PJW hash function is a non-cryptographic hash function created by Peter J. Weinberger of AT&T Bell Labs.
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]
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 returnh
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;
}
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.[4]
Non-cryptographic hash functions
