lua math.random伪随机问题浅析

-- 获取当前系统时间(秒)作为随机种子
math.randomseed(os.time())

-- 有三种方式：
-- 1. 不带参数调用时，获取的是[0,1)范围内的随机浮点数
-- 2. 带一个整型参数时，获取的是[1,n]范围内的随机整数
-- 3. 带两个整型参数m,n时，获取的是[m,n]范围内随机整数
-- 请注意Lua5.3以后，参数一定要为整数,否则会返回错误：bad argument #1 to ‘random‘ (number has no integer representation)
math.random(10, 30)

static const luaL_Reg mathlib[] = {
  {"abs",   math_abs},
  {"acos",  math_acos},
  {"asin",  math_asin},
  {"atan",  math_atan},
  {"ceil",  math_ceil},
  {"cos",   math_cos},
  {"deg",   math_deg},
  {"exp",   math_exp},
  {"tointeger", math_toint},
  {"floor", math_floor},
  {"fmod",   math_fmod},
  {"ult",   math_ult},
  {"log",   math_log},
  {"max",   math_max},
  {"min",   math_min},
  {"modf",   math_modf},
  {"rad",   math_rad},
  {"random",     math_random},　　　　// 关于math.random的调用方法:math_random
  {"randomseed", math_randomseed},　 // 关于math.randomseed的调用方法:math_randomseed
  {"sin",   math_sin},
  {"sqrt",  math_sqrt},
  {"tan",   math_tan},
  {"type", math_type},
  /* placeholders */
  {"pi", NULL},
  {"huge", NULL},
  {"maxinteger", NULL},
  {"mininteger", NULL},
  {NULL, NULL}
};

/*
** This function uses ‘double‘ (instead of ‘lua_Number‘) to ensure that
** all bits from ‘l_rand‘ can be represented, and that ‘RANDMAX + 1.0‘
** will keep full precision (ensuring that ‘r‘ is always less than 1.0.)
*/
static int math_random (lua_State *L) {
  lua_Integer low, up;
  double r = (double)l_rand() * (1.0 / ((double)L_RANDMAX + 1.0));
  switch (lua_gettop(L)) {  /* check number of arguments */
    case 0: {  /* no arguments */
      lua_pushnumber(L, (lua_Number)r);  /* Number between 0 and 1 */
      return 1;
    }
    case 1: {  /* only upper limit */
      low = 1;
      up = luaL_checkinteger(L, 1);
      break;
    }
    case 2: {  /* lower and upper limits */
      low = luaL_checkinteger(L, 1);
      up = luaL_checkinteger(L, 2);
      break;
    }
    default: return luaL_error(L, "wrong number of arguments");
  }
  /* random integer in the interval [low, up] */
  luaL_argcheck(L, low <= up, 1, "interval is empty");
  luaL_argcheck(L, low >= 0 || up <= LUA_MAXINTEGER + low, 1,
                   "interval too large");
  r *= (double)(up - low) + 1.0;
  lua_pushinteger(L, (lua_Integer)r + low);
  return 1;
}

static int math_randomseed (lua_State *L) {
  l_srand((unsigned int)(lua_Integer)luaL_checknumber(L, 1));
  (void)l_rand(); /* discard first value to avoid undesirable correlations */
  return 0;
}

#if !defined(l_rand
#if defined(LUA_USE_POSIX)
#define l_rand()    random()
#define l_srand(x)    srandom(x)
#define L_RANDMAX    2147483647    /* (2^31 - 1), following POSIX */
#else
// windows平台等
#define l_rand()    rand()
#define l_srand(x)    srand(x)
#define L_RANDMAX    RAND_MAX         // 0x7fff
#endif
#endif

// 参考来自：The Standard C Library 中第337页
static unsigned long int next = 1;
 
/* RAND_MAX assumed to be 32767 */
int rand(void)
 {
    next = next * 1103515245 + 12345;
    return((unsigned)(next/65536) % 32768);
}
 
void srand(unsigned seed) {
    next = seed;
}

-- 随机种子数值为100，每次执行的结果都是：1 1 1 3 4 2 4 1 4 1
math.randomseed(100)
-- 随机种子数字为os.time()，每次执行结果会发生改变
math.randomseed(os.time())
local randNum = {}
for i = 1,10 do
    table.insert(randNum,math.random(1,5))
end
print(table.concat(randNum," "))

-- 将os.time()获取的系统秒数数值翻转(低位变高位)，再取高6位，这样即使time变化很小
-- 由于低位变高位，数值就会变化很大，这样1秒内进行多次运行的话，效果会好些
local next = tostring(os.time()):reverse():sub(1, 6)
math.randomseed(next )