Fix pgbench's getrand() function so that min and max have approximately

tglsfdc · tglsfdc · commit 9537739f7fcd · 2008-03-10T01:23:04.000Z
the same chance of being selected as do numbers between them.  Problem
noted by Greg Stark; fix by Alexey Klyukin.
diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c
@@ -1,5 +1,5 @@
 /*
- * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.75 2007/12/11 02:31:49 tgl Exp $
+ * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.76 2008/03/10 01:23:04 tgl Exp $
  *
  * pgbench: a simple benchmark program for PostgreSQL
  * written by Tatsuo Ishii
@@ -191,11 +191,15 @@ usage(void)
 	fprintf(stderr, "(initialize mode): pgbench -i [-h hostname][-p port][-s scaling_factor] [-F fillfactor] [-U login][-d][dbname]\n");
 }
 
-/* random number generator */
+/* random number generator: uniform distribution from min to max inclusive */
 static int
 getrand(int min, int max)
 {
-	return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5);
+	/*
+	 * Odd coding is so that min and max have approximately the same chance of
+	 * being selected as do numbers between them.
+	 */
+	return min + (int) (((max - min + 1) * (double) random()) / (MAX_RANDOM_VALUE + 1.0));
 }
 
 /* call PQexec() and exit() on failure */

Original file line number	Diff line number	Diff line change
`@@ -1,5 +1,5 @@`
`1`	`1`	`/*`
`2`		`- * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.75 2007/12/11 02:31:49 tgl Exp $`
	`2`	`+ * $PostgreSQL: pgsql/contrib/pgbench/pgbench.c,v 1.76 2008/03/10 01:23:04 tgl Exp $`
`3`	`3`	`*`
`4`	`4`	`* pgbench: a simple benchmark program for PostgreSQL`
`5`	`5`	`* written by Tatsuo Ishii`
`@@ -191,11 +191,15 @@ usage(void)`
`191`	`191`	`fprintf(stderr, "(initialize mode): pgbench -i [-h hostname][-p port][-s scaling_factor] [-F fillfactor] [-U login][-d][dbname]\n");`
`192`	`192`	`}`
`193`	`193`
`194`		`-/* random number generator */`
	`194`	`+/* random number generator: uniform distribution from min to max inclusive */`
`195`	`195`	`static int`
`196`	`196`	`getrand(int min, int max)`
`197`	`197`	`{`
`198`		`- return min + (int) (((max - min) * (double) random()) / MAX_RANDOM_VALUE + 0.5);`
	`198`	`+ /*`
	`199`	`+ * Odd coding is so that min and max have approximately the same chance of`
	`200`	`+ * being selected as do numbers between them.`
	`201`	`+ */`
	`202`	`+ return min + (int) (((max - min + 1) * (double) random()) / (MAX_RANDOM_VALUE + 1.0));`
`199`	`203`	`}`
`200`	`204`
`201`	`205`	`/* call PQexec() and exit() on failure */`