import binascii
import os
import mmap
import sys
import errno

from cStringIO import StringIO

# in py 2.4, StringIO is only StringI, without write support.
# Hence we must use the python implementation for this
if sys.version_info[1] < 5:
	from StringIO import StringIO
# END handle python 2.4

try:
	import async.mod.zlib as zlib
except ImportError:
	import zlib
# END try async zlib

from async import ThreadPool

try:
    import hashlib
except ImportError:
    import sha

try:
	from struct import unpack_from
except ImportError:
	from struct import unpack, calcsize
	__calcsize_cache = dict()
	def unpack_from(fmt, data, offset=0):
		try:
			size = __calcsize_cache[fmt]
		except KeyError:
			size = calcsize(fmt)
			__calcsize_cache[fmt] = size
		# END exception handling
		return unpack(fmt, data[offset : offset + size])
	# END own unpack_from implementation


#{ Globals

# A pool distributing tasks, initially with zero threads, hence everything 
# will be handled in the main thread
pool = ThreadPool(0)

#} END globals


#{ Aliases

hex_to_bin = binascii.a2b_hex
bin_to_hex = binascii.b2a_hex

# errors
ENOENT = errno.ENOENT

# os shortcuts
exists = os.path.exists
mkdir = os.mkdir
chmod = os.chmod
isdir = os.path.isdir
isfile = os.path.isfile
rename = os.rename
remove = os.remove
dirname = os.path.dirname
basename = os.path.basename
join = os.path.join
read = os.read
write = os.write
close = os.close
fsync = os.fsync

# constants
NULL_HEX_SHA = "0"*40
NULL_BIN_SHA = "\0"*20

#} END Aliases

#{ compatibility stuff ... 

class _RandomAccessStringIO(object):
	"""Wrapper to provide required functionality in case memory maps cannot or may 
	not be used. This is only really required in python 2.4"""
	__slots__ = '_sio'
	
	def __init__(self, buf=''):
		self._sio = StringIO(buf)
		
	def __getattr__(self, attr):
		return getattr(self._sio, attr)
	
	def __len__(self):
		return len(self.getvalue())
		
	def __getitem__(self, i):
		return self.getvalue()[i]
		
	def __getslice__(self, start, end):
		return self.getvalue()[start:end]
	
#} END compatibility stuff ...

#{ Routines

def make_sha(source=''):
    """A python2.4 workaround for the sha/hashlib module fiasco 
    :note: From the dulwich project """
    try:
        return hashlib.sha1(source)
    except NameError:
        sha1 = sha.sha(source)
        return sha1

def allocate_memory(size):
	""":return: a file-protocol accessible memory block of the given size"""
	if size == 0:
		return _RandomAccessStringIO('')
	# END handle empty chunks gracefully
	
	try:
		return mmap.mmap(-1, size)	# read-write by default
	except EnvironmentError:
		# setup real memory instead
		# this of course may fail if the amount of memory is not available in
		# one chunk - would only be the case in python 2.4, being more likely on 
		# 32 bit systems.
		return _RandomAccessStringIO("\0"*size)
	# END handle memory allocation
	

def file_contents_ro(fd, stream=False, allow_mmap=True):
	""":return: read-only contents of the file represented by the file descriptor fd
	:param fd: file descriptor opened for reading
	:param stream: if False, random access is provided, otherwise the stream interface
		is provided.
	:param allow_mmap: if True, its allowed to map the contents into memory, which 
		allows large files to be handled and accessed efficiently. The file-descriptor
		will change its position if this is False"""
	try:
		if allow_mmap:
			# supports stream and random access
			try:
				return mmap.mmap(fd, 0, access=mmap.ACCESS_READ)
			except EnvironmentError:
				# python 2.4 issue, 0 wants to be the actual size
				return mmap.mmap(fd, os.fstat(fd).st_size, access=mmap.ACCESS_READ)
			# END handle python 2.4
	except OSError:
		pass
	# END exception handling
	
	# read manully
	contents = os.read(fd, os.fstat(fd).st_size)
	if stream:
		return _RandomAccessStringIO(contents)
	return contents
	
def file_contents_ro_filepath(filepath, stream=False, allow_mmap=True, flags=0):
	"""Get the file contents at filepath as fast as possible
	:return: random access compatible memory of the given filepath
	:param stream: see ``file_contents_ro``
	:param allow_mmap: see ``file_contents_ro``
	:param flags: additional flags to pass to os.open
	:raise OSError: If the file could not be opened
	:note: for now we don't try to use O_NOATIME directly as the right value needs to be 
		shared per database in fact. It only makes a real difference for loose object 
		databases anyway, and they use it with the help of the ``flags`` parameter"""
	fd = os.open(filepath, os.O_RDONLY|getattr(os, 'O_BINARY', 0)|flags)
	try:
		return file_contents_ro(fd, stream, allow_mmap)
	finally:
		close(fd)
	# END assure file is closed
	
def to_hex_sha(sha):
	""":return: hexified version  of sha"""
	if len(sha) == 40:
		return sha
	return bin_to_hex(sha)
	
def to_bin_sha(sha):
	if len(sha) == 20:
		return sha
	return hex_to_bin(sha)


#} END routines


#{ Utilities

class LazyMixin(object):
	"""
	Base class providing an interface to lazily retrieve attribute values upon
	first access. If slots are used, memory will only be reserved once the attribute
	is actually accessed and retrieved the first time. All future accesses will
	return the cached value as stored in the Instance's dict or slot.
	"""
	
	__slots__ = tuple()
	
	def __getattr__(self, attr):
		"""
		Whenever an attribute is requested that we do not know, we allow it 
		to be created and set. Next time the same attribute is reqeusted, it is simply
		returned from our dict/slots. """
		self._set_cache_(attr)
		# will raise in case the cache was not created
		return object.__getattribute__(self, attr)

	def _set_cache_(self, attr):
		"""
		This method should be overridden in the derived class. 
		It should check whether the attribute named by attr can be created
		and cached. Do nothing if you do not know the attribute or call your subclass
		
		The derived class may create as many additional attributes as it deems 
		necessary in case a git command returns more information than represented 
		in the single attribute."""
		pass

	
class LockedFD(object):
	"""
	This class facilitates a safe read and write operation to a file on disk.
	If we write to 'file', we obtain a lock file at 'file.lock' and write to 
	that instead. If we succeed, the lock file will be renamed to overwrite 
	the original file.
	
	When reading, we obtain a lock file, but to prevent other writers from 
	succeeding while we are reading the file.
	
	This type handles error correctly in that it will assure a consistent state 
	on destruction.
	
	:note: with this setup, parallel reading is not possible"""
	__slots__ = ("_filepath", '_fd', '_write')
	
	def __init__(self, filepath):
		"""Initialize an instance with the givne filepath"""
		self._filepath = filepath
		self._fd = None
		self._write = None			# if True, we write a file
	
	def __del__(self):
		# will do nothing if the file descriptor is already closed
		if self._fd is not None:
			self.rollback()
		
	def _lockfilepath(self):
		return "%s.lock" % self._filepath
		
	def open(self, write=False, stream=False):
		"""
		Open the file descriptor for reading or writing, both in binary mode.
		
		:param write: if True, the file descriptor will be opened for writing. Other
			wise it will be opened read-only.
		:param stream: if True, the file descriptor will be wrapped into a simple stream 
			object which supports only reading or writing
		:return: fd to read from or write to. It is still maintained by this instance
			and must not be closed directly
		:raise IOError: if the lock could not be retrieved
		:raise OSError: If the actual file could not be opened for reading
		:note: must only be called once"""
		if self._write is not None:
			raise AssertionError("Called %s multiple times" % self.open)
		
		self._write = write
		
		# try to open the lock file
		binary = getattr(os, 'O_BINARY', 0)
		lockmode = 	os.O_WRONLY | os.O_CREAT | os.O_EXCL | binary
		try:
			fd = os.open(self._lockfilepath(), lockmode, 0600)
			if not write:
				os.close(fd)
			else:
				self._fd = fd
			# END handle file descriptor
		except OSError:
			raise IOError("Lock at %r could not be obtained" % self._lockfilepath())
		# END handle lock retrieval
		
		# open actual file if required
		if self._fd is None:
			# we could specify exlusive here, as we obtained the lock anyway
			try:
				self._fd = os.open(self._filepath, os.O_RDONLY | binary)
			except:
				# assure we release our lockfile
				os.remove(self._lockfilepath())
				raise
			# END handle lockfile
		# END open descriptor for reading
		
		if stream:
			# need delayed import
			from stream import FDStream
			return FDStream(self._fd)
		else:
			return self._fd
		# END handle stream
		
	def commit(self):
		"""When done writing, call this function to commit your changes into the 
		actual file. 
		The file descriptor will be closed, and the lockfile handled.
		:note: can be called multiple times"""
		self._end_writing(successful=True)
		
	def rollback(self):
		"""Abort your operation without any changes. The file descriptor will be 
		closed, and the lock released.
		:note: can be called multiple times"""
		self._end_writing(successful=False)
		
	def _end_writing(self, successful=True):
		"""Handle the lock according to the write mode """
		if self._write is None:
			raise AssertionError("Cannot end operation if it wasn't started yet")
		
		if self._fd is None:
			return
		
		os.close(self._fd)
		self._fd = None
		
		lockfile = self._lockfilepath()
		if self._write and successful:
			# on windows, rename does not silently overwrite the existing one
			if sys.platform == "win32":
				if isfile(self._filepath):
					os.remove(self._filepath)
				# END remove if exists
			# END win32 special handling
			os.rename(lockfile, self._filepath)
			
			# assure others can at least read the file - the tmpfile left it at rw--
			# We may also write that file, on windows that boils down to a remove-
			# protection as well
			chmod(self._filepath, 0644)
		else:
			# just delete the file so far, we failed
			os.remove(lockfile)
		# END successful handling

#} END utilities
