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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/>.

    //! Disk-backed `HashDB` implementation.

    use common::*;
    use hashdb::*;
    use kvdb::{Database, DBTransaction};

    /// A `HashDB` which can manage a short-term journal potentially containing many forks of mutually
    /// exclusive actions.
    pub trait JournalDB: HashDB {
    	/// Return a copy of ourself, in a box.
    	fn boxed_clone(&self) -> Box<JournalDB>;

    	/// Returns heap memory size used
    	fn mem_used(&self) -> usize;

    	/// Returns the size of journalled state in memory.
    	/// This function has a considerable speed requirement --
    	/// it must be fast enough to call several times per block imported.
    	fn journal_size(&self) -> usize { 0 }

    	/// Check if this database has any commits
    	fn is_empty(&self) -> bool;

    	/// Get the earliest era in the DB. None if there isn't yet any data in there.
    	fn earliest_era(&self) -> Option<u64> { None }

    	/// Get the latest era in the DB. None if there isn't yet any data in there.
    	fn latest_era(&self) -> Option<u64>;

    	/// Journal recent database operations as being associated with a given era and id.
    	// TODO: give the overlay to this function so journaldbs don't manage the overlays themeselves.
    	fn journal_under(&mut self, batch: &mut DBTransaction, now: u64, id: &H256) -> Result<u32, UtilError>;

    	/// Mark a given block as canonical, indicating that competing blocks' states may be pruned out.
    	fn mark_canonical(&mut self, batch: &mut DBTransaction, era: u64, id: &H256) -> Result<u32, UtilError>;

    	/// Commit all queued insert and delete operations without affecting any journalling -- this requires that all insertions
    	/// and deletions are indeed canonical and will likely lead to an invalid database if that assumption is violated.
    	///
    	/// Any keys or values inserted or deleted must be completely independent of those affected
    	/// by any previous `commit` operations. Essentially, this means that `inject` can be used
    	/// either to restore a state to a fresh database, or to insert data which may only be journalled
    	/// from this point onwards.
    	fn inject(&mut self, batch: &mut DBTransaction) -> Result<u32, UtilError>;

    	/// State data query
    	fn state(&self, _id: &H256) -> Option<Bytes>;

    	/// Whether this database is pruned.
    	fn is_pruned(&self) -> bool { true }

    	/// Get backing database.
    	fn backing(&self) -> &Arc<Database>;

    	/// Clear internal strucutres. This should called after changes have been written
    	/// to the backing strage
    	fn flush(&self) {}

    	/// Consolidate all the insertions and deletions in the given memory overlay.
    	fn consolidate(&mut self, overlay: ::memorydb::MemoryDB);

    	/// Commit all changes in a single batch
    	#[cfg(test)]
    	fn commit_batch(&mut self, now: u64, id: &H256, end: Option<(u64, H256)>) -> Result<u32, UtilError> {
    		let mut batch = self.backing().transaction();
    		let mut ops = self.journal_under(&mut batch, now, id)?;

    		if let Some((end_era, canon_id)) = end {
    			ops += self.mark_canonical(&mut batch, end_era, &canon_id)?;
    		}

    		let result = self.backing().write(batch).map(|_| ops).map_err(Into::into);
    		self.flush();
    		result
    	}

    	/// Inject all changes in a single batch.
    	#[cfg(test)]
    	fn inject_batch(&mut self) -> Result<u32, UtilError> {
    		let mut batch = self.backing().transaction();
    		let res = self.inject(&mut batch)?;
    		self.backing().write(batch).map(|_| res).map_err(Into::into)
    	}
    }

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