PJW hash function

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PJW hash function is a non-cryptographic hash function created by Peter J. Weinberger of AT&T Bell Labs.

Other versions[edit]

A variant of PJW hash had been used to create ElfHash or Elf64 hash that is used in Unix object files with ELF format.

Allen Holub has created a portable version of PJW hash algorithm that had a bug and ended up in several textbooks, as the author of one of these textbooks later admitted.[1]

Algorithm[edit]

PJW hash algorithm involves shifting the previous hash and adding the current byte followed by moving the high bits:[2]

algorithm PJW_hash(s) is
    uint h := 0
    bits := uint size in bits
    for i := 1 to |S| do
        h := h << bits/8 + s[i]
        high := get top bits/8 bits of h from left
        if high ≠ 0 then
            h := h xor (high >> bits * 3/4)
            h := h & ~high
    return h

Implementation[edit]

Below is the algorithm implementation used in Unix ELF format:[3]

unsigned long ElfHash(const unsigned char *s)
{
    unsigned long   h = 0, high;
    while (*s)
    {
        h = (h << 4) + *s++;
        if (high = h & 0xF0000000)
            h ^= high >> 24;
        h &= ~high;
    }
    return h;
}

See also[edit]

Non-cryptographic hash functions

References[edit]

  1. ^ Binstock, Andrew (1996). "Hashing Rehashed". Dr. Dobb's.
  2. ^ "Hash Functions". www.cs.hmc.edu. Retrieved 2015-06-10.
  3. ^ CORPORATE UNIX Press (1993). System V application binary interface. ISBN 0-13-100439-5.