"""Generally useful Python code.

But strictly no third party module dependencies.
"""

#   Copyright (C) 2011 Stephen Fairchild (s-fairchild@users.sourceforge.net)
#
#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation, either version 2 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program in the file entitled COPYING.
#   If not, see <http://www.gnu.org/licenses/>.

__all__ = ["Singleton", "PolicedAttributes", "FixedAttributes",
                "PathStr", "SlotObject", "string_multireplace"]

import os
import uuid
import re
import glob
import shutil
import threading
from functools import wraps

def cmp(a, b):
    """ cmp was removed in python3, so added this simple replacement """
    return (a > b) - ( a < b)


class Singleton(type):
    """Enforce the singleton pattern upon the user class."""

    def __init__(cls, name, bases, dict_):
        super(Singleton, cls).__init__(name, bases, dict_)
        cls._instance = None

    def __call__(cls, *args, **kwds):
        if cls._instance is not None:
            # Return an existing instance.
            return cls._instance
        else:
            # No existing instance so instantiate just this once.
            cls._instance = super(Singleton, cls).__call__(*args, **kwds)
            return cls._instance


def _pa_rlock(func):
    """Policed Attributes helper for thread locking."""

    @wraps(func)
    def _wrapper(cls, *args, **kwds):
        """Wrapper with locking feature. Performs rlock."""

        rlock = type.__getattribute__(cls, "_rlock")
        try:
            rlock.acquire()
            return func(cls, *args, **kwds)
        finally:
            rlock.release()
    return _wrapper


class FixedAttributes(type):
    """Implements a namespace class of constants."""

    def __setattr__(cls, name, value):
        raise AttributeError("attribute is locked")

    def __call__(cls, *args, **kwds):
        raise TypeError("%s object is not callable" % cls.__name__)


class PolicedAttributes(FixedAttributes):
    """Polices data access to a namespace class.

    Prevents write access to attributes after they have been read.
    Envisioned useful for the implementation of "safe" global variables.
    """

    def __new__(mcs, name, bases, dict_):
        @classmethod
        @_pa_rlock
        def peek(cls, attr, callback, *args, **kwds):
            """Allow read + write within a callback.

            Typical use might be to append to an existing string.
            No modification ban is placed or bypassed.
            """

            if attr not in type.__getattribute__(cls, "_banned"):
                new = callback(
                        super(PolicedAttributes, cls).__getattribute__(attr),
                        *args, **kwds)
                type.__setattr__(attr, new)
            else:
                raise AttributeError("attribute is locked")

        dict_["peek"] = peek
        dict_["_banned"] = set()
        dict_["_rlock"] = threading.RLock()
        return super(PolicedAttributes, mcs).__new__(mcs, name, bases, dict_)

    @_pa_rlock
    def __getattribute__(cls, name):
        type.__getattribute__(cls, "_banned").add(name)
        return type.__getattribute__(cls, name)

    @_pa_rlock
    def __setattr__(cls, name, value):
        if name in type.__getattribute__(cls, "_banned"):
            FixedAttributes.__setattr__(cls, name, value)
        type.__setattr__(cls, name, value)


class PathStrMeta(type):
    """PathStr() returns None if called with None."""

    def __call__(cls, arg):
        if arg is None:
            return None
        else:
            return cls.__new__(cls, arg)


class PathStr(str, metaclass=PathStrMeta):
    """A data type to perform path joins using the / operator.

    In this case the higher precedence of / is unfortunate.
    """

    def __truediv__(self, other):
        return PathStr(os.path.join(str(self), other))

    def __add__(self, other):
        return PathStr(str.__add__(self, other))

    def __repr__(self):
        return "PathStr('%s')" % self


class SlotObject(object):
    """A mutable object containing an immutable object."""

    __slots__ = ['value']

    def __init__(self, value):
        self.value = value

    def __str__(self):
        return str(self.value)

    def __int__(self):
        return int(self.value)

    def __float__(self):
        return float(self.value)

    def __repr__(self):
        return "SlotObject(%s)" % repr(self.value)

    def __getattr__(self, what):
        """Universal getter for get_ prefix."""

        def assign(value):
            """Returned by set_ prefix call. A setter function."""

            self.value = value

        if what.startswith("get_"):
            return lambda : self.value
        elif what.startswith("set_"):
            return assign
        else:
            object.__getattribute__(self, what)


def string_multireplace(part, table, encoding='utf-8'):
    """Replace multiple items in a string.

    Table is a sequence of 2 tuples of from, to strings.

    TOREVIEW: I added scanning the table and parts to ensure
    that everything is encoded as (by default) utf-8. This
    was to try and keep most of the bytes/string conversion
    in one place. It's kinda ugly but works. I'm thinking a
    more pythonic solution would be to create a new list/tuple class
    that automatically does this.
    """

    if not table:
        return part

    nt = []
    for r in table:
        nr = []
        for c in r:
            if type(c) is bytes:
                c = c.decode(encoding, 'replace')
            nr.append(c)
        nt.append(nr)

    table = nt
    parts = part.split(table[0][0])
    t_next = table[1:]

    for i, each in enumerate(parts):
        if each is type(bytes):
            each = each.decode(encoding, 'replace')
        if i is type(bytes):
            i = i.decode(encoding, 'replace')
        parts[i] = string_multireplace(each, t_next)

    return table[0][1].join(parts)


class LinkUUIDRegistry(dict, metaclass=Singleton):
    """Manage substitute hard links for data files."""

    link_re = re.compile(
            r"\{[a-fA-F0-9]{8}-([a-fA-F0-9]{4}-){3}[a-fA-F0-9]{12}\}")
    link_dir = None

    def add(self, uuid_, pathname):
        if os.path.exists(pathname):
            self[uuid_] = pathname
        else:
            print("LinkUUIDRegistry: pathname does not exist", pathname)

    def remove(self, uuid_):
        try:
            del self[uuid_]
        except KeyError:
            print("LinkUUIDRegisty: remove -- UUID does not exist: {%s}" % uuid_)

    def _purge(self, where):
        """Clean orphaned hard links from the links directory."""

        basedir, dirs, files = next(os.walk(where))
        for filename in files:
            match = self.link_re.match(filename)
            try:
                if match is None or str(uuid.UUID(match.group(0))) not in self:
                    os.unlink(os.path.join(basedir, filename))
            except EnvironmentError as e:
                print("LinkUUIDRegistry: link purge failed: %s" % e)


    def _save(self, where, copy):
        """Write new hard links to the links directory.

        Existing links are kept as they are. To unlink them could delete the
        only copy of the link source.
        """

        # Create the links directory as needed.
        if not os.path.isdir(where):
            try:
                os.mkdir(where)
            except EnvironmentError as e:
                print("LinkUUIDRegistry: link directory creation failed:", e)
                return

        for uuid_, source in self.items():
            ext = os.path.splitext(source)[1]
            if copy:
                cmd = shutil.copyfile
            else:
                cmd = os.link
            try:
                cmd(source, os.path.join(where, "{%s}%s" % (uuid_, ext)))
            except EnvironmentError as e:
                if e.errno != 17:
                    print("LinkUUIDRegistry: link failed:", e)
            except shutil.Error:
                pass

    def update(self, where, copy=False):
        """Update the hard links in the links directory."""

        self._save(where, copy)
        # Purge after save because the link source may just be in the
        # links directory itself.
        self._purge(where)
        self.link_dir = where

    def get_link_filename(self, uuid_):
        """Check in the links directory for a specific UUID filename."""

        if self.link_dir is not None:
            matches = glob.glob(os.path.join(self.link_dir, "{%s}.*" % uuid_))
            if len(matches) == 1:
                return os.path.basename(matches[0])

        # Link does not exist e.g. can't hard-link across filesystems
        # or was not made due to policy.
        # For a return value of None the caller must substitute the
        # pre-existing pathname to preserve functionality.
        return None