dbmac.py

"""A slow but simple dbm clone for the Mac.

For database spam, spam.dir contains the index (a text file),
spam.bak *may* contain a backup of the index (also a text file),
while spam.dat contains the data (a binary file).

XXX TO DO:

- reclaim free space (currently, space once occupied by deleted or expanded
items is never reused)

- support concurrent access (currently, if two processes take turns making
updates, they can mess up the index)

- support efficient access to large databases (currently, the whole index
is read when the database is opened, and some updates rewrite the whole index)

- support opening for read-only (flag = 'm')

"""

_os = __import__('os')
import __builtin__

_open = __builtin__.open

_BLOCKSIZE = 512

class _Database:

	def __init__(self, file):
		self._dirfile = file + '.dir'
		self._datfile = file + '.dat'
		self._bakfile = file + '.bak'
		# Mod by Jack: create data file if needed
		try:
			f = _open(self._datfile, 'r')
		except IOError:
			f = _open(self._datfile, 'w')
		f.close()
		self._update()
	
	def _update(self):
		self._index = {}
		try:
			f = _open(self._dirfile)
		except IOError:
			pass
		else:
			while 1:
				line = f.readline()
				if not line: break
				key, (pos, siz) = eval(line)
				self._index[key] = (pos, siz)
			f.close()

	def _commit(self):
		try: _os.unlink(self._bakfile)
		except _os.error: pass
		try: _os.rename(self._dirfile, self._bakfile)
		except _os.error: pass
		f = _open(self._dirfile, 'w')
		for key, (pos, siz) in self._index.items():
			f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
		f.close()
	
	def __getitem__(self, key):
		pos, siz = self._index[key]	# may raise KeyError
		f = _open(self._datfile, 'rb')
		f.seek(pos)
		dat = f.read(siz)
		f.close()
		return dat
	
	def _addval(self, val):
		f = _open(self._datfile, 'rb+')
		f.seek(0, 2)
		pos = f.tell()
## Does not work under MW compiler
##		pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
##		f.seek(pos)
		npos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
		f.write('\0'*(npos-pos))
		pos = npos
		
		f.write(val)
		f.close()
		return (pos, len(val))
	
	def _setval(self, pos, val):
		f = _open(self._datfile, 'rb+')
		f.seek(pos)
		f.write(val)
		f.close()
		return pos, (val)
	
	def _addkey(self, key, (pos, siz)):
		self._index[key] = (pos, siz)
		f = _open(self._dirfile, 'a')
		f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
		f.close()
	
	def __setitem__(self, key, val):
		if not type(key) == type('') == type(val):
			raise TypeError, "dbmac keys and values must be strings"
		if not self._index.has_key(key):
			(pos, siz) = self._addval(val)
			self._addkey(key, (pos, siz))
		else:
			pos, siz = self._index[key]
			oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
			newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
			if newblocks <= oldblocks:
				pos, siz = self._setval(pos, val)
				self._index[key] = pos, siz
			else:
				pos, siz = self._addval(val)
				self._index[key] = pos, siz
			self._addkey(key, (pos, siz))
	
	def __delitem__(self, key):
		del self._index[key]
		self._commit()
	
	def keys(self):
		return self._index.keys()
	
	def has_key(self, key):
		return self._index.has_key(key)
	
	def __len__(self):
		return len(self._index)
	
	def close(self):
		self._index = self._datfile = self._dirfile = self._bakfile = None


def open(file, flag = None, mode = None):
	# flag, mode arguments are currently ignored
	return _Database(file)