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'math.random(n,m)' interval restricted in size, to avoid using conversion 

between 'double' and 'unsigned' (which can be slow and unreliable)
This commit is contained in:
Roberto Ierusalimschy 2014-04-17 13:09:40 -03:00
parent 0172c9552c
commit cbe4998bc2
1 changed files with 12 additions and 8 deletions
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18
lmathlib.c
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@ -1,5 +1,5 @@
/* /*
** $Id: lmathlib.c,v 1.95 2014/04/03 14:18:19 roberto Exp roberto $ ** $Id: lmathlib.c,v 1.97 2014/04/10 17:53:33 roberto Exp roberto $
** Standard mathematical library ** Standard mathematical library
** See Copyright Notice in lua.h ** See Copyright Notice in lua.h
*/ */
@ -228,12 +228,14 @@ static int math_max (lua_State *L) {
return 1; return 1;
} }
/*
** This function uses 'double' (instead of 'lua_Number') to ensure that
** all bits from 'l_rand' can be represented, and that 'RAND_MAX + 1.0'
** will keep full precision (ensuring that 'r' is always less than 1.0.)
*/
static int math_random (lua_State *L) { static int math_random (lua_State *L) {
lua_Integer low, up; lua_Integer low, up;
lua_Number r = (lua_Number)l_rand() * (1.0 / ((lua_Number)RAND_MAX + 1.0)); double r = (double)l_rand() * (1.0 / ((double)RAND_MAX + 1.0));
if (r >= 1.0) /* can occur if lua_Number has no precision for RAND_MAX */
r = 0.0; /* statistically irrelevant; avoids out-of-range results */
switch (lua_gettop(L)) { /* check number of arguments */ switch (lua_gettop(L)) { /* check number of arguments */
case 0: { /* no arguments */ case 0: { /* no arguments */
lua_pushnumber(L, r); /* Number between 0 and 1 */ lua_pushnumber(L, r); /* Number between 0 and 1 */
@ -253,8 +255,10 @@ static int math_random (lua_State *L) {
} }
/* random integer in the interval [low, up] */ /* random integer in the interval [low, up] */
luaL_argcheck(L, low <= up, 1, "interval is empty"); luaL_argcheck(L, low <= up, 1, "interval is empty");
r *= ((lua_Number)((lua_Unsigned)up - (lua_Unsigned)low) + 1.0); luaL_argcheck(L, (lua_Unsigned)up - low <= (lua_Unsigned)LUA_MAXINTEGER,
lua_pushinteger(L, (lua_Integer)((lua_Unsigned)r + (lua_Unsigned)low)); 1, "interval too large");
r *= (double)(up - low) + 1.0;
lua_pushinteger(L, (lua_Integer)r + low);
return 1; return 1;
} }