Module: DJB2

Defined in:
lib/djb2.rb,
lib/djb2/version.rb

Defined Under Namespace

Classes: Error

Constant Summary collapse

VERSION = 
"0.2.0"

Class Method Summary collapse

Class Method Details

.digest(string) ⇒ Integer

Computes the djb2 hash (xor variant) of the given string.

The hash is computed using 64-bit arithmetic split into two 32-bit halves to keep all intermediate values within Ruby's Fixnum range, avoiding Bignum allocation in the hot loop. This makes the implementation YJIT-friendly: the JIT can emit efficient native code for the entire loop without any object allocations.

Parameters:

  • string (String) —

    the string to hash (binary-safe, operates on raw bytes)

Returns:

  • (Integer) —

    a 64-bit unsigned integer hash value

Raises:

  • (TypeError) —

    if the argument is not a String



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# File 'lib/djb2.rb', line 19

def self.digest(string)
  raise TypeError, "no implicit conversion of #{string.class} into String" unless string.is_a?(String)

  hi = 0       # upper 32 bits of the hash
  lo = 5381    # lower 32 bits of the hash
  i = 0
  len = string.bytesize

  # Process 4 bytes at a time to reduce loop overhead.
  stop = len - (len & 3)
  while i < stop
    # Multiply (hi:lo) by 33 using: x * 33 = (x << 5) + x
    # lo half: must mask (lo << 5) to 32 bits BEFORE adding lo, so that
    # the carry into the hi half is correct (0 or 1, never more).
    t = ((lo << 5) & 0xFFFFFFFF) + lo
    # hi half: (hi << 5) can be up to 37 bits, but the total expression
    # still fits in a Fixnum (< 62 bits), so we only mask at the end.
    hi = ((hi << 5) + (lo >> 27) + hi + (t >> 32)) & 0xFFFFFFFF
    # XOR the current byte into the lo half.
    lo = (t & 0xFFFFFFFF) ^ string.getbyte(i)

    t = ((lo << 5) & 0xFFFFFFFF) + lo
    hi = ((hi << 5) + (lo >> 27) + hi + (t >> 32)) & 0xFFFFFFFF
    lo = (t & 0xFFFFFFFF) ^ string.getbyte(i + 1)

    t = ((lo << 5) & 0xFFFFFFFF) + lo
    hi = ((hi << 5) + (lo >> 27) + hi + (t >> 32)) & 0xFFFFFFFF
    lo = (t & 0xFFFFFFFF) ^ string.getbyte(i + 2)

    t = ((lo << 5) & 0xFFFFFFFF) + lo
    hi = ((hi << 5) + (lo >> 27) + hi + (t >> 32)) & 0xFFFFFFFF
    lo = (t & 0xFFFFFFFF) ^ string.getbyte(i + 3)

    i += 4
  end

  # Handle remaining 0-3 bytes.
  while i < len
    t = ((lo << 5) & 0xFFFFFFFF) + lo
    hi = ((hi << 5) + (lo >> 27) + hi + (t >> 32)) & 0xFFFFFFFF
    lo = (t & 0xFFFFFFFF) ^ string.getbyte(i)
    i += 1
  end

  # Combine the two halves into a 64-bit result. Using multiply instead
  # of (hi << 32) | lo avoids a YJIT side exit caused by left shift overflow.
  hi * 0x100000000 + lo
end