"""Utility code for constructing importers, etc."""
from _frozen_importlib import _resolve_name
from _frozen_importlib import _find_spec

import sys


def resolve_name(name, package):
    """Resolve a relative module name to an absolute one."""
    if not name.startswith('.'):
        return name
    elif not package:
        raise ValueError(f'no package specified for {repr(name)} '
                         '(required for relative module names)')
    level = 0
    for character in name:
        if character != '.':
            break
        level += 1
    return _resolve_name(name[level:], package, level)


def _find_spec_from_path(name, path=None):
    """Return the spec for the specified module.

    First, sys.modules is checked to see if the module was already imported. If
    so, then sys.modules[name].__spec__ is returned. If that happens to be
    set to None, then ValueError is raised. If the module is not in
    sys.modules, then sys.meta_path is searched for a suitable spec with the
    value of 'path' given to the finders. None is returned if no spec could
    be found.

    Dotted names do not have their parent packages implicitly imported. You will
    most likely need to explicitly import all parent packages in the proper
    order for a submodule to get the correct spec.

    """
    if name not in sys.modules:
        return _find_spec(name, path)
    else:
        module = sys.modules[name]
        if module is None:
            return None
        try:
            spec = module.__spec__
        except AttributeError:
            raise ValueError('{}.__spec__ is not set'.format(name)) from None
        else:
            if spec is None:
                raise ValueError('{}.__spec__ is None'.format(name))
            return spec


def find_spec(name, package=None):
    """Return the spec for the specified module.

    First, sys.modules is checked to see if the module was already imported. If
    so, then sys.modules[name].__spec__ is returned. If that happens to be
    set to None, then ValueError is raised. If the module is not in
    sys.modules, then sys.meta_path is searched for a suitable spec with the
    value of 'path' given to the finders. None is returned if no spec could
    be found.

    If the name is for submodule (contains a dot), the parent module is
    automatically imported.

    The name and package arguments work the same as importlib.import_module().
    In other words, relative module names (with leading dots) work.

    """
    fullname = resolve_name(name, package) if name.startswith('.') else name
    if fullname not in sys.modules:
        parent_name = fullname.rpartition('.')[0]
        if parent_name:
            parent = __import__(parent_name, fromlist=['__path__'])
            try:
                parent_path = parent.__path__
            except AttributeError as e:
                raise ModuleNotFoundError(
                    f"__path__ attribute not found on {parent_name!r} "
                    f"while trying to find {fullname!r}", name=fullname) from e
        else:
            parent_path = None
        return _find_spec(fullname, parent_path)
    else:
        module = sys.modules[fullname]
        if module is None:
            return None
        try:
            spec = module.__spec__
        except AttributeError:
            raise ValueError('{}.__spec__ is not set'.format(name)) from None
        else:
            if spec is None:
                raise ValueError('{}.__spec__ is None'.format(name))
            return spec