Class: MammothHasher

Inherits:
Object
  • Object
show all
Defined in:
lib/mammoth-hasher.rb

Class Method Summary collapse

Class Method Details

.hash(filename, debug = false) ⇒ Object

Raises:

  • (ArgumentError) 


4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
 
# File 'lib/mammoth-hasher.rb', line 4

def self.hash filename, debug=false
  time_start = Time.now if debug

  # we check that the file exist
  raise ArgumentError, "give the filename as a parameter (got nil)" if filename == nil
  raise ArgumentError, "filename must be a string" if ! filename.is_a? String
  filename = File.expand_path filename
  raise ArgumentError, "#{filename} does not exist" if ! File.exist? filename

  # algorithm parameters
  # WARNING: if you change them, the resulting hash will be different !
  number_of_chunks = 100
  length_of_chunks = 100

  # we get the file size (in bytes), used as PRNG (Pseudo Random Number Generator)
  filesize = File.size filename

  # if the file is not a big file, it's quicker to compute
  # the MD5 of the whole file than to apply our custom algorithm
  if filesize <= number_of_chunks*length_of_chunks
    file = File.open(filename, 'r')
    final_hash = Digest::MD5.file(file).hexdigest
    file.close
    puts (Time.now - time_start).to_s + " seconds" if debug
    return final_hash
  end

  # we initialize the PRNG
  prng = Random.new filesize

  # we get 1000 numbers between 0 and filesize-size_of_chunk
  offsets = []
  for i in 0..number_of_chunks
    offsets << prng.rand(filesize - length_of_chunks)
  end

  # we sort the offsets in ascending order
  # (in order to optimize the way the file will be read (in only one direction))
  offsets.sort

  # we compute the hashes of several parts of the file
  hashes = ""
  # first, we compute the hash of the first bytes of the file,
  # because that's where the magic number indicating the file type is
  # so making sure that it's still the same may be safer
  hashes << Digest::MD5.new.hexdigest(File.read(filename, 100))
  # for each offset, we compute the hash of the following bytes
  # and we concatenate these hashes
  for offset in offsets
    hashes += Digest::MD5.new.hexdigest(File.read(filename, length_of_chunks, offset))
  end

  # we compute the final hash, which is the hash of the concatenation
  # of the previous hashes
  final_hash = Digest::MD5.new.hexdigest hashes

  puts (Time.now - time_start).to_s + " seconds" if debug

  return final_hash
end