pub fn new(protection_set: SharedProtectionSet) -> OperationCache {

        OperationCache {

            protection_set,

            caches1: vec![Cache::new()],

            caches2: vec![Cache::new(); 4],

            caches3: vec![Cache::new()],

        }

    }

    pub fn clear(&mut self) {

        for cache in self.caches1.iter_mut() {

            cache.clear();

        }

        for cache in self.caches2.iter_mut() {

            cache.clear();

        }

        for cache in self.caches3.iter_mut() {

            cache.clear();

        }

    }

    pub fn len(&self) -> usize {

        let mut result: usize = 0;

        for cache in self.caches1.iter() {

            result += cache.len();

        }

        for cache in self.caches2.iter() {

            result += cache.len();

        }

        for cache in self.caches3.iter() {

            result += cache.len();

        }

        result

    }

    pub fn limit(&mut self, size: usize) {

        for cache in self.caches1.iter_mut() {

            cache.limit(size / 4);

        }

        for cache in self.caches2.iter_mut() {

            cache.limit(size / 4);

        }

        for cache in self.caches3.iter_mut() {

            cache.limit(size / 4);

        }

    }

    fn get_cache1(&mut self, operator: &UnaryFunction) -> &mut Cache<LddIndex, usize> {

        match operator {

            UnaryFunction::Len => &mut self.caches1[0],

        }

    }

    fn get_cache2(&mut self, operator: &BinaryOperator) -> &mut Cache<(LddIndex, LddIndex), LddIndex> {

        match operator {

            BinaryOperator::Union => &mut self.caches2[0],

            BinaryOperator::Merge => &mut self.caches2[1],

            BinaryOperator::Minus => &mut self.caches2[2],

            BinaryOperator::Project => &mut self.caches2[3],

    }

    fn get_cache3(&mut self, operator: &TernaryOperator) -> &mut Cache<(LddIndex, LddIndex, LddIndex), LddIndex> {

        match operator {

            TernaryOperator::RelationalProduct => &mut self.caches3[0],

        }

    }

    fn create(&mut self, index: LddIndex) -> Ldd {

        Ldd::new(&self.protection_set, index)

    }

    pub fn new() -> Cache<K, V, RandomState> {

        Cache {

            table: vec![Default::default(); 1024],

            hash_builder: RandomState::default(),

        }

    }

    pub fn clear(&mut self) {

        let capacity = self.table.len();

        self.table.clear();

        self.table.resize(capacity, Default::default());

    }

    pub fn limit(&mut self, size: usize) {

        let power_of_two = size.next_power_of_two();

        self.table.clear();

        self.table.resize(power_of_two, Default::default());

    }

    pub fn len(&self) -> usize {

        self.table.len()

    }

    pub fn get(&mut self, key: &K) -> Option<&V> {

        debug_assert!(*key != K::default(), "The key may never be equal to its default value.");

        let index = self.hash_builder.hash_one(key) % (self.table.len() as u64);

        let entry = &self.table[index as usize];

        if entry.0 == *key { Some(&entry.1) } else { None }

    }

    pub fn insert(&mut self, key: K, value: V) {

        debug_assert!(key != K::default(), "The key may never be equal to its default value.");

        let index = self.hash_builder.hash_one(&key) % (self.table.len() as u64);

        self.table[index as usize] = (key, value);

    }

    fn clone(&self) -> Self {

        Cache {

            table: self.table.clone(),

            hash_builder: self.hash_builder.clone(),

        }

    }

pub fn cache_unary_function<F>(storage: &mut Storage, operator: UnaryFunction, a: &LddRef, f: F) -> usize

where

    F: Fn(&mut Storage, &LddRef<'_>) -> usize,

    let key = a.index();

    if let Some(result) = storage.operation_cache().get_cache1(&operator).get(&key) {

        *result

        let result = f(storage, a);

        storage.operation_cache().get_cache1(&operator).insert(key, result);

        result

}

pub fn cache_binary_op<F>(storage: &mut Storage, operator: BinaryOperator, a: &LddRef, b: &LddRef, f: F) -> Ldd

where

    F: Fn(&mut Storage, &LddRef<'_>, &LddRef<'_>) -> Ldd,

    let key = (a.index(), b.index());

    if let Some(result) = storage.operation_cache().get_cache2(&operator).get(&key) {

        let result = *result; // Necessary to decouple borrow from storage and the call to create.

        storage.operation_cache().create(result)

        let result = f(storage, a, b);

        storage

            .operation_cache()

            .get_cache2(&operator)

            .insert(key, result.index());

        result

}

pub fn cache_comm_binary_op<F>(storage: &mut Storage, operator: BinaryOperator, a: &LddRef, b: &LddRef, f: F) -> Ldd

where

    F: Fn(&mut Storage, &LddRef<'_>, &LddRef<'_>) -> Ldd,

    if a.index() < b.index() {

        cache_binary_op(storage, operator, a, b, f)

        cache_binary_op(storage, operator, b, a, f)

}

pub fn cache_terniary_op<F>(

    storage: &mut Storage,

    operator: TernaryOperator,

    a: &LddRef,

    b: &LddRef,

    c: &LddRef,

    f: F,

) -> Ldd

where

    F: Fn(&mut Storage, &LddRef<'_>, &LddRef<'_>, &LddRef<'_>) -> Ldd,

    let key = (a.index(), b.index(), c.index());

    if let Some(result) = storage.operation_cache().get_cache3(&operator).get(&key) {

        let result = *result; // Necessary to decouple borrow from storage and the call to create.

        storage.operation_cache().create(result)

        let result = f(storage, a, b, c);

        storage

            .operation_cache()

            .get_cache3(&operator)

            .insert(key, result.index());

        result

}