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PJW hash function

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PJW hash function is a <a href="Non-cryptographic_hash_function" title="Non-cryptographic hash function">non-cryptographic hash function</a> created by <a href="Peter_J._Weinberger" title="Peter J. Weinberger">Peter J. Weinberger</a> of AT&T Bell Labs.

<div class="mw-heading mw-heading2"><h2 id="Other_versions">Other versions</h2></div>

A variant of PJW hash had been used to create ElfHash or Elf64 hash that is used in Unix object files with <a href="Executable_and_Linkable_Format" title="Executable and Linkable Format">ELF</a> format.

<a href="Allen_Holub" title="Allen Holub">Allen Holub</a> 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.<a href="#cite_note-1">[1]</a>

<div class="mw-heading mw-heading2"><h2 id="Algorithm">Algorithm</h2></div>

PJW hash algorithm involves shifting the previous hash and adding the current byte followed by moving the high bits:<a href="#cite_note-2">[2]</a>

<pre>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

</pre>

<div class="mw-heading mw-heading2"><h2 id="Implementation">Implementation</h2></div>

Below is the algorithm implementation used in Unix ELF format:<a href="#cite_note-3">[3]</a>

<div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre>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;

}

</pre></div>

This C code incorrectly assumes that <code>long</code> is a 32-bit data type. When <code>long</code> is wider than 32 bits, as it is on many <a href="64-bit_computing" title="64-bit computing">64-bit</a> systems, the code contains a bug.<a href="#cite_note-4">[4]</a>

<a href="Non-cryptographic_hash_functions" class="mw-redirect" title="Non-cryptographic hash functions">Non-cryptographic hash functions</a>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFBinstock1996" class="citation journal cs1">Binstock, Andrew (1996). <a rel="nofollow" class="external text" href="http://www.drdobbs.com/database/hashing-rehashed/184409859">"Hashing Rehashed"</a>. <a href="Dr._Dobb's" class="mw-redirect" title="Dr. Dobb's">Dr. Dobb's</a>.</cite>

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<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.cs.hmc.edu/~geoff/classes/hmc.cs070.200101/homework10/hashfuncs.html">"Hash Functions"</a>. www.cs.hmc.edu. Retrieved 2015-06-10.</cite>

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<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite id="CITEREFCORPORATE_UNIX_Press1993" class="citation book cs1">CORPORATE UNIX Press (1993). <a rel="nofollow" class="external text" href="https://archive.org/details/systemvapplicati00unix">System V application binary interface</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-13-100439-5</bdi>.</cite>

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<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://maskray.me/blog/2023-04-12-elf-hash-function">"ELF hash function may overflow"</a>. 12 April 2023. Retrieved 2023-04-14.</cite>

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This article is issued from <a class="external text" title="Last edited on 2024-10-25" href="https://en.wikipedia.org/wiki/?title=PJW_hash_function&oldid=1253333688">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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