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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/>.

    //! Database of byte-slices keyed to their Keccak hash.
    use hash::*;
    use std::collections::HashMap;
    use elastic_array::ElasticArray128;

    /// `HashDB` value type.
    pub type DBValue = ElasticArray128<u8>;

    /// Trait modelling datastore keyed by a 32-byte Keccak hash.
    pub trait HashDB: AsHashDB + Send + Sync {
    	/// Get the keys in the database together with number of underlying references.
    	fn keys(&self) -> HashMap<H256, i32>;

    	/// Look up a given hash into the bytes that hash to it, returning None if the
    	/// hash is not known.
    	///
    	/// # Examples
    	/// ```rust
    	/// extern crate ethcore_util;
    	/// use ethcore_util::hashdb::*;
    	/// use ethcore_util::memorydb::*;
    	/// fn main() {
    	///   let mut m = MemoryDB::new();
    	///   let hello_bytes = "Hello world!".as_bytes();
    	///   let hash = m.insert(hello_bytes);
    	///   assert_eq!(m.get(&hash).unwrap(), hello_bytes);
    	/// }
    	/// ```
    	fn get(&self, key: &H256) -> Option<DBValue>;

    	/// Check for the existance of a hash-key.
    	///
    	/// # Examples
    	/// ```rust
    	/// extern crate ethcore_util;
    	/// use ethcore_util::hashdb::*;
    	/// use ethcore_util::memorydb::*;
    	/// use ethcore_util::sha3::*;
    	/// fn main() {
    	///   let mut m = MemoryDB::new();
    	///   let hello_bytes = "Hello world!".as_bytes();
    	///   assert!(!m.contains(&hello_bytes.sha3()));
    	///   let key = m.insert(hello_bytes);
    	///   assert!(m.contains(&key));
    	///   m.remove(&key);
    	///   assert!(!m.contains(&key));
    	/// }
    	/// ```
    	fn contains(&self, key: &H256) -> bool;

    	/// Insert a datum item into the DB and return the datum's hash for a later lookup. Insertions
    	/// are counted and the equivalent number of `remove()`s must be performed before the data
    	/// is considered dead.
    	///
    	/// # Examples
    	/// ```rust
    	/// extern crate ethcore_util;
    	/// use ethcore_util::hashdb::*;
    	/// use ethcore_util::memorydb::*;
    	/// use ethcore_util::hash::*;
    	/// fn main() {
    	///   let mut m = MemoryDB::new();
    	///   let key = m.insert("Hello world!".as_bytes());
    	///   assert!(m.contains(&key));
    	/// }
    	/// ```
    	fn insert(&mut self, value: &[u8]) -> H256;

    	/// Like `insert()` , except you provide the key and the data is all moved.
    	fn emplace(&mut self, key: H256, value: DBValue);

    	/// Remove a datum previously inserted. Insertions can be "owed" such that the same number of `insert()`s may
    	/// happen without the data being eventually being inserted into the DB.
    	///
    	/// # Examples
    	/// ```rust
    	/// extern crate ethcore_util;
    	/// use ethcore_util::hashdb::*;
    	/// use ethcore_util::memorydb::*;
    	/// use ethcore_util::sha3::*;
    	/// fn main() {
    	///   let mut m = MemoryDB::new();
    	///   let d = "Hello world!".as_bytes();
    	///   let key = &d.sha3();
    	///   m.remove(key);	// OK - we now owe an insertion.
    	///   assert!(!m.contains(key));
    	///   m.insert(d);	// OK - now it's "empty" again.
    	///   assert!(!m.contains(key));
    	///   m.insert(d);	// OK - now we've
    	///   assert_eq!(m.get(key).unwrap(), d);
    	/// }
    	/// ```
    	fn remove(&mut self, key: &H256);
    }

    /// Upcast trait.
    pub trait AsHashDB {
    	/// Perform upcast to HashDB for anything that derives from HashDB.
    	fn as_hashdb(&self) -> &HashDB;
    	/// Perform mutable upcast to HashDB for anything that derives from HashDB.
    	fn as_hashdb_mut(&mut self) -> &mut HashDB;
    }

    impl<T: HashDB> AsHashDB for T {
    	fn as_hashdb(&self) -> &HashDB {
    		self
    	}
    	fn as_hashdb_mut(&mut self) -> &mut HashDB {
    		self
    	}
    }

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