PJW hash function
{{Short description|Computing algorithm}}
PJW hash function is a non-cryptographic hash function created by Peter J. Weinberger of AT&T Bell Labs.
Other versions
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.{{Cite journal |url = http://www.drdobbs.com/database/hashing-rehashed/184409859|title = Hashing Rehashed |journal=Dr. Dobb's |last = Binstock|first = Andrew|year = 1996}}
Algorithm
PJW hash algorithm involves shifting the previous hash and adding the current byte followed by moving the high bits:{{Cite web|title = Hash Functions|url = http://www.cs.hmc.edu/~geoff/classes/hmc.cs070.200101/homework10/hashfuncs.html|website = www.cs.hmc.edu|accessdate = 2015-06-10}}
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
Below is the algorithm implementation used in Unix ELF format:{{Cite book|title = System V application binary interface|last = CORPORATE UNIX Press|publisher = |year = 1993|isbn = 0-13-100439-5|location = |pages = |url-access = registration|url = https://archive.org/details/systemvapplicati00unix}}
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;
}
This C code incorrectly assumes that long is a 32-bit data type. When long is wider than 32 bits, as it is on many 64-bit systems, the code contains a bug.{{Cite web |url=https://maskray.me/blog/2023-04-12-elf-hash-function |title=ELF hash function may overflow |date=12 April 2023 |access-date=2023-04-14}}