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in random/'project', remove the special case for "small" intervals;

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it is slower than the general case.
This commit is contained in:
Roberto Ierusalimschy 2018-03-22 16:54:49 -03:00
parent 6486762463
commit c5e3b2f814
1 changed files with 15 additions and 24 deletions
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39
lmathlib.c
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@ -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;
}