ORIGIN '~beta/basiclib/betaenv'; LIB_DEF 'texthash' '../lib';  lib:attributes  (* This is a pattern implementing a hash of a text into an integer. * * Usage: * ph: @honeyman; * ... * do ph.init; * ... * 'Some text' > ph.hash > hashValue * * This version is a BETA implementation of a hash function found in: * * C News Source, which contains the following statement: * * "dbz.c V3.2 * * Copyright 1988 Jon Zeeff (zeeff@btech.annarbor.mi.us) * You can use this code in any manner, as long as you leave my name on it * and don't hold me responsible for any problems with it. * * Hacked on by gdb@ninja.UUCP (David Butler); Sun Jun 5 00:27:08 CDT 1988 * * Various improvments + INCORE by moraes@ai.toronto.edu (Mark Moraes) * * Major reworking by Henry Spencer as part of the C News project." * * The following text is the original comment before the hash function: * * This is a simplified version of the pathalias hashing function. * Thanks to Steve Belovin and Peter Honeyman * * hash a string into a long int. 31 bit crc (from andrew appel). * the crc table is computed at run time by crcinit()  we could * precompute, but it takes 1 clock tick on a 750. * * This fast table calculation works only if POLY is a prime polynomial * in the field of integers modulo 2. Since the coefficients of a * 32bit polynomial won't fit in a 32bit word, the highorder bit is * implicit. IT MUST ALSO BE THE CASE that the coefficients of orders * 31 down to 25 are zero. Happily, we have candidates, from * E. J. Watson, "Primitive Polynomials (Mod 2)", Math. Comp. 16 (1962): * x^32 + x^7 + x^5 + x^3 + x^2 + x^1 + x^0 * x^31 + x^3 + x^0 * * We reverse the bits to get: * 111101010000000000000000000000001 but drop the last 1 * f 5 0 0 0 0 0 0 * 010010000000000000000000000000001 ditto, for 31bit crc * 4 8 0 0 0 0 0 0 *) honeyman: (* BETA VERSION *) (# POLY: (# exit 0x48000000 #); CrcTable: [128]@Integer; init: (# j,sum: @Integer; do (for i:128 repeat 0>sum; 6>j; loop: (# do (if ((i1) %Band (1 %sll j)) then (sum %Bxor (POLY %srl j))>sum; if); (if j>0 then j1>j; restart loop if); #); sum>CrcTable[i]; for); #); hash: @ (# t: ^Text; sum: @Integer; enter t[] do 0>sum; (for i:t.lgth repeat (sum %srl 7) %Bxor CrcTable[((sum %Bxor t.T[i]) %Band 0x7f) + 1] >sum; for); exit sum #); #)
13.19 Texthash Interface  © 19902002 Mjølner Informatics 
[Modified: Saturday January 22^{nd} 2000 at 0:18]
