import sys

from packaging import version as _version


def _check_version(actver, version, cmp_op):
    """
    Check version string of an active module against a required version.

    If dev/prerelease tags result in TypeError for string-number comparison,
    it is assumed that the dependency is satisfied.
    Users on dev branches are responsible for keeping their own packages up to
    date.
    """
    try:
        if cmp_op == '>':
            return _version.parse(actver) > _version.parse(version)
        elif cmp_op == '>=':
            return _version.parse(actver) >= _version.parse(version)
        elif cmp_op == '=':
            return _version.parse(actver) == _version.parse(version)
        elif cmp_op == '<':
            return _version.parse(actver) < _version.parse(version)
        else:
            return False
    except TypeError:
        return True


def get_module_version(module_name):
    """Return module version or None if version can't be retrieved."""
    mod = __import__(module_name, fromlist=[module_name.rpartition('.')[-1]])
    return getattr(mod, '__version__', getattr(mod, 'VERSION', None))


def is_installed(name, version=None):
    """Test if *name* is installed.

    Parameters
    ----------
    name : str
        Name of module or "python"
    version : str, optional
        Version string to test against.
        If version is not None, checking version
        (must have an attribute named '__version__' or 'VERSION')
        Version may start with =, >=, > or < to specify the exact requirement

    Returns
    -------
    out : bool
        True if `name` is installed matching the optional version.
    """
    if name.lower() == 'python':
        actver = sys.version[:6]
    else:
        try:
            actver = get_module_version(name)
        except ImportError:
            return False
    if version is None:
        return True
    else:
        # since version_requirements is in the critical import path,
        # we lazy import re
        import re

        match = re.search('[0-9]', version)
        assert match is not None, "Invalid version number"
        symb = version[: match.start()]
        if not symb:
            symb = '='
        assert symb in ('>=', '>', '=', '<'), f"Invalid version condition '{symb}'"
        version = version[match.start() :]
        return _check_version(actver, version, symb)


def require(name, version=None):
    """Return decorator that forces a requirement for a function or class.

    Parameters
    ----------
    name : str
        Name of module or "python".
    version : str, optional
        Version string to test against.
        If version is not None, checking version
        (must have an attribute named '__version__' or 'VERSION')
        Version may start with =, >=, > or < to specify the exact requirement

    Returns
    -------
    func : function
        A decorator that raises an ImportError if a function is run
        in the absence of the input dependency.
    """
    # since version_requirements is in the critical import path, we lazy import
    # functools
    import functools

    def decorator(obj):
        @functools.wraps(obj)
        def func_wrapped(*args, **kwargs):
            if is_installed(name, version):
                return obj(*args, **kwargs)
            else:
                msg = f'"{obj}" in "{obj.__module__}" requires "{name}'
                if version is not None:
                    msg += f" {version}"
                raise ImportError(msg + '"')

        return func_wrapped

    return decorator


def get_module(module_name, version=None):
    """Return a module object of name *module_name* if installed.

    Parameters
    ----------
    module_name : str
        Name of module.
    version : str, optional
        Version string to test against.
        If version is not None, checking version
        (must have an attribute named '__version__' or 'VERSION')
        Version may start with =, >=, > or < to specify the exact requirement

    Returns
    -------
    mod : module or None
        Module if *module_name* is installed matching the optional version
        or None otherwise.
    """
    if not is_installed(module_name, version):
        return None
    return __import__(module_name, fromlist=[module_name.rpartition('.')[-1]])