# [BitTorrent] Re: Really random?

Sat Jul 17 14:37:07 EDT 2004

```Hi Arvid,

Quick question... Does this algorithm garuantee that by the time the
nth piece comes in, that every piece received whose index is < n is in
its correct (final) position on the hard drive? I'm interested in an
efficient way to maintain this invariant.

The simplest algorithm I could come up with that doesn't pre-allocate
written follows below. It does not, however, adhere to the invariant
mentioned above -- on the average, it tends to do more sorting as the
file nears completion.

*** BEGIN PYTHON CODE ***

from random import shuffle

global completed_file, piece_map, pieces_completed, max_shifts

while next_piece_index < pieces_completed:
old_piece_index = piece_map[next_piece_index]
old_piece = completed_file[next_piece_index]
piece_map[next_piece_index] = next_piece_index
completed_file[next_piece_index] = next_piece
next_piece_index = old_piece_index
next_piece = old_piece

piece_map[pieces_completed] = next_piece_index
completed_file.append(next_piece)
pieces_completed += 1

initial_file = 'abcdefghijklmnopqrstuvwxyz'
piece_order = range(len(initial_file))
shuffle(piece_order)

completed_file = []
piece_map = [-1] * len(initial_file)
pieces_completed = 0

for next_piece_index in piece_order:

print 'completed file = ' + ''.join(completed_file)

*** END PYTHON CODE ***

Note that the "file" is just the string 'abcdefghijklmnopqrstuvwxyz',
where each letter represents a piece.

Thanks,
Michael

> This is, roughly, the algorithm I employ in libtorrent, I'm pretty
sure
> it's the same for all clients supporting this kind of storage.
>
> storing a piece:
>
> 1. let A be a newly downloaded piece, with index n
> 2. let s be the number of slots allocated in the file we're
> 3. if n >= s then allocate a new slot and put the piece there
> 4. if n < s then allocate a new slot, move the data at
>     slot n to the new slot and put A in slot n
>
> allocating a new slot:
>
> 1. if there's an unassigned slot (a slot that doesn't
>     contain any piece), return that slot index.
> 2. append the new slot at the end of the file
> 3. let i be the index of newly allocated slot
> 4.1. move the data at slot j to slot i
> 4.2. return slot index j as the newly allocated free slot
> 5. return i as the newly allocated slot
>
> I wrote this from the top of my head, I may very well have
forgotten
> something, feel free to correct any mistakes.
>
> --
> Arvid Norberg

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