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    pub const WINDOW_SIZE: usize = 32768;
    pub const WINDOW_MASK: usize = WINDOW_SIZE - 1;
    #[cfg(test)]
    pub const HASH_BYTES: usize = 3;
    const HASH_SHIFT: u16 = 5;
    const HASH_MASK: u16 = WINDOW_MASK as u16;

    /// Helper struct to let us allocate both head and prev in the same block.
    struct Tables {
        /// Starts of hash chains (in prev)
        pub head: [u16; WINDOW_SIZE],
        /// Link to previous occurence of this hash value
        pub prev: [u16; WINDOW_SIZE],
    }

    impl Default for Tables {
        #[inline]
        fn default() -> Tables {
            Tables {
                head: [0;WINDOW_SIZE],
                prev: [0;WINDOW_SIZE],
            }
        }
    }

    impl Tables {
        #[inline]
        fn fill_prev(&mut self) {
            self.prev.copy_from_slice(&self.head);
        }
    }

    /// Create and box the hash chains.
    fn create_tables() -> Box<Tables> {
        // Using default here is a trick to get around the lack of box syntax on stable rust.
        //
        // Box::new([0u16,n]) ends up creating an temporary array on the stack which is not optimised
        // but using default, which simply calls `box value` internally allows us to get around this.
        //
        // We could use vec instead, but using a boxed array helps the compiler optimise
        // away bounds checks as `n & WINDOW_MASK < WINDOW_SIZE` will always be true.
        let mut t: Box<Tables> = Box::default();

        for (n, b) in t.head.iter_mut().enumerate() {
            *b = n as u16;
        }

        t.fill_prev();

        t
    }

    /// Returns a new hash value based on the previous value and the next byte
    #[inline]
    pub fn update_hash(current_hash: u16, to_insert: u8) -> u16 {
        update_hash_conf(current_hash, to_insert, HASH_SHIFT, HASH_MASK)
    }

    #[inline]
    fn update_hash_conf(current_hash: u16, to_insert: u8, shift: u16, mask: u16) -> u16 {
        ((current_hash << shift) ^ (to_insert as u16)) & mask
    }

    #[inline]
    fn reset_array(arr: &mut [u16; WINDOW_SIZE]) {
        for (n, b) in arr.iter_mut().enumerate() {
            *b = n as u16;
        }
    }

    pub struct ChainedHashTable {
        // Current running hash value of the last 3 bytes
        current_hash: u16,
        // Hash chains.
        c: Box<Tables>,
        // Used for testing
        // count: DebugCounter,
    }

    impl ChainedHashTable {
        pub fn new() -> ChainedHashTable {
            ChainedHashTable {
                current_hash: 0,
                c: create_tables(),
                //count: DebugCounter::default(),
            }
        }

        #[cfg(test)]
        pub fn from_starting_values(v1: u8, v2: u8) -> ChainedHashTable {
            let mut t = ChainedHashTable::new();
            t.current_hash = update_hash(t.current_hash, v1);
            t.current_hash = update_hash(t.current_hash, v2);
            t
        }

        /// Resets the hash value and hash chains
        pub fn reset(&mut self) {
            self.current_hash = 0;
            reset_array(&mut self.c.head);
            {
                let h = self.c.head;
                let mut c = self.c.prev;
                c[..].copy_from_slice(&h[..]);
            }
            /*if cfg!(debug_assertions) {
                self.count.reset();
            }*/
        }

        pub fn add_initial_hash_values(&mut self, v1: u8, v2: u8) {
            self.current_hash = update_hash(self.current_hash, v1);
            self.current_hash = update_hash(self.current_hash, v2);
        }

        /// Insert a byte into the hash table
        #[inline]
        pub fn add_hash_value(&mut self, position: usize, value: u8) {
            // Check that all bytes are input in order and at the correct positions.
            // Disabled for now as it breaks when sync flushing.
            /*debug_assert_eq!(
                position & WINDOW_MASK,
                self.count.get() as usize & WINDOW_MASK
            );*/
            debug_assert!(
                position < WINDOW_SIZE * 2,
                "Position is larger than 2 * window size! {}",
                position
            );
            // Storing the hash in a temporary variable here makes the compiler avoid the
            // bounds checks in this function.
            let new_hash = update_hash(self.current_hash, value);

            self.add_with_hash(position, new_hash);

            // Update the stored hash value with the new hash.
            self.current_hash = new_hash;
        }

        /// Directly set the current hash value
        #[inline]
        pub fn set_hash(&mut self, hash: u16) {
            self.current_hash = hash;
        }

        /// Update the tables directly, providing the hash.
        #[inline]
        pub fn add_with_hash(&mut self, position: usize, hash: u16) {
            /*if cfg!(debug_assertions) {
                self.count.add(1);
            }*/

            self.c.prev[position & WINDOW_MASK] = self.c.head[hash as usize];

            // Ignoring any bits over 16 here is deliberate, as we only concern ourselves about
            // where in the buffer (which is 64k bytes) we are referring to.
            self.c.head[hash as usize] = position as u16;
        }

        // Get the head of the hash chain for the current hash value
        #[cfg(test)]
        #[inline]
        pub fn current_head(&self) -> u16 {
            self.c.head[self.current_hash as usize]
        }

        #[inline]
        pub fn current_hash(&self) -> u16 {
            self.current_hash
        }

        #[inline]
        pub fn get_prev(&self, bytes: usize) -> u16 {
            self.c.prev[bytes & WINDOW_MASK]
        }

        #[cfg(test)]
        #[inline]
        pub fn farthest_next(&self, match_pos: usize, match_len: usize) -> usize {
            let to_check = match_len.saturating_sub(2);

            let mut n = 0;
            let mut smallest_prev =
                self.get_prev(match_pos);
            let mut smallest_pos = 0;
            while n < to_check {
                let prev =
                    self.get_prev(match_pos + n);
                if prev < smallest_prev {
                    smallest_prev = prev;
                    smallest_pos = n;
                }
                n += 1;
            }
            smallest_pos
        }

        #[inline]
        fn slide_value(b: u16, pos: u16, bytes: u16) -> u16 {
            if b >= bytes {
                b - bytes
            } else {
                pos
            }
        }

        #[inline]
        fn slide_table(table: &mut [u16; WINDOW_SIZE], bytes: u16) {
            for (n, b) in table.iter_mut().enumerate() {
                *b = ChainedHashTable::slide_value(*b, n as u16, bytes);
            }
        }

        pub fn slide(&mut self, bytes: usize) {
            /*if cfg!(debug_assertions) && bytes != WINDOW_SIZE {
                // This should only happen in tests in this file.
                self.count.reset();
            }*/
            ChainedHashTable::slide_table(&mut self.c.head, bytes as u16);
            ChainedHashTable::slide_table(&mut self.c.prev, bytes as u16);
        }
    }

    #[cfg(test)]
    pub fn filled_hash_table(data: &[u8]) -> ChainedHashTable {
        assert!(data.len() <= (WINDOW_SIZE * 2) + 2);
        let mut hash_table = ChainedHashTable::from_starting_values(data[0], data[1]);
        for (n, b) in data[2..].iter().enumerate() {
            hash_table.add_hash_value(n, *b);
        }
        hash_table
    }

    #[cfg(test)]
    mod test {
        use super::{filled_hash_table, ChainedHashTable};

        #[test]
        fn chained_hash() {
            use std::str;

            let test_string = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do \
                               eiusmod tempor. rum. incididunt ut labore et dolore magna aliqua. Ut \
                               enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi \
                               ut aliquip ex ea commodo consequat. rum. Duis aute irure dolor in \
                               reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla \
                               pariatur. Excepteur sint occaecat cupidatat non proident, sunt in \
                               culpa qui officia deserunt mollit anim id est laborum.";

            let test_data = test_string.as_bytes();

            let current_bytes = &test_data[test_data.len() - super::HASH_BYTES..test_data.len()];

            let num_iters = test_string
                .matches(str::from_utf8(current_bytes).unwrap())
                .count();

            let hash_table = filled_hash_table(test_data);

            // Test that the positions in the chain are valid
            let mut prev_value = hash_table.get_prev(hash_table.current_head() as usize) as usize;
            let mut count = 0;
            let mut current = hash_table.current_head() as usize;
            while current != prev_value {
                count += 1;
                current = prev_value;
                prev_value = hash_table.get_prev(prev_value) as usize;
            }
            // There should be at least as many occurences of the hash of the checked bytes as the
            // numbers of occurences of the checked bytes themselves. As the hashes are not large enough
            // to store 8 * 3 = 24 bits, there could be more with different input data.
            assert!(count >= num_iters);
        }

        #[test]
        fn table_unique() {
            let mut test_data = Vec::new();
            test_data.extend(0u8..255);
            test_data.extend(255u8..0);
            let hash_table = filled_hash_table(&test_data);
            let prev_pos = hash_table.get_prev(hash_table.current_head() as usize);
            // Since all sequences in the input are unique, there shouldn't be any previous values.
            assert_eq!(prev_pos, hash_table.current_hash());
        }

        #[test]
        fn table_slide() {
            use std::fs::File;
            use std::io::Read;

            let window_size = super::WINDOW_SIZE;
            let window_size16 = super::WINDOW_SIZE as u16;

            let mut input = Vec::new();

            let mut f = File::open("tests/pg11.txt").unwrap();

            f.read_to_end(&mut input).unwrap();

            let mut hash_table = filled_hash_table(&input[..window_size + 2]);

            for (n, b) in input[2..window_size + 2].iter().enumerate() {
                hash_table.add_hash_value(n + window_size, *b);
            }

            hash_table.slide(window_size);

            {
                let max_head = hash_table.c.head.iter().max().unwrap();
                // After sliding there should be no hashes referring to values
                // higher than the window size
                assert!(*max_head < window_size16);
                assert!(*max_head > 0);
                let pos = hash_table.get_prev(hash_table.current_head() as usize);
                // There should be a previous occurence since we inserted the data 3 times
                assert!(pos < window_size16);
                assert!(pos > 0);
            }

            for (n, b) in input[2..(window_size / 2)].iter().enumerate() {
                hash_table.add_hash_value(n + window_size, *b);
            }

            // There should hashes referring to values in the upper part of the input window
            // at this point
            let max_prev = hash_table.c.prev.iter().max().unwrap();
            assert!(*max_prev > window_size16);

            let mut pos = hash_table.current_head();
            // There should be a previous occurence since we inserted the data 3 times
            assert!(pos > window_size16);
            let end_byte = input[(window_size / 2) - 1 - 2];
            let mut iterations = 0;
            while pos > window_size16 && iterations < 5000 {
                assert_eq!(input[pos as usize & window_size - 1], end_byte);

                pos = hash_table.get_prev(pos as usize);
                iterations += 1;
            }
        }

        #[test]
        /// Ensure that the initial hash values are correct.
        fn initial_chains() {
            let t = ChainedHashTable::new();
            for (n, &b) in t.c.head.iter().enumerate() {
                assert_eq!(n, b as usize);
            }
            for (n, &b) in t.c.prev.iter().enumerate() {
                assert_eq!(n, b as usize);
            }
        }
    }

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