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The random selection in function linear() could deliver a value equal to max

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if geqo_rand() returns exactly 1.0, resulting in failure due to indexing
off the end of the pool array.  Also, since this is using inexact float math,
it seems wise to guard against roundoff error producing values slightly
outside the expected range.  Per report from bug@zedware.org.
This commit is contained in:
Tom Lane 2005-06-14 14:21:31 +00:00
parent 4f3ae012fd
commit a1f08fcf80
1 changed files with 25 additions and 12 deletions
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37
src/backend/optimizer/geqo/geqo_selection.c
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@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $Id: geqo_selection.c,v 1.15 2003/08/04 02:39:59 momjian Exp $
* $Id: geqo_selection.c,v 1.15.4.1 2005/06/14 14:21:31 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -44,10 +44,11 @@
static int linear(int max, double bias);
/* geqo_selection
*
/*
* geqo_selection
* according to bias described by input parameters,
* second genes are selected from the pool
* first and second genes are selected from the pool
*/
void
geqo_selection(Chromosome *momma, Chromosome *daddy, Pool *pool, double bias)
@ -55,28 +56,27 @@ geqo_selection(Chromosome *momma, Chromosome *daddy, Pool *pool, double bias)
int first,
second;
first = (int) linear(pool->size, bias);
second = (int) linear(pool->size, bias);
first = linear(pool->size, bias);
second = linear(pool->size, bias);
if (pool->size > 1)
{
while (first == second)
second = (int) linear(pool->size, bias);
second = linear(pool->size, bias);
}
geqo_copy(momma, &pool->data[first], pool->string_length);
geqo_copy(daddy, &pool->data[second], pool->string_length);
}
/* linear
/*
* linear
* generates random integer between 0 and input max number
* using input linear bias
*
* probability distribution function is: f(x) = bias - 2(bias - 1)x
* bias = (prob of first rule) / (prob of middle rule)
*
*/
static int
linear(int pool_size, double bias) /* bias is y-intercept of linear
* distribution */
@ -84,8 +84,21 @@ linear(int pool_size, double bias) /* bias is y-intercept of linear
double index; /* index between 0 and pop_size */
double max = (double) pool_size;
index = max * (bias - sqrt((bias * bias) - 4.0 * (bias - 1.0) * geqo_rand()))
/ 2.0 / (bias - 1.0);
/*
* If geqo_rand() returns exactly 1.0 then we will get exactly max from
* this equation, whereas we need 0 <= index < max. Also it seems possible
* that roundoff error might deliver values slightly outside the range;
* in particular avoid passing a value slightly less than 0 to sqrt().
* If we get a bad value just try again.
*/
do {
double sqrtval;
sqrtval = (bias * bias) - 4.0 * (bias - 1.0) * geqo_rand();
if (sqrtval > 0.0)
sqrtval = sqrt(sqrtval);
index = max * (bias - sqrtval) / 2.0 / (bias - 1.0);
} while (index < 0.0 || index >= max);
return (int) index;
}