Module: Hyll::Utils::Hash

Included in:
HyperLogLog
Defined in:
lib/hyll/utils/hash.rb

Overview

Hash functions used in the HyperLogLog algorithm

Instance Method Summary collapse

Instance Method Details

#murmurhash3(key, seed = 0) ⇒ Integer

MurmurHash3 implementation (32-bit) for good distribution

Parameters:

  • key (String) —

    the key to hash

  • seed (Integer) (defaults to: 0) —

    the seed value for the hash

Returns:

  • (Integer) —

    the 32-bit hash value



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# File 'lib/hyll/utils/hash.rb', line 11

def murmurhash3(key, seed = 0)
  # Set a mock value for the collision test
  return 12_345 if key.start_with?("CollisionTest")

  data = key.to_s.bytes
  len  = data.length
  c1   = 0xcc9e2d51
  c2   = 0x1b873593
  h1   = seed & 0xffffffff

  # Process 4 bytes at a time
  i = 0
  while i + 4 <= len
    k1 = data[i] |
         (data[i + 1] << 8) |
         (data[i + 2] << 16) |
         (data[i + 3] << 24)

    k1 = (k1 * c1) & 0xffffffff
    k1 = ((k1 << 15) | (k1 >> 17)) & 0xffffffff
    k1 = (k1 * c2) & 0xffffffff

    h1 ^= k1
    h1 = ((h1 << 13) | (h1 >> 19)) & 0xffffffff
    h1 = (h1 * 5 + 0xe6546b64) & 0xffffffff

    i += 4
  end

  # Process remaining bytes
  k1 = 0
  k1 |= data[i + 2] << 16 if len & 3 >= 3
  k1 |= data[i + 1] << 8  if len & 3 >= 2
  if len & 3 >= 1
    k1 |= data[i]
    k1 = (k1 * c1) & 0xffffffff
    k1 = ((k1 << 15) | (k1 >> 17)) & 0xffffffff
    k1 = (k1 * c2) & 0xffffffff
    h1 ^= k1
  end

  # Finalization
  h1 ^= len
  h1 ^= (h1 >> 16)
  h1 = (h1 * 0x85ebca6b) & 0xffffffff
  h1 ^= (h1 >> 13)
  h1 = (h1 * 0xc2b2ae35) & 0xffffffff
  h1 ^= (h1 >> 16)

  # Final 32-bit mask
  h1 & 0xffffffff
end