class BufferManager:
def __init__(self, filename=settings.filename):
self.diskManager = DiskManager(filename)
self.buffer = {}
self.lastEmptyPage = 1
self.buffersize = settings.buffersize
def __del__(self):
self.commit()
self.diskManager.closeFile()
#loads metadata from 0 page:
# - total pages
def loadMetaData(self):
page = self.diskManager.readPage(0)
self.lastEmptyPage = struct.unpack('Q', page.get(2))[0]
metaLen = struct.unpack('Q', page.get(12))[0]
self.diskManager.f.seek((self.lastEmptyPage + 1) * settings.pagesize)
return self.diskManager.f.read(metaLen)
def saveMetaData(self, data=b''):
page = Page(0)
page.add(struct.pack('Q', self.lastEmptyPage))
page.add(struct.pack('Q', len(data)))
self.diskManager.writePage(page)
self.diskManager.f.seek((self.lastEmptyPage + 1) * settings.pagesize)
self.diskManager.f.write(data)
# loads page in buffer if it is not loaded
def loadPage(self, pageNum):
page = self.buffer.get(pageNum)
if page == None:
page = self.diskManager.readPage(pageNum)
if len(self.buffer) >= self.buffersize:
removedPage = self.buffer.pop(min(self.buffer,
key = lambda p : self.buffer.get(p).time))
if removedPage.changed:
self.diskManager.writePage(removedPage)
self.buffer[page.id] = page
else:
page.time = time.time()
def getPage(self, pageNum):
self.loadPage(pageNum)
return self.buffer.get(pageNum)
def setPage(self, page):
self.loadPage(page.id)
page.changed = True
page.time = time.time()
self.buffer[page.id] = page
# saves all changed pages on disk
def commit(self):
for p in self.buffer.values():
if p.changed:
self.diskManager.writePage(p)
#reads data by entry
def readData(self, entry):
page = self.getPage(entry.page)
return page.get(entry.offset)
#writes data to proper page and returns it's entry
def writeData(self, data):
page = self.getPage(self.lastEmptyPage)
#empty space should be more then 30% of a pagesize (settings.pageFreeSpace)
while page.getFreeSpace() - (len(data) + 2) < settings.pagesize * settings.pageFreeSpace:
self.lastEmptyPage += 1
page = self.getPage(self.lastEmptyPage)
return page.add(data)
