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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity is free software: you can redistribute it and/or modify
    // it under the terms of the GNU General Public License as published by
    // the Free Software Foundation, either version 3 of the License, or
    // (at your option) any later version.

    // Parity is distributed in the hope that it will be useful,
    // but WITHOUT ANY WARRANTY; without even the implied warranty of
    // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    // GNU General Public License for more details.

    // You should have received a copy of the GNU General Public License
    // along with Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Lru-cache related utilities as quick-and-dirty wrappers around the lru-cache
    //! crate.
    // TODO: push changes upstream in a clean way.

    use heapsize::HeapSizeOf;
    use lru_cache::LruCache;

    use std::hash::Hash;

    const INITIAL_CAPACITY: usize = 4;

    /// An LRU-cache which operates on memory used.
    pub struct MemoryLruCache<K: Eq + Hash, V: HeapSizeOf> {
    	inner: LruCache<K, V>,
    	cur_size: usize,
    	max_size: usize,
    }

    impl<K: Eq + Hash, V: HeapSizeOf> MemoryLruCache<K, V> {
    	/// Create a new cache with a maximum size in bytes.
    	pub fn new(max_size: usize) -> Self {
    		MemoryLruCache {
    			inner: LruCache::new(INITIAL_CAPACITY),
    			max_size: max_size,
    			cur_size: 0,
    		}
    	}

    	/// Insert an item.
    	pub fn insert(&mut self, key: K, val: V) {
    		let cap = self.inner.capacity();

    		// grow the cache as necessary; it operates on amount of items
    		// but we're working based on memory usage.
    		if self.inner.len() == cap && self.cur_size < self.max_size {
    			self.inner.set_capacity(cap * 2);
    		}

    		// account for any element displaced from the cache.
    		if let Some(lru) = self.inner.insert(key, val) {
    			self.cur_size -= lru.heap_size_of_children();
    		}

    		// remove elements until we are below the memory target.
    		while self.cur_size > self.max_size {
    			match self.inner.remove_lru() {
    				Some((_, v)) => self.cur_size -= v.heap_size_of_children(),
    				_ => break,
    			}
    		}
    	}

    	/// Get a reference to an item in the cache. It is a logic error for its
    	/// heap size to be altered while borrowed.
    	pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
    		self.inner.get_mut(key)
    	}

    	/// Currently-used size of values in bytes.
    	pub fn current_size(&self) -> usize {
    		self.cur_size
    	}
    }

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