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Ensure that Math.random() really covers [0,1) interval (#1417)

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Legacy implementation made incorrect assumptions on how many bits
of useful information is returned by libc's `rand()`. If `RAND_MAX`
had more than 16 useful bits, it assumed that `rand()` could return
32 useful random bits. However, e.g., jerry-libc's `RAND_MAX` is
`0x7fffffff`, which denotes 31 useful bits only. The consequence
was that `Math.random()` covered only the lower half of the
standard-mandated `[0,1)` interval.

This path fixes the error and always uses the exact value of
`RAND_MAX` to compute the random value, which will thus fully cover
`[0,1)`.

Fixes #1414

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu
This commit is contained in:
Akos Kiss
2016-11-01 15:43:16 +01:00
committed by GitHub
parent 36edae3257
commit b31074cc6b
1 changed files with 3 additions and 20 deletions
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jerry-core/ecma/builtin-objects/ecma-builtin-math.c
+3 -20
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@@ -507,28 +507,11 @@ static ecma_value_t
ecma_builtin_math_object_random (ecma_value_t this_arg) /**< 'this' argument */
{
JERRY_UNUSED (this_arg);
uint32_t rnd = 1;
uint32_t reps_count;
#if RAND_MAX < 0x100
reps_count = 4;
#elif RAND_MAX < 0x10000
reps_count = 2;
#else /* RAND_MAX >= 0x10000 */
reps_count = 1;
#endif /* RAND_MAX < 0x100 */
for (uint32_t i = 0; i < reps_count; i++)
{
uint32_t next_rand = (uint32_t) rand ();
rnd *= next_rand;
}
const ecma_number_t rand_max = (ecma_number_t) RAND_MAX;
const ecma_number_t rand_max_min_1 = (ecma_number_t) (RAND_MAX - 1);
const uint32_t max_uint32 = (uint32_t) -1;
ecma_number_t rand = (ecma_number_t) rnd;
rand /= (ecma_number_t) max_uint32;
rand *= (ecma_number_t) (max_uint32 - 1) / (ecma_number_t) max_uint32;
return ecma_make_number_value (rand);
return ecma_make_number_value (((ecma_number_t) rand ()) / rand_max * rand_max_min_1 / rand_max);
} /* ecma_builtin_math_object_random */
/**