Module: StringDerivedRandom

Extended by:
StringDerivedRandom
Included in:
StringDerivedRandom
Defined in:
lib/string_derived_random.rb,
lib/string_derived_random/version.rb

Defined Under Namespace

Classes: Error

Constant Summary collapse

VERSION = 
"0.1.0"

Instance Method Summary collapse

Instance Method Details

#is_one_in?(n, string_seed:) ⇒ Boolean

For quick bucketing of “process one in N” objects

Returns:

  • (Boolean)

Raises:

  • (ArgumentError) 


9
10
11
12
 
# File 'lib/string_derived_random.rb', line 9

def is_one_in?(n, string_seed:)
  raise ArgumentError if n < 1
  new(string_seed).rand(1..n) == 1
end

#new(string_seed) ⇒ Random

The objective here is getting a stable random number based on a given string. We use the string to generate a big Integer that can be used to seed a Random object at creation. This allows us to experiment by, say, applying something to ‘every 100th of some class of objects having this identifier’, with the result being exactly the same for that identifier, every time.

Parameters:

  • string_seed (#to_s) —

    An object that can be converted into a String

Returns:

  • (Random) —

    A seeded Random object



25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
 
# File 'lib/string_derived_random.rb', line 25

def new(string_seed)
  # Compute a digest that gives us a good
  # distribution of values. Doesn't matter which
  # digest gets used since it is the distribution
  # that matters, as well as consistency and a broader
  # range of possible values
  digest_bytes = Digest::SHA256.digest(string_seed.to_s)

  # We get 256 bits of information from the digest.
  # The Mersenne twister can be seeded with values
  # in a certain range, after which the seed rolls over.
  # For the gory details read https://stackoverflow.com/a/20082583/153886
  # but basically the range is this:
  #
  # 2 ** ( 624 * 32 )
  #
  # which is actually 19968 bits of available values, quite a bit more
  # than what the SHA256 gives us - which is only 256 bits.
  # So for our purposes we can just use those values and relax, as there
  # is no requirement for this seed to be cryptographically-secure.
  # https://stackoverflow.com/a/17556861/153886 for the bytes -> bigint part
  seed_int = digest_bytes.bytes.inject {|a, b| (a << 8) + b }
  Random.new(seed_int)
end