PJW hash function

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     return h 

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