"""Simulate having a file of data simply as a big list of byte-arrays
in memory."""


class PageNotPinnedException(Exception):
    pass
class PagePinnedException(Exception):
    pass


class Frame:

    def __init__(self):
        self.fileName = None
        self.pageNum = None
        self.pinCount = 0
        self.dirty = False
        self.page = None


class BufferManager:

    def __init__(self, poolSize):
        """Creates a buffer manager with the specified size. Feel free
        to add other instance variables if you find them useful."""
        self.__bufferPool = [None]*poolSize

    def poolSize(self):
        """Returns the pool size."""

        return 0

    def pinPage(self, pinPageId, diskfile):
        """Checks if this page is in buffer pool. If it is, returns a
        pointer to it. Otherwise, it finds an available frame for this
        page, makes a copy of the page (make sure you do this, it's
        easy in Python just to get a pointer copy) and pins it. Writes
        out the old page, if it is dirty, before reading.

        pinPageId is the page id for the page to be pinned.

        diskfile is a pointer to the array of data that is simulating
        the disk file.

        Return a reference to the page in the buffer pool. If the
        buffer pool is full, return None."""

        return None

    def unpinPage(self, unpinPageId, diskfile, dirty):
        """If the pin count for this page is greater than 0, it is
        decremented. If the pin count becomes zero, it is appropriately
        included in a group of replacement candidates.

        unpinPageId the page id for the page to be unpinned.

        diskfile is a pointer to the array of data that is simulating
        the disk file.

        dirty indicates whether or not the page is dirty. If False,
        then the page does not actually need to be written back to
        disk.

        Raises PageNotPinnedException if the page is not pinned, or if
        the page id is invalid in some other way."""

        pass


    def newPage(self, numPages,diskfile):
        """Requests a run of pages from the underlying database, then
        finds a frame in the buffer pool for the first page and pins
        it. If the buffer pool is full, no new pages are allocated
        from the database.

     
        numPages is the number of pages in the run to be allocated.

        diskfile is a pointer to the array of data that is simulating
        the disk file.

        Return a tuple consisting of the first page id of the run, and
        a reference to the page which has been pinned in the buffer
        pool. Return None if there is not enough space in the buffer
        pool for the first page."""

        return None

    def freePage(self,pageId, diskfile):
        """Deallocates a page from the underlying database (sets its
        pointer to None). Verifies that page is not pinned.

        pageId is the page id to be deallocated.

        diskfile is a pointer to the array of data that is simulating
        the disk file.

        Raise PagePinnedException if the page is pinned."""

        pass

    def flushPage(self, pageId, diskfile):
        """Flushes page from the buffer pool to the underlying database if
        it is dirty. If page is not dirty, it is not flushed,
        especially since an undirty page may hang around even after the
        underlying database has been erased. If the page is not in the
        buffer pool, do nothing, since the page is effectively flushed
        already.

        pageId is the page id to be flushed.
        
        diskfile is a pointer to the array of data that is simulating
        the disk file."""

        pass
        
    def flushAllPages(self):
        """Flushes all dirty pages from the buffer pool to the underlying
        databases. If page is not dirty, it is not flushed, especially
        since an undirty page may hang around even after the underlying
        database has been erased."""

        pass

    def findFrame(self, pageId, diskfile):
        """Returns buffer pool location for a particular pageId. This
        method is just used for testing purposes: it probably doesn't
        serve a real purpose in an actual database system.

        pageId the page id to be looked up.

        diskfile is a pointer to the array of data that is simulating
        the disk file.

        Return the frame location for the page of interest. Return -1
        if the page is not in the pool."""

        return -1