# Copyright (c) 2016-2017, the ElectrumX authors
#
# All rights reserved.
#
# See the file "LICENCE" for information about the copyright
# and warranty status of this software.

'''Backend database abstraction.'''

import os
from functools import partial
from typing import Type, Sequence, TYPE_CHECKING

import electrumx.lib.util as util

if TYPE_CHECKING:
    from rocksdb import DB as rocksdb_db
    from plyvel import DB as plyvel_db


def db_class(name: str) -> Type['Storage']:
    '''Returns a DB engine class.'''
    for db_class in util.subclasses(Storage):
        if db_class.__name__.lower() == name.lower():
            db_class.import_module()
            return db_class
    raise RuntimeError(f'unrecognised DB engine "{name}"')


def list_db_engine_choices() -> Sequence[str]:
    return [db_class.__name__.lower() for db_class in util.subclasses(Storage)]


class Storage:
    '''Abstract base class of the DB backend abstraction.'''

    def __init__(self, name: str, for_sync: bool):
        self.is_new = not os.path.exists(name)
        self.for_sync = for_sync or self.is_new
        self.open(name, create=self.is_new)

    @classmethod
    def import_module(cls) -> None:
        '''Import the DB engine module.'''
        raise NotImplementedError

    def open(self, name: str, create: bool):
        '''Open an existing database or create a new one.'''
        raise NotImplementedError

    def close(self) -> None:
        '''Close an existing database.'''
        raise NotImplementedError

    def get(self, key: bytes) -> bytes:
        raise NotImplementedError

    def put(self, key: bytes, value: bytes) -> None:
        raise NotImplementedError

    def write_batch(self):
        '''Return a context manager that provides `put` and `delete`.

        Changes should only be committed when the context manager
        closes without an exception.
        '''
        raise NotImplementedError

    def iterator(self, prefix: bytes = b'', reverse: bool = False):
        '''Return an iterator that yields (key, value) pairs from the
        database sorted by key.

        If `prefix` is set, only keys starting with `prefix` will be
        included.  If `reverse` is True the items are returned in
        reverse order.

        The iterator supports .seek(key), which moves it just left of `key`.
        - if forward-iterating
            - if `key` is present, it will be the next item
            - if `key` is not present, the next item will be the smallest still greater than `key`
        - if reverse-iterating
            - the next item will be the largest still smaller than `key`
        '''
        raise NotImplementedError


class LevelDB(Storage):
    '''LevelDB database engine.'''

    db: 'plyvel_db'

    @classmethod
    def import_module(cls):
        try:
            import plyvel
        except ImportError as e:
            raise Exception(f"{e}. For the leveldb DB-backend, you should install ElectrumX with the [leveldb] pip extra.") from e

        cls.module = plyvel

    def open(self, name, create):
        mof = 512 if self.for_sync else 128
        # Use snappy compression (the default)
        self.db = self.module.DB(name, create_if_missing=create,
                                 max_open_files=mof)
        self.close = self.db.close
        self.get = self.db.get
        self.put = self.db.put
        self.write_batch = partial(self.db.write_batch, transaction=True,
                                   sync=True)

    def iterator(self, prefix=b'', reverse=False):
        return LevelDBIterator(db=self.db, prefix=prefix, reverse=reverse)


class LevelDBIterator:
    '''An iterator for LevelDB.'''

    def __init__(self, *, db: 'plyvel_db', prefix: bytes, reverse: bool):
        self.prefix = prefix
        self.iterator = db.iterator(prefix=prefix, reverse=reverse)

    def __iter__(self):
        return self

    def __next__(self):
        k, v = next(self.iterator)
        if not k.startswith(self.prefix):
            raise StopIteration
        return k, v

    def seek(self, key: bytes) -> None:
        self.iterator.seek(key)


class RocksDB(Storage):
    '''RocksDB database engine.'''

    db: 'rocksdb_db'

    @classmethod
    def import_module(cls):
        try:
            import rocksdb
        except ImportError as e:
            raise Exception(f"{e}. For the rocksdb DB-backend, you should install ElectrumX with the [rocksdb] pip extra.") from e
        cls.module = rocksdb

    def open(self, name, create):
        mof = 512 if self.for_sync else 128
        # Use snappy compression (the default)
        options = self.module.Options(create_if_missing=create,
                                      use_fsync=True,
                                      target_file_size_base=33554432,
                                      max_open_files=mof)
        self.db = self.module.DB(name, options)
        self.close = self.db.close
        self.get = self.db.get
        self.put = self.db.put

    def write_batch(self):
        return RocksDBWriteBatch(self.db)

    def iterator(self, prefix=b'', reverse=False):
        return RocksDBIterator(db=self.db, prefix=prefix, reverse=reverse)


class RocksDBWriteBatch:
    '''A write batch for RocksDB.'''

    def __init__(self, db: 'rocksdb_db'):
        self.batch = RocksDB.module.WriteBatch()
        self.db = db

    def __enter__(self):
        return self.batch

    def __exit__(self, exc_type, exc_val, exc_tb):
        if not exc_val:
            self.db.write(self.batch)


class RocksDBIterator:
    '''An iterator for RocksDB.'''

    def __init__(self, *, db: 'rocksdb_db', prefix: bytes, reverse: bool):
        self.prefix = prefix
        self._is_reverse = reverse
        if reverse:
            self.iterator = reversed(db.iteritems())
            nxt_prefix = util.increment_byte_string(prefix)
            if nxt_prefix:
                self.iterator.seek(nxt_prefix)
                try:
                    next(self.iterator)
                except StopIteration:
                    self.iterator.seek(nxt_prefix)
            else:
                self.iterator.seek_to_last()
        else:
            self.iterator = db.iteritems()
            self.iterator.seek(prefix)

    def __iter__(self):
        return self

    def __next__(self):
        k, v = next(self.iterator)
        if not k.startswith(self.prefix):
            raise StopIteration
        return k, v

    def seek(self, key: bytes) -> None:
        self.iterator.seek(key)
        if self._is_reverse:
            try:
                next(self)
            except StopIteration:
                self.iterator.seek_to_last()