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//! The `cache` module declare the cache functionality for webmachine is
//! executing in. Basically implements in-memory and Dummy cache.
//! Inspired by [any-cache].
//! 
//! TODO: 
//! [ ] - partitioning
//! [ ] - fnv
//! [ ] - POLICY in key
//! [ ] - policy implementation (LFU, LRU, etc.)
//! [ ] - async loader
//! 
//! [any-cache]: https://github.com/phaazon/any-cache

use std::{
    any::{Any, TypeId},
    collections::hash_map::{DefaultHasher, HashMap},
    hash::{Hash, Hasher},
};

/// A cache that can store arbitrary values and namespace them by key types.
pub trait Cache {
    /// Save item in cache
    fn save<K>(&mut self, key: K, value: K::Target)
    where
        K::Target: Any + 'static,
        K: CacheKey;

    /// Get item from cache
    fn get<K>(&self, key: &K) -> Option<&K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey;

    /// Remove item from cache
    fn remove<K>(&mut self, key: &K) -> Option<K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey;

    /// Clear cache
    fn clear(&mut self);
}

/// A key that is usable in a cache.
///
/// Cache keys are required to declare the type of values they reference. This is needed to
/// implement type-level namespacing.
pub trait CacheKey: 'static + Hash {
    /// Target type for cache key
    type Target;
}

/// An implementation of a cache with a `HashMap`.
pub struct HashCache {
    items: HashMap<u64, Box<dyn Any>>,
}

impl HashCache {
    /// Constructor
    pub fn new() -> Self {
        HashCache {
            items: HashMap::new(),
        }
    }
}

impl Default for HashCache {
    fn default() -> Self {
        Self::new()
    }
}

impl Cache for HashCache {
    fn save<K>(&mut self, key: K, value: K::Target)
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
        let mut hasher = DefaultHasher::new();
        key.hash(&mut hasher);
        TypeId::of::<K>().hash(&mut hasher);
        self.items.insert(hasher.finish(), Box::new(value));
    }

    fn get<K>(&self, key: &K) -> Option<&K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
        let mut hasher = DefaultHasher::new();
        key.hash(&mut hasher);
        TypeId::of::<K>().hash(&mut hasher);
        self.items
            .get(&hasher.finish())
            .and_then(|a| a.downcast_ref::<K::Target>())
    }

    fn remove<K>(&mut self, key: &K) -> Option<K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
        let mut hasher = DefaultHasher::new();
        key.hash(&mut hasher);
        TypeId::of::<K>().hash(&mut hasher);
        self.items
            .remove(&hasher.finish())
            .and_then(|anybox| anybox.downcast().ok())
            .map(|b| *b)
    }

    fn clear(&mut self) {
        self.items.clear();
    }
}

/// An implementation of a cache that actually doesn’t cache at all.
pub struct DummyCache;

impl DummyCache {
    /// Constructor
    pub fn new() -> Self {
        DummyCache
    }
}

impl Default for DummyCache {
    fn default() -> Self {
        DummyCache
    }
}

impl Cache for DummyCache {
    fn save<K>(&mut self, _: K, _: K::Target)
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
    }

    fn get<K>(&self, _: &K) -> Option<&K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
        None
    }

    fn remove<K>(&mut self, _: &K) -> Option<K::Target>
    where
        K::Target: Any + 'static,
        K: CacheKey,
    {
        None
    }

    fn clear(&mut self) {}
}