from distutils.version import LooseVersion
import functools
import re
import sys


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.

    Copyright (C) 2013  The IPython Development Team

    Distributed under the terms of the BSD License.
    """
    try:
        if cmp_op == '>':
            return LooseVersion(actver) > LooseVersion(version)
        elif cmp_op == '>=':
            return LooseVersion(actver) >= LooseVersion(version)
        elif cmp_op == '=':
            return LooseVersion(actver) == LooseVersion(version)
        elif cmp_op == '<':
            return LooseVersion(actver) < LooseVersion(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.

    Notes
    -----
    Original Copyright (C) 2009-2011 Pierre Raybaut
    Licensed under the terms of the MIT License.
    """
    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:
        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 ('>=', '>', '=', '<'),\
            "Invalid version condition '%s'" % 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.
    """
    def decorator(obj):
        @functools.wraps(obj)
        def func_wrapped(*args, **kwargs):
            if is_installed(name, version):
                return obj(*args, **kwargs)
            else:
                msg = '"%s" in "%s" requires "%s'
                msg = msg % (obj, obj.__module__, name)
                if not version is None:
                    msg += " %s" % 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]])
