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    //! Key-Value store abstraction with `RocksDB` backend.

    use std::collections::HashMap;
    use std::path::Path;
    use rocksdb::{
    	DB, Writable, WriteBatch, WriteOptions, IteratorMode, DBIterator,
    	Options, DBCompactionStyle, BlockBasedOptions, Cache, Column, ReadOptions
    };
    use bytes::Bytes;
    use kv::{Transaction, RawTransaction, RawOperation, Location, Value, KeyValueDatabase, Key, KeyState, RawKeyValue,
    RawKey};

    const DB_BACKGROUND_FLUSHES: i32 = 2;
    const DB_BACKGROUND_COMPACTIONS: i32 = 2;

    /// Compaction profile for the database settings
    #[derive(Clone, Copy, PartialEq, Debug)]
    pub struct CompactionProfile {
    	/// L0-L1 target file size
    	pub initial_file_size: u64,
    	/// L2-LN target file size multiplier
    	pub file_size_multiplier: i32,
    	/// rate limiter for background flushes and compactions, bytes/sec, if any
    	pub write_rate_limit: Option<u64>,
    }

    impl Default for CompactionProfile {
    	/// Default profile suitable for most storage
    	fn default() -> CompactionProfile {
    		CompactionProfile::ssd()
    	}
    }

    impl CompactionProfile {
    	/// Default profile suitable for SSD storage
    	pub fn ssd() -> CompactionProfile {
    		CompactionProfile {
    			initial_file_size: 32 * 1024 * 1024,
    			file_size_multiplier: 2,
    			write_rate_limit: None,
    		}
    	}

    	/// Slow HDD compaction profile
    	pub fn hdd() -> CompactionProfile {
    		CompactionProfile {
    			initial_file_size: 192 * 1024 * 1024,
    			file_size_multiplier: 1,
    			write_rate_limit: Some(8 * 1024 * 1024),
    		}
    	}
    }

    /// Database configuration
    #[derive(Clone)]
    pub struct DatabaseConfig {
    	/// Max number of open files.
    	pub max_open_files: i32,
    	/// Cache sizes (in MiB) for specific columns.
    	pub cache_sizes: HashMap<Option<u32>, usize>,
    	/// Specific number of bloom filter bits, if any
    	pub bloom_filters: HashMap<Option<u32>, u8>,
    	/// Compaction profile
    	pub compaction: CompactionProfile,
    	/// Set number of columns
    	pub columns: Option<u32>,
    	/// Should we keep WAL enabled?
    	pub wal: bool,
    }

    impl DatabaseConfig {
    	/// Create new `DatabaseConfig` with default parameters and specified set of columns.
    	/// Note that cache sizes must be explicitly set.
    	pub fn with_columns(columns: Option<u32>) -> Self {
    		let mut config = Self::default();
    		config.columns = columns;
    		config
    	}

    	/// Set the column cache size in MiB.
    	pub fn set_cache(&mut self, col: Option<u32>, size: usize) {
    		self.cache_sizes.insert(col, size);
    	}
    }

    impl Default for DatabaseConfig {
    	fn default() -> DatabaseConfig {
    		DatabaseConfig {
    			cache_sizes: HashMap::new(),
    			bloom_filters: HashMap::new(),
    			max_open_files: 512,
    			compaction: CompactionProfile::default(),
    			columns: None,
    			wal: true,
    		}
    	}
    }

    /// Database iterator for flushed data only
    pub struct DatabaseIterator {
    	iter: DBIterator,
    }

    impl<'a> Iterator for DatabaseIterator {
    	type Item = (Box<[u8]>, Box<[u8]>);

        fn next(&mut self) -> Option<Self::Item> {
    		self.iter.next()
    	}
    }

    struct DBAndColumns {
    	db: DB,
    	cfs: Vec<Column>,
    }

    /// Key-Value database.
    pub struct Database {
    	db: DBAndColumns,
    	write_opts: WriteOptions,
    	read_opts: ReadOptions,
    }

    impl KeyValueDatabase for Database {
    	fn write(&self, tx: Transaction) -> Result<(), String> {
    		Database::write(self, (&tx).into())
    	}

    	fn get(&self, key: &Key) -> Result<KeyState<Value>, String> {
    		match Database::get(self, &key.into())? {
    			Some(value) => Ok(KeyState::Insert(Value::for_key(key, &value)?)),
    			None => Ok(KeyState::Unknown)
    		}
    	}
    }

    impl Database {
    	/// Open database with default settings.
    	pub fn open_default<P>(path: P) -> Result<Database, String> where P: AsRef<Path> {
    		Database::open(DatabaseConfig::default(), path)
    	}

    	/// Open database file. Creates if it does not exist.
    	pub fn open<P>(config: DatabaseConfig, path: P) -> Result<Database, String> where P: AsRef<Path>{
    		let path = path.as_ref().to_string_lossy();
    		// default cache size for columns not specified.
    		const DEFAULT_CACHE: usize = 2;

    		let mut opts = Options::new();
    		if let Some(rate_limit) = config.compaction.write_rate_limit {
    			try!(opts.set_parsed_options(&format!("rate_limiter_bytes_per_sec={}", rate_limit)));
    		}
    		try!(opts.set_parsed_options(&format!("max_total_wal_size={}", 64 * 1024 * 1024)));
    		try!(opts.set_parsed_options("verify_checksums_in_compaction=0"));
    		opts.set_max_open_files(config.max_open_files);
    		opts.create_if_missing(true);
    		opts.set_use_fsync(false);

    		opts.set_max_background_flushes(DB_BACKGROUND_FLUSHES);
    		opts.set_max_background_compactions(DB_BACKGROUND_COMPACTIONS);

    		// compaction settings
    		opts.set_compaction_style(DBCompactionStyle::DBUniversalCompaction);
    		opts.set_target_file_size_base(config.compaction.initial_file_size);
    		opts.set_target_file_size_multiplier(config.compaction.file_size_multiplier);

    		let mut cf_options = Vec::with_capacity(config.columns.unwrap_or(0) as usize);
    		let cfnames: Vec<_> = (0..config.columns.unwrap_or(0)).map(|c| format!("col{}", c)).collect();
    		let cfnames: Vec<&str> = cfnames.iter().map(|n| n as &str).collect();

    		for col in 0 .. config.columns.unwrap_or(0) {
    			let mut opts = Options::new();
    			opts.set_compaction_style(DBCompactionStyle::DBUniversalCompaction);
    			opts.set_target_file_size_base(config.compaction.initial_file_size);
    			opts.set_target_file_size_multiplier(config.compaction.file_size_multiplier);

    			let col_opt = config.columns.map(|_| col);

    			{
    				let mut block_opts = BlockBasedOptions::new();
    				let cache_size = config.cache_sizes.get(&col_opt).cloned().unwrap_or(DEFAULT_CACHE);
    				if let Some(bloom_bits) = config.bloom_filters.get(&col_opt) {
    					block_opts.set_filter(bloom_bits.clone() as i32);
    				}
    				// all goes to read cache.
    				block_opts.set_cache(Cache::new(cache_size * 1024 * 1024));
    				opts.set_block_based_table_factory(&block_opts);
    			}

    			cf_options.push(opts);
    		}

    		let mut write_opts = WriteOptions::new();
    		if !config.wal {
    			write_opts.disable_wal(true);
    		}
    		let mut read_opts = ReadOptions::new();
    		read_opts.set_verify_checksums(false);

    		let mut cfs: Vec<Column> = Vec::new();
    		let db = match config.columns {
    			Some(columns) => {
    				match DB::open_cf(&opts, &path, &cfnames, &cf_options) {
    					Ok(db) => {
    						cfs = cfnames.iter().map(|n| db.cf_handle(n)
    							.expect("rocksdb opens a cf_handle for each cfname; qed")).collect();
    						assert!(cfs.len() == columns as usize);
    						Ok(db)
    					}
    					Err(_) => {
    						// retry and create CFs
    						match DB::open_cf(&opts, &path, &[], &[]) {
    							Ok(mut db) => {
    								cfs = try!(cfnames.iter().enumerate().map(|(i, n)| db.create_cf(n, &cf_options[i])).collect());
    								Ok(db)
    							},
    							err @ Err(_) => err,
    						}
    					}
    				}
    			},
    			None => DB::open(&opts, &path)
    		};

    		let db = match db {
    			Ok(db) => db,
    			Err(ref s) if s.starts_with("Corruption:") => {
    				info!("{}", s);
    				info!("Attempting DB repair for {}", path);
    				try!(DB::repair(&opts, &path));

    				match cfnames.is_empty() {
    					true => try!(DB::open(&opts, &path)),
    					false => try!(DB::open_cf(&opts, &path, &cfnames, &cf_options))
    				}
    			},
    			Err(s) => { return Err(s); }
    		};
    		Ok(Database {
    			db: DBAndColumns{ db: db, cfs: cfs },
    			write_opts: write_opts,
    			read_opts: read_opts,
    		})
    	}

    	/// Commit transaction to database.
    	pub fn write(&self, tx: RawTransaction) -> Result<(), String> {
    		let DBAndColumns { ref db, ref cfs } = self.db;
    		let batch = WriteBatch::new();
    		for op in tx.operations.into_iter() {
    			match op {
    				RawOperation::Insert(RawKeyValue { location, key, value }) => match location {
    					Location::DB => batch.put(&key, &value)?,
    					Location::Column(col) => batch.put_cf(cfs[col as usize], &key, &value)?,
    				},
    				RawOperation::Delete(RawKey { location, key }) => match location {
    					Location::DB => batch.delete(&key)?,
    					Location::Column(col) => batch.delete_cf(cfs[col as usize], &key)?,
    				},
    			}
    		}
    		db.write_opt(batch, &self.write_opts)
    	}

    	/// Get value by key.
    	pub fn get(&self, key: &RawKey) -> Result<Option<Bytes>, String> {
    		let DBAndColumns { ref db, ref cfs } = self.db;
    		match key.location {
    			Location::DB => {
    				let value = db.get_opt(&key.key, &self.read_opts)?;
    				Ok(value.map(|v| (&*v).into()))
    			},
    			Location::Column(col) => {
    				let value = db.get_cf_opt(cfs[col as usize], &key.key, &self.read_opts)?;
    				Ok(value.map(|v| (&*v).into()))
    			}
    		}
    	}

    	/// Close the database
    	pub fn close(self) {}

    	pub fn iter(&self, location: Location) -> DatabaseIterator {
    		let DBAndColumns { ref db, ref cfs } = self.db;
    		match location {
    			Location::DB => DatabaseIterator {
    				iter: db.iterator_opt(IteratorMode::Start, &self.read_opts)
    			},
    			Location::Column(column) => DatabaseIterator {
    				iter: db.iterator_cf_opt(cfs[column as usize], IteratorMode::Start, &self.read_opts)
    					.expect("iterator params are valid; qed")
    			}
    		}
    	}
    }

    #[cfg(test)]
    mod tests {
    	extern crate tempdir;

    	use self::tempdir::TempDir;
    	use kv::{RawTransaction, Location};
    	use super::*;

    	fn test_db(config: DatabaseConfig) {
    		let tempdir = TempDir::new("").unwrap();
    		let db = Database::open(config, tempdir.path()).unwrap();

    		let key1 = b"key1";
    		let key2 = b"key2";
    		let key3 = b"key3";

    		let mut batch = RawTransaction::default();
    		batch.insert_raw(Location::DB, key1, b"cat");
    		batch.insert_raw(Location::DB, key2, b"dog");
    		db.write(batch).unwrap();

    		assert_eq!(&*db.get(&RawKey::new(Location::DB,key1 as &[u8])).unwrap().unwrap(), b"cat");

    		let contents: Vec<_> = db.iter(Location::DB).collect();
    		assert_eq!(contents.len(), 2);
    		assert_eq!(&*contents[0].0, &*key1);
    		assert_eq!(&*contents[0].1, b"cat");
    		assert_eq!(&*contents[1].0, &*key2);
    		assert_eq!(&*contents[1].1, b"dog");

    		let mut batch = RawTransaction::default();
    		batch.delete_raw(Location::DB, key1);
    		db.write(batch).unwrap();

    		assert_eq!(db.get(&RawKey::new(Location::DB, key1 as &[u8])).unwrap(), None);

    		let mut batch = RawTransaction::default();
    		batch.insert_raw(Location::DB, key1, b"cat");
    		db.write(batch).unwrap();

    		let mut transaction = RawTransaction::default();
    		transaction.insert_raw(Location::DB, key3, b"elephant");
    		transaction.delete_raw(Location::DB, key1);
    		db.write(transaction).unwrap();
    		assert_eq!(&*db.get(&RawKey::new(Location::DB, key3 as &[u8])).unwrap().unwrap(), b"elephant");
    	}

    	#[test]
    	fn kvdb() {
    		let tempdir = TempDir::new("").unwrap();
    		let _ = Database::open_default(tempdir.path()).unwrap();
    		test_db(DatabaseConfig::default());
    	}
    }

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