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Added comments to scm_c_random_bignum

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This commit is contained in:
Mikael Djurfeldt 2003-03-03 13:19:46 +00:00
parent 25ad768128
commit ea7f6344cf
1 changed files with 26 additions and 3 deletions
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29
libguile/random.c
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@ -1,4 +1,4 @@
/* Copyright (C) 1999,2000,2001 Free Software Foundation, Inc.
/* Copyright (C) 1999,2000,2001, 2003 Free Software Foundation, Inc.
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2, or (at your option)
@ -265,6 +265,21 @@ scm_c_random (scm_t_rstate *state, unsigned long m)
return r;
}
/*
SCM scm_c_random_bignum (scm_t_rstate *state, SCM m)
Takes a random state (source of random bits) and a bignum m.
Returns a bignum b, 0 <= b < m.
It does this by allocating a bignum b with as many base 65536 digits
as m, filling b with random bits (in 32 bit chunks) up to the most
significant 1 in m, and, finally checking if the resultant b is too
large (>= m). If too large, we simply repeat the process again. (It
is important to throw away all generated random bits if b >= m,
otherwise we'll end up with a distorted distribution.)
*/
SCM
scm_c_random_bignum (scm_t_rstate *state, SCM m)
{
@ -272,7 +287,10 @@ scm_c_random_bignum (scm_t_rstate *state, SCM m)
int i, nd;
LONG32 *bits, mask, w;
nd = SCM_NUMDIGS (m);
/* calculate mask for most significant digit */
/* Compute a 32-bit bitmask for use when filling bits into the most
significant long word in b's bit space. A combination of
conditionals and an 8-bit lookup table (scm_masktab) is used.
*/
#if SIZEOF_INT == 4
/* 16 bit digits */
if (nd & 1)
@ -298,12 +316,13 @@ scm_c_random_bignum (scm_t_rstate *state, SCM m)
? scm_masktab[w >> 16] << 16 | 0xffff
: scm_masktab[w >> 24] << 24 | 0xffffff));
}
/* allocate b and assign the bit space address to the variable "bits" */
b = scm_i_mkbig (nd, 0);
bits = (LONG32 *) SCM_BDIGITS (b);
do
{
i = nd;
/* treat most significant digit specially */
/* generate the most significant bits using the mask */
#if SIZEOF_INT == 4
/* 16 bit digits */
if (i & 1)
@ -328,9 +347,13 @@ scm_c_random_bignum (scm_t_rstate *state, SCM m)
/* now fill up the rest of the bignum */
while (i)
bits[--i] = scm_the_rng.random_bits (state);
/* use scm_i_normbig to cut away leading zeros and/or convert to inum */
b = scm_i_normbig (b);
if (SCM_INUMP (b))
return b;
/* if b >= m, regenerate b (it is important to regenerates all
bits in order not to get a distorted distribution)
*/
} while (scm_bigcomp (b, m) <= 0);
return b;
}