* Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $Id: geqo_selection.c,v 1.12 2001/01/24 19:42:57 momjian Exp $
+ * $Id: geqo_selection.c,v 1.12.4.1 2005/06/14 14:21:43 tgl Exp $
*
*-------------------------------------------------------------------------
*/
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)
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 */
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;
}
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