commit c5e3b2f81402773466f2afc8e87839509f65facc
parent 64867624630b4effacb3d3fb54708ecda19f0221
Author: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Thu, 22 Mar 2018 16:54:22 -0300
in random/'project', remove the special case for "small" intervals;
it is slower than the general case.
Diffstat:
| M | lmathlib.c | | | 39 | +++++++++++++++------------------------ |
1 file changed, 15 insertions(+), 24 deletions(-)
diff --git a/lmathlib.c b/lmathlib.c
@@ -1,5 +1,5 @@
/*
-** $Id: lmathlib.c,v 1.125 2018/03/12 12:39:03 roberto Exp roberto $
+** $Id: lmathlib.c,v 1.126 2018/03/16 14:18:18 roberto Exp roberto $
** Standard mathematical library
** See Copyright Notice in lua.h
*/
@@ -422,27 +422,18 @@ typedef struct {
/*
** Project the random integer 'ran' into the interval [0, n].
-** Because 'ran' has 2^B possible values, the projection can only
-** be uniform when the size of the interval [0, n] is a power of 2
-** (exact division). With the fairest possible projection (e.g.,
-** '(ran % (n + 1))'), the maximum bias is 1 in 2^B/n.
-** For a "small" 'n', this bias is acceptable. (Here, we accept
-** a maximum bias of 0.0001%.) For a larger 'n', we first
-** compute 'lim', the smallest (2^b - 1) not smaller than 'n',
-** to get a uniform projection into [0,lim]. If the result is
-** inside [0, n], we are done. Otherwise, we try we another
-** 'ran' until we have a result inside the interval.
+** Because 'ran' has 2^B possible values, the projection can only be
+** uniform when the size of the interval [0, n] is a power of 2 (exact
+** division). To get a uniform projection into [0,lim], we first
+** compute 'lim', the smallest (2^b - 1) not smaller than 'n'. If the
+** random number is inside [0, n], we are done. Otherwise, we try with
+** another 'ran' until we have a result inside the interval.
*/
-
-#define MAXBIAS 1000000
-
static lua_Unsigned project (lua_Unsigned ran, lua_Unsigned n,
RanState *state) {
- if (n < LUA_MAXUNSIGNED / MAXBIAS)
- return ran % (n + 1);
- else {
+ lua_Unsigned lim = n;
+ if ((lim & (lim + 1)) > 0) { /* 'lim + 1' is not a power of 2? */
/* compute the smallest (2^b - 1) not smaller than 'n' */
- lua_Unsigned lim = n;
lim |= (lim >> 1);
lim |= (lim >> 2);
lim |= (lim >> 4);
@@ -451,13 +442,13 @@ static lua_Unsigned project (lua_Unsigned ran, lua_Unsigned n,
#if (LUA_MAXINTEGER >> 30 >> 2) > 0
lim |= (lim >> 32); /* integer type has more than 32 bits */
#endif
- lua_assert((lim & (lim + 1)) == 0 /* 'lim + 1' is a power of 2 */
- && lim >= n /* not smaller than 'n' */
- && (lim >> 1) < n); /* it is the smallest one */
- while ((ran & lim) > n)
- ran = I2UInt(xorshift128plus(state->s));
- return ran & lim;
}
+ lua_assert((lim & (lim + 1)) == 0 /* 'lim + 1' is a power of 2 */
+ && lim >= n /* not smaller than 'n' */
+ && (lim == 0 || (lim >> 1) < n)); /* it is the smallest one */
+ while ((ran & lim) > n)
+ ran = I2UInt(xorshift128plus(state->s));
+ return ran & lim;
}
