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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/>.

    //! DB Migration module.
    #[cfg(test)]
    mod tests;

    use std::collections::BTreeMap;
    use std::fs;
    use std::fmt;
    use std::path::{Path, PathBuf};
    use std::sync::Arc;

    use ::kvdb::{CompactionProfile, Database, DatabaseConfig, DBTransaction};

    /// Migration config.
    #[derive(Clone)]
    pub struct Config {
    	/// Defines how many elements should be migrated at once.
    	pub batch_size: usize,
    	/// Database compaction profile.
    	pub compaction_profile: CompactionProfile,
    }

    impl Default for Config {
    	fn default() -> Self {
    		Config {
    			batch_size: 1024,
    			compaction_profile: Default::default(),
    		}
    	}
    }

    /// A batch of key-value pairs to be written into the database.
    pub struct Batch {
    	inner: BTreeMap<Vec<u8>, Vec<u8>>,
    	batch_size: usize,
    	column: Option<u32>,
    }

    impl Batch {
    	/// Make a new batch with the given config.
    	pub fn new(config: &Config, col: Option<u32>) -> Self {
    		Batch {
    			inner: BTreeMap::new(),
    			batch_size: config.batch_size,
    			column: col,
    		}
    	}

    	/// Insert a value into the batch, committing if necessary.
    	pub fn insert(&mut self, key: Vec<u8>, value: Vec<u8>, dest: &mut Database) -> Result<(), Error> {
    		self.inner.insert(key, value);
    		if self.inner.len() == self.batch_size {
    			self.commit(dest)?;
    		}
    		Ok(())
    	}

    	/// Commit all the items in the batch to the given database.
    	pub fn commit(&mut self, dest: &mut Database) -> Result<(), Error> {
    		if self.inner.is_empty() { return Ok(()) }

    		let mut transaction = DBTransaction::new(dest);

    		for keypair in &self.inner {
    			transaction.put(self.column, &keypair.0, &keypair.1);
    		}

    		self.inner.clear();
    		dest.write(transaction).map_err(Error::Custom)
    	}
    }

    /// Migration error.
    #[derive(Debug)]
    pub enum Error {
    	/// Error returned when it is impossible to add new migration rules.
    	CannotAddMigration,
    	/// Error returned when migration from specific version can not be performed.
    	MigrationImpossible,
    	/// Io Error.
    	Io(::std::io::Error),
    	/// Custom error.
    	Custom(String),
    }

    impl fmt::Display for Error {
    	fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
    		match *self {
    			Error::CannotAddMigration => write!(f, "Cannot add migration"),
    			Error::MigrationImpossible => write!(f, "Migration impossible"),
    			Error::Io(ref err) => write!(f, "{}", err),
    			Error::Custom(ref err) => write!(f, "{}", err),
    		}
    	}
    }

    impl From<::std::io::Error> for Error {
    	fn from(e: ::std::io::Error) -> Self {
    		Error::Io(e)
    	}
    }

    impl From<String> for Error {
    	fn from(e: String) -> Self {
    		Error::Custom(e)
    	}
    }

    /// A generalized migration from the given db to a destination db.
    pub trait Migration: 'static {
    	/// Number of columns in the database before the migration.
    	fn pre_columns(&self) -> Option<u32> { self.columns() }
    	/// Number of columns in database after the migration.
    	fn columns(&self) -> Option<u32>;
    	/// Version of the database after the migration.
    	fn version(&self) -> u32;
    	/// Migrate a source to a destination.
    	fn migrate(&mut self, source: Arc<Database>, config: &Config, destination: &mut Database, col: Option<u32>) -> Result<(), Error>;
    }

    /// A simple migration over key-value pairs.
    pub trait SimpleMigration: 'static {
    	/// Number of columns in database after the migration.
    	fn columns(&self) -> Option<u32>;
    	/// Version of database after the migration.
    	fn version(&self) -> u32;
    	/// Should migrate existing object to new database.
    	/// Returns `None` if the object does not exist in new version of database.
    	fn simple_migrate(&mut self, key: Vec<u8>, value: Vec<u8>) -> Option<(Vec<u8>, Vec<u8>)>;
    }

    impl<T: SimpleMigration> Migration for T {
    	fn columns(&self) -> Option<u32> { SimpleMigration::columns(self) }

    	fn version(&self) -> u32 { SimpleMigration::version(self) }

    	fn migrate(&mut self, source: Arc<Database>, config: &Config, dest: &mut Database, col: Option<u32>) -> Result<(), Error> {
    		let mut batch = Batch::new(config, col);

    		for (key, value) in source.iter(col) {
    			if let Some((key, value)) = self.simple_migrate(key.to_vec(), value.to_vec()) {
    				batch.insert(key, value, dest)?;
    			}
    		}

    		batch.commit(dest)
    	}
    }

    /// Get the path where all databases reside.
    fn database_path(path: &Path) -> PathBuf {
    	let mut temp_path = path.to_owned();
    	temp_path.pop();
    	temp_path
    }

    enum TempIndex {
    	One,
    	Two,
    }

    impl TempIndex {
    	fn swap(&mut self) {
    		match *self {
    			TempIndex::One => *self = TempIndex::Two,
    			TempIndex::Two => *self = TempIndex::One,
    		}
    	}

    	// given the path to the old database, get the path of this one.
    	fn path(&self, db_root: &Path) -> PathBuf {
    		let mut buf = db_root.to_owned();

    		match *self {
    			TempIndex::One => buf.push("temp_migration_1"),
    			TempIndex::Two => buf.push("temp_migration_2"),
    		};

    		buf
    	}
    }

    /// Manages database migration.
    pub struct Manager {
    	config: Config,
    	migrations: Vec<Box<Migration>>,
    }

    impl Manager {
    	/// Creates new migration manager with given configuration.
    	pub fn new(config: Config) -> Self {
    		Manager {
    			config: config,
    			migrations: vec![],
    		}
    	}

    	/// Adds new migration rules.
    	pub fn add_migration<T>(&mut self, migration: T) -> Result<(), Error> where T: Migration {
    		let is_new = match self.migrations.last() {
    			Some(last) => migration.version() > last.version(),
    			None => true,
    		};

    		match is_new {
    			true => Ok(self.migrations.push(Box::new(migration))),
    			false => Err(Error::CannotAddMigration),
    		}
    	}

    	/// Performs migration in order, starting with a source path, migrating between two temporary databases,
    	/// and producing a path where the final migration lives.
    	pub fn execute(&mut self, old_path: &Path, version: u32) -> Result<PathBuf, Error> {
    		let config = self.config.clone();
    		let migrations = self.migrations_from(version);
    		trace!(target: "migration", "Total migrations to execute for version {}: {}", version, migrations.len());
    		if migrations.is_empty() { return Err(Error::MigrationImpossible) };

    		let columns = migrations.get(0).and_then(|m| m.pre_columns());

    		trace!(target: "migration", "Expecting database to contain {:?} columns", columns);
    		let mut db_config = DatabaseConfig {
    			max_open_files: 64,
    			cache_sizes: Default::default(),
    			compaction: config.compaction_profile,
    			columns: columns,
    			wal: true,
    		};

    		let db_root = database_path(old_path);
    		let mut temp_idx = TempIndex::One;
    		let mut temp_path = temp_idx.path(&db_root);

    		// start with the old db.
    		let old_path_str = old_path.to_str().ok_or(Error::MigrationImpossible)?;
    		let mut cur_db = Arc::new(Database::open(&db_config, old_path_str).map_err(Error::Custom)?);

    		for migration in migrations {
    			trace!(target: "migration", "starting migration to version {}", migration.version());
    			// Change number of columns in new db
    			let current_columns = db_config.columns;
    			db_config.columns = migration.columns();

    			// open the target temporary database.
    			temp_path = temp_idx.path(&db_root);
    			let temp_path_str = temp_path.to_str().ok_or(Error::MigrationImpossible)?;
    			let mut new_db = Database::open(&db_config, temp_path_str).map_err(Error::Custom)?;

    			// perform the migration from cur_db to new_db.
    			match current_columns {
    				// migrate only default column
    				None => migration.migrate(cur_db.clone(), &config, &mut new_db, None)?,
    				Some(v) => {
    					// Migrate all columns in previous DB
    					for col in 0..v {
    						migration.migrate(cur_db.clone(), &config, &mut new_db, Some(col))?
    					}
    				}
    			}
    			// next iteration, we will migrate from this db into the other temp.
    			cur_db = Arc::new(new_db);
    			temp_idx.swap();

    			// remove the other temporary migration database.
    			let _ = fs::remove_dir_all(temp_idx.path(&db_root));
    		}
    		Ok(temp_path)
    	}

    	/// Returns true if migration is needed.
    	pub fn is_needed(&self, version: u32) -> bool {
    		match self.migrations.last() {
    			Some(last) => version < last.version(),
    			None => false,
    		}
    	}

    	/// Find all needed migrations.
    	fn migrations_from(&mut self, version: u32) -> Vec<&mut Box<Migration>> {
    		self.migrations.iter_mut().filter(|m| m.version() > version).collect()
    	}
    }

    /// Prints a dot every `max` ticks
    pub struct Progress {
    	current: usize,
    	max: usize,
    }

    impl Default for Progress {
    	fn default() -> Self {
    		Progress {
    			current: 0,
    			max: 100_000,
    		}
    	}
    }

    impl Progress {
    	/// Tick progress meter.
    	pub fn tick(&mut self) {
    		self.current += 1;
    		if self.current == self.max {
    			self.current = 0;
    			flush!(".");
    		}
    	}
    }

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