"""

Part of the bsie module.
A copy of the license is provided with the project.
Author: Matthias Baumgartner, 2022
"""
# imports
import importlib
import logging
import typing

# bsie imports
from bsie import base
from bsie.base import errors
from bsie.utils.bsfs import URI, typename
from bsie.utils import bsfs

# inner-module imports
from . import pipeline

# exports
__all__: typing.Sequence[str] = (
    'ExtractorBuilder',
    'PipelineBuilder',
    'ReaderBuilder',
    )


## code ##

logger = logging.getLogger(__name__)

def _safe_load(module_name: str, class_name: str):
    """Get a class from a module. Raise BuilderError if anything goes wrong."""
    try:
        # load the module
        module = importlib.import_module(module_name)
    except Exception as err:
        # cannot import module
        raise errors.LoaderError(f'cannot load module {module_name}') from err

    try:
        # get the class from the module
        cls = getattr(module, class_name)
    except Exception as err:
        # cannot find the class
        raise errors.LoaderError(f'cannot load class {class_name} from module {module_name}') from err

    return cls


def _unpack_name(name):
    """Split a name into its module and class component (dot-separated)."""
    if not isinstance(name, str):
        raise TypeError(name)
    if '.' not in name:
        raise ValueError('name must be a qualified class name.')
    module_name, class_name = name[:name.rfind('.')], name[name.rfind('.')+1:]
    if module_name == '':
        raise ValueError('name must be a qualified class name.')
    return module_name, class_name


class ReaderBuilder():
    """Build `bsie.base.Reader` instances.

    Readers are defined via their qualified class name
    (e.g., bsie.reader.path.Path) and optional keyword
    arguments that are passed to the constructor via
    the *kwargs* argument (name as key, kwargs as value).
    The ReaderBuilder keeps a cache of previously built
    reader instances, as they are anyway built with
    identical keyword arguments.

    """

    # keyword arguments
    kwargs: typing.Dict[str, typing.Dict[str, typing.Any]]

    # cached readers
    cache: typing.Dict[str, base.reader.Reader]

    def __init__(self, kwargs: typing.Dict[str, typing.Dict[str, typing.Any]]):
        self.kwargs = kwargs
        self.cache = {}

    def build(self, name: str) -> base.Reader:
        """Return an instance for the qualified class name."""
        # return cached instance
        if name in self.cache:
            return self.cache[name]

        # check name and get module/class components
        module_name, class_name = _unpack_name(name)

        # import reader class
        cls = _safe_load(module_name, class_name)

        # get kwargs
        kwargs = self.kwargs.get(name, {})
        if not isinstance(kwargs, dict):
            raise TypeError(f'expected a kwargs dict, found {bsfs.typename(kwargs)}')

        try: # build, cache, and return instance
            obj = cls(**kwargs)
            # cache instance
            self.cache[name] = obj
            # return instance
            return obj

        except Exception as err:
            raise errors.BuilderError(f'failed to build reader {name} due to {bsfs.typename(err)}: {err}') from err


class ExtractorBuilder():
    """Build `bsie.base.Extractor instances.

    It is permissible to build multiple instances of the same extractor
    (typically with different arguments), hence the ExtractorBuilder
    receives a list of build specifications. Each specification is
    a dict with a single key (extractor's qualified name) and a dict
    to be used as keyword arguments.
    Example: [{'bsie.extractor.generic.path.Path': {}}, ]

    """

    # build specifications
    specs: typing.List[typing.Dict[str, typing.Dict[str, typing.Any]]]

    def __init__(self, specs: typing.List[typing.Dict[str, typing.Dict[str, typing.Any]]]):
        self.specs = specs

    def __iter__(self) -> typing.Iterator[int]:
        """Iterate over extractor specifications."""
        return iter(range(len(self.specs)))

    def build(self, index: int) -> base.Extractor:
        """Return an instance of the n'th extractor (n=*index*)."""
        # get build instructions
        specs = self.specs[index]

        # check specs structure. expecting[{name: {kwargs}}]
        if not isinstance(specs, dict):
            raise TypeError(f'expected a dict, found {bsfs.typename(specs)}')
        if len(specs) != 1:
            raise TypeError(f'expected a dict of length one, found {len(specs)}')

        # get name and args from specs
        name = next(iter(specs.keys()))
        kwargs = specs[name]

        # check kwargs structure
        if not isinstance(kwargs, dict):
            raise TypeError(f'expected a dict, found {bsfs.typename(kwargs)}')

        # check name and get module/class components
        module_name, class_name = _unpack_name(name)

        # import extractor class
        cls = _safe_load(module_name, class_name)

        try: # build and return instance
            return cls(**kwargs)

        except Exception as err:
            raise errors.BuilderError(f'failed to build extractor {name} due to {bsfs.typename(err)}: {err}') from err


class PipelineBuilder():
    """Build `bsie.tools.pipeline.Pipeline` instances."""

    # Prefix to be used in the Pipeline.
    prefix: bsfs.Namespace

    # builder for Readers.
    rbuild: ReaderBuilder

    # builder for Extractors.
    ebuild: ExtractorBuilder

    def __init__(
            self,
            prefix: bsfs.Namespace,
            reader_builder: ReaderBuilder,
            extractor_builder: ExtractorBuilder,
            ):
        self.prefix = prefix
        self.rbuild = reader_builder
        self.ebuild = extractor_builder

    def build(self) -> pipeline.Pipeline:
        """Return a Pipeline instance."""
        ext2rdr = {}

        for eidx in self.ebuild:
            # build extractor
            try:
                ext = self.ebuild.build(eidx)

            except errors.LoaderError as err: # failed to load extractor; skip
                logger.error('failed to load extractor: %s', err)
                continue

            except errors.BuilderError as err: # failed to build instance; skip
                logger.error(str(err))
                continue

            try:
                # get reader required by extractor
                if ext.CONTENT_READER is not None:
                    rdr = self.rbuild.build(ext.CONTENT_READER)
                else:
                    rdr = None
                # store extractor
                ext2rdr[ext] = rdr

            except errors.LoaderError as err: # failed to load reader
                logger.error('failed to load reader: %s', err)

            except errors.BuilderError as err: # failed to build reader
                logger.error(str(err))

        return pipeline.Pipeline(self.prefix, ext2rdr)



## EOF ##