make sure even user-supplied RNGs have 0 < X < 1

git-svn-id: https://svn.r-project.org/R/trunk@39398 00db46b3-68df-0310-9c12-caf00c1e9a41

src/library/stats/man/Uniform.Rd

@@ -40,8 +40,9 @@ runif(n, min=0, max=1)
   \eqn{X \equiv u}{X == u} is assumed, although there is no density in
   that case and \code{dunif} will return \code{NaN} (the error condition).
 
-  \code{runif} will not generate either of the extreme values (unless
-  \code{max = min}.
+  \code{runif} will not generate either of the extreme values unless
+  \code{max = min} or \code{max-min} is small compared to \code{min},
+  and in particular not for the default arguments.
 }
 \references{
   Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)

src/nmath/runif.c

@@ -29,6 +29,11 @@ double runif(double a, double b)
 
     if (a == b)
 	return a;
-    else
-	return a + (b - a) * unif_rand();
+    else {
+	double u;
+	/* This is true of all builtin generators, but protect against
+	   user-supplied ones */
+	do {u = unif_rand();} while (u <= 0 || u >= 1);
+	return a + (b - a) * u;
+    }
 }