tahoe-lafs/src/allmydata/chunk.py

"""
Read and write chunks from files.

Version 1.0.0.

A file is divided into blocks, each of which has size L{BLOCK_SIZE}
(except for the last block, which may be smaller).  Blocks are encoded
into chunks.  One publishes the hash of the entire file.  Clients
who want to download the file first obtain the hash, then the clients
can receive chunks in any order.  Cryptographic hashing is used to
verify each received chunk before writing to disk.  Thus it is
impossible to download corrupt data if one has the correct file hash.

One obtains the hash of a complete file via
L{CompleteChunkFile.file_hash}.  One can read chunks from a complete
file by the sequence operations of C{len()} and subscripting on a
L{CompleteChunkFile} object.  One can open an empty or partially
downloaded file with L{PartialChunkFile}, and read and write chunks
to this file.  A chunk will fail to write if its contents and index
are not consistent with the overall file hash passed to
L{PartialChunkFile} when the partial chunk file was first created.

The chunks have an overhead of less than 4% for files of size
less than C{10**20} bytes.

Benchmarks:

 - On a 3 GHz Pentium 3, it took 3.4 minutes to first make a
   L{CompleteChunkFile} object for a 4 GB file.  Up to 10 MB of
   memory was used as the constructor ran.  A metafile filename
   was passed to the constructor, and so the hash information was
   written to the metafile.  The object used a negligible amount
   of memory after the constructor was finished.
 - Creation of L{CompleteChunkFile} objects in future runs of the
   program took negligible time, since the hash information was
   already stored in the metafile.

@var BLOCK_SIZE:     Size of a block.  See L{BlockFile}.
@var MAX_CHUNK_SIZE: Upper bound on the size of a chunk.
                     See L{CompleteChunkFile}.

free (adj.): unencumbered; not under the control of others
Written by Connelly Barnes in 2005 and released into the
public domain  with no warranty of any kind, either expressed
or implied.  It probably won't make your computer catch on fire,
or eat  your children, but it might.  Use at your own risk.
"""

from allmydata.util.hashutil import tagged_hash, tagged_pair_hash

__version__ = '1.0.0-allmydata'

BLOCK_SIZE     = 65536
MAX_CHUNK_SIZE = BLOCK_SIZE + 4096

def roundup_pow2(x):
  """
  Round integer C{x} up to the nearest power of 2.
  """
  ans = 1
  while ans < x:
    ans *= 2
  return ans


class CompleteBinaryTreeMixin:
  """
  Adds convenience methods to a complete binary tree.

  Assumes the total number of elements in the binary tree may be
  accessed via C{__len__}, and that each element can be retrieved
  using list subscripting.

  Tree is indexed like so::


                      0
                 /        \
              1               2
           /    \          /    \
         3       4       5       6
        / \     / \     / \     / \
       7   8   9   10  11  12  13  14

  """
  def parent(self, i):
    """
    Index of the parent of C{i}.
    """
    if i < 1 or (hasattr(self, '__len__') and i >= len(self)):
      raise IndexError('index out of range: ' + repr(i))
    return (i - 1) // 2

  def lchild(self, i):
    """
    Index of the left child of C{i}.
    """
    ans = 2 * i + 1
    if i < 0 or (hasattr(self, '__len__') and ans >= len(self)):
      raise IndexError('index out of range: ' + repr(i))
    return ans

  def rchild(self, i):
    """
    Index of right child of C{i}.
    """
    ans = 2 * i + 2
    if i < 0 or (hasattr(self, '__len__') and ans >= len(self)):
      raise IndexError('index out of range: ' + repr(i))
    return ans

  def sibling(self, i):
    """
    Index of sibling of C{i}.
    """
    parent = self.parent(i)
    if self.lchild(parent) == i:
      return self.rchild(parent)
    else:
      return self.lchild(parent)

  def needed_for(self, i):
    """
    Return a list of node indices that are necessary for the hash chain.
    """
    if i < 0 or i >= len(self):
      raise IndexError('index out of range: ' + repr(i))
    needed = []
    here = i
    while here != 0:
      needed.append(self.sibling(here))
      here = self.parent(here)
    return needed


class HashTree(CompleteBinaryTreeMixin, list):
  """
  Compute Merkle hashes at any node in a complete binary tree.

  Tree is indexed like so::


                      0
                 /        \
              1               2
           /    \          /    \
         3       4       5       6
        / \     / \     / \     / \
       7   8   9   10  11  12  13  14  <- List passed to constructor.

  """
  def __init__(self, L):
    """
    Create complete binary tree from list of hash strings.

    The list is augmented by hashes so its length is a power of 2, and
    then this is used as the bottom row of the hash tree.

    The augmenting is done so that if the augmented element is at
    index C{i}, then its value is C{hash(tagged_hash('Merkle tree empty leaf', '%d'%i))}.
    """
    # Augment the list.
    start = len(L)
    end   = roundup_pow2(len(L))
    L     = L + [None] * (end - start)
    for i in range(start, end):
      L[i] = tagged_hash('Merkle tree empty leaf', "%d"%i)
    # Form each row of the tree.
    rows = [L]
    while len(rows[-1]) != 1:
      last = rows[-1]
      rows += [[tagged_pair_hash('Merkle tree internal node', last[2*i], last[2*i+1]) for i in xrange(len(last)//2)]]
    # Flatten the list of rows into a single list.
    rows.reverse()
    self[:] = sum(rows, [])