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    #![allow(dead_code)]

    use std::borrow::Borrow;

    /// Flat (Vec) backed map
    ///
    /// This preserves insertion order
    #[derive(Clone, Debug, PartialEq, Eq)]
    pub(crate) struct FlatMap<K, V> {
        keys: Vec<K>,
        values: Vec<V>,
    }

    impl<K: PartialEq + Eq, V> FlatMap<K, V> {
        pub(crate) fn new() -> Self {
            Default::default()
        }

        pub(crate) fn insert(&mut self, key: K, mut value: V) -> Option<V> {
            for (index, existing) in self.keys.iter().enumerate() {
                if *existing == key {
                    std::mem::swap(&mut self.values[index], &mut value);
                    return Some(value);
                }
            }

            self.insert_unchecked(key, value);
            None
        }

        pub(crate) fn insert_unchecked(&mut self, key: K, value: V) {
            self.keys.push(key);
            self.values.push(value);
        }

        pub(crate) fn extend_unchecked(&mut self, iter: impl IntoIterator<Item = (K, V)>) {
            for (key, value) in iter {
                self.insert_unchecked(key, value);
            }
        }

        pub fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool
        where
            K: Borrow<Q>,
            Q: Eq,
        {
            for existing in &self.keys {
                if existing.borrow() == key {
                    return true;
                }
            }
            false
        }

        pub fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
        where
            K: Borrow<Q>,
            Q: std::hash::Hash + Eq,
        {
            self.remove_entry(key).map(|(_, v)| v)
        }

        pub fn remove_entry<Q: ?Sized>(&mut self, key: &Q) -> Option<(K, V)>
        where
            K: Borrow<Q>,
            Q: std::hash::Hash + Eq,
        {
            let index = some!(self
                .keys
                .iter()
                .enumerate()
                .find_map(|(i, k)| (k.borrow() == key).then(|| i)));
            let key = self.keys.remove(index);
            let value = self.values.remove(index);
            Some((key, value))
        }

        pub(crate) fn is_empty(&self) -> bool {
            self.keys.is_empty()
        }

        pub fn entry(&mut self, key: K) -> Entry<K, V> {
            for (index, existing) in self.keys.iter().enumerate() {
                if *existing == key {
                    return Entry::Occupied(OccupiedEntry { v: self, index });
                }
            }
            Entry::Vacant(VacantEntry { v: self, key })
        }

        pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
        where
            K: Borrow<Q>,
            Q: Eq,
        {
            for (index, existing) in self.keys.iter().enumerate() {
                if existing.borrow() == k {
                    return Some(&self.values[index]);
                }
            }
            None
        }

        pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
        where
            K: Borrow<Q>,
            Q: Eq,
        {
            for (index, existing) in self.keys.iter().enumerate() {
                if existing.borrow() == k {
                    return Some(&mut self.values[index]);
                }
            }
            None
        }

        pub fn keys(&self) -> std::slice::Iter<'_, K> {
            self.keys.iter()
        }

        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                keys: self.keys.iter(),
                values: self.values.iter(),
            }
        }

        pub fn iter_mut(&mut self) -> IterMut<K, V> {
            IterMut {
                keys: self.keys.iter_mut(),
                values: self.values.iter_mut(),
            }
        }
    }

    impl<K: PartialEq + Eq, V> Default for FlatMap<K, V> {
        fn default() -> Self {
            Self {
                keys: Default::default(),
                values: Default::default(),
            }
        }
    }

    pub enum Entry<'a, K: 'a, V: 'a> {
        Vacant(VacantEntry<'a, K, V>),
        Occupied(OccupiedEntry<'a, K, V>),
    }

    impl<'a, K: 'a, V: 'a> Entry<'a, K, V> {
        pub fn or_insert(self, default: V) -> &'a mut V {
            match self {
                Entry::Occupied(entry) => &mut entry.v.values[entry.index],
                Entry::Vacant(entry) => {
                    entry.v.keys.push(entry.key);
                    entry.v.values.push(default);
                    entry.v.values.last_mut().unwrap()
                }
            }
        }

        pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
            match self {
                Entry::Occupied(entry) => &mut entry.v.values[entry.index],
                Entry::Vacant(entry) => {
                    entry.v.keys.push(entry.key);
                    entry.v.values.push(default());
                    entry.v.values.last_mut().unwrap()
                }
            }
        }
    }

    pub struct VacantEntry<'a, K: 'a, V: 'a> {
        v: &'a mut FlatMap<K, V>,
        key: K,
    }

    pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
        v: &'a mut FlatMap<K, V>,
        index: usize,
    }

    pub struct Iter<'a, K: 'a, V: 'a> {
        keys: std::slice::Iter<'a, K>,
        values: std::slice::Iter<'a, V>,
    }

    impl<'a, K, V> Iterator for Iter<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<(&'a K, &'a V)> {
            match self.keys.next() {
                Some(k) => {
                    let v = self.values.next().unwrap();
                    Some((k, v))
                }
                None => None,
            }
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.keys.size_hint()
        }
    }

    impl<'a, K, V> DoubleEndedIterator for Iter<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
            match self.keys.next_back() {
                Some(k) => {
                    let v = self.values.next_back().unwrap();
                    Some((k, v))
                }
                None => None,
            }
        }
    }

    impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {}

    pub struct IterMut<'a, K: 'a, V: 'a> {
        keys: std::slice::IterMut<'a, K>,
        values: std::slice::IterMut<'a, V>,
    }

    impl<'a, K, V> Iterator for IterMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
            match self.keys.next() {
                Some(k) => {
                    let v = self.values.next().unwrap();
                    Some((k, v))
                }
                None => None,
            }
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.keys.size_hint()
        }
    }

    impl<'a, K, V> DoubleEndedIterator for IterMut<'a, K, V> {
        fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
            match self.keys.next_back() {
                Some(k) => {
                    let v = self.values.next_back().unwrap();
                    Some((k, v))
                }
                None => None,
            }
        }
    }

    impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {}