminate between a match and a non-match.
    ///
    /// For example, in the literals `abc` and `xyz`, the bytes [\x00-`], [d-w]
    /// and [{-\xFF] never discriminate between a match and a non-match, precisely
    /// because they never occur in the literals anywhere.
    ///
    /// Note though that this does not necessarily compute the minimal set of
    /// equivalence classes. For example, in the literals above, the byte ranges
    /// [\x00-`], [d-w] and [{-\xFF] are all treated as distinct equivalence
    /// classes even though they could be treated a single class. The reason for
    /// this is implementation complexity. In the future, we should endeavor to
    /// compute the minimal equivalence classes since they can have a rather large
    /// impact on the size of the DFA.
    ///
    /// The representation here is 256 booleans, all initially set to false. Each
    /// boolean maps to its corresponding byte based on position. A `true` value
    /// indicates the end of an equivalence class, where its corresponding byte
    /// and all of the bytes corresponding to all previous contiguous `false`
    /// values are in the same equivalence class.
    ///
    /// This particular representation only permits contiguous ranges of bytes to
    /// be in the same equivalence class, which means that we can never discover
    /// the true minimal set of equivalence classes.
    #[derive(Debug)]
    pub struct ByteClassBuilder(Vec<bool>);

    impl ByteClassBuilder {
        /// Create a new builder of byte classes where all bytes are part of the
        /// same equivalence class.
        pub fn new() -> ByteClassBuilder {
            ByteClassBuilder(vec![false; 256])
        }

        /// Indicate the the range of byte given (inclusive) can discriminate a
        /// match between it and all other bytes outside of the range.
        pub fn set_range(&mut self, start: u8, end: u8) {
            debug_assert!(start <= end);
            if start > 0 {
                self.0[start as usize - 1] = true;
            }
            self.0[end as usize] = true;
        }

        /// Build byte classes that map all byte values to their corresponding
        /// equivalence class. The last mapping indicates the largest equivalence
        /// class identifier (which is never bigger than 255).
        pub fn build(&self) -> ByteClasses {
            let mut classes = ByteClasses::empty();
            let mut class = 0u8;
            let mut i = 0;
            loop {
                classes.set(i as u8, class as u8);
                if i >= 255 {
                    break;
                }
                if self.0[i] {
                    class = class.checked_add(1).unwrap();
                }
                i += 1;
            }
            classes
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn byte_classes() {
            let mut set = ByteClassBuilder::new();
            set.set_range(b'a', b'z');

            let classes = set.build();
            assert_eq!(classes.get(0), 0);
            assert_eq!(classes.get(1), 0);
            assert_eq!(classes.get(2), 0);
            assert_eq!(classes.get(b'a' - 1), 0);
            assert_eq!(classes.get(b'a'), 1);
            assert_eq!(classes.get(b'm'), 1);
            assert_eq!(classes.get(b'z'), 1);
            assert_eq!(classes.get(b'z' + 1), 2);
            assert_eq!(classes.get(254), 2);
            assert_eq!(classes.get(255), 2);

            let mut set = ByteClassBuilder::new();
            set.set_range(0, 2);
            set.set_range(4, 6);
            let classes = set.build();
            assert_eq!(classes.get(0), 0);
            assert_eq!(classes.get(1), 0);
            assert_eq!(classes.get(2), 0);
            assert_eq!(classes.get(3), 1);
            assert_eq!(classes.get(4), 2);
            assert_eq!(classes.get(5), 2);
            assert_eq!(classes.get(6), 2);
            assert_eq!(classes.get(7), 3);
            assert_eq!(classes.get(255), 3);
        }

        #[test]
        fn full_byte_classes() {
            let mut set = ByteClassBuilder::new();
            for i in 0..256u16 {
                set.set_range(i as u8, i as u8);
            }
            assert_eq!(set.build().alphabet_len(), 256);
        }
    }

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