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COMMON: Change PRNG Function to Xorshift* (#3341)

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The old RNG method had non-standard periods, ranging from some seeds looping on themselves (seed = 1184201285) to some seeds having periods as low as 11 or 48, as listed in https://github.com/scummvm/scummvm/pull/3340. This is a problem even for games that run the RNG once a frame, as the possibilities for random events is greatly reduced should the initial seed be in one of these sets of small periods.

Xorshift* is a standard, fast, non-cryptographic PRNG with academic backing that has period 2^32-1 (all seeds lead to another seed except 0, which is excluded from the initial seeds).  Many different flavors are possible, as listed in the paper, but the choice implemented in this pull request uses only a single 32-bit integer as a state, like the old PRNG.

Co-authored-by: Thierry Crozat <criezy@scummvm.org>
Co-authored-by: Filippos Karapetis <bluegr@gmail.com>
This commit is contained in:
TehGelly 2021-09-09 14:46:08 -04:00 committed by GitHub
parent 17917f3613
commit 0992fa9c3b
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2 changed files with 23 additions and 8 deletions
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21
common/random.cpp
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@ -48,21 +48,20 @@ RandomSource::RandomSource(const String &name) {
}
void RandomSource::setSeed(uint32 seed) {
if (seed == 0)
seed++;
_randSeed = seed;
}
uint RandomSource::getRandomNumber(uint max) {
_randSeed = 0xDEADBF03 * (_randSeed + 1);
_randSeed = (_randSeed >> 13) | (_randSeed << 19);
scrambleSeed();
if (max == UINT_MAX)
return _randSeed;
return _randSeed % (max + 1);
return (_randSeed * 0xDEADBF03);
return (_randSeed * 0xDEADBF03) % (max + 1);
}
uint RandomSource::getRandomBit() {
_randSeed = 0xDEADBF03 * (_randSeed + 1);
_randSeed = (_randSeed >> 13) | (_randSeed << 19);
scrambleSeed();
return _randSeed & 1;
}
@ -73,5 +72,13 @@ uint RandomSource::getRandomNumberRng(uint min, uint max) {
int RandomSource::getRandomNumberRngSigned(int min, int max) {
return getRandomNumber(max - min) + min;
}
inline void RandomSource::scrambleSeed() {
//marsaglia's paper says that any of 81 triplets are feasible
//(11,21,13) was chosen, with (cba) and (>>,<<,>>)
_randSeed ^= _randSeed >> 13;
_randSeed ^= _randSeed << 21;
_randSeed ^= _randSeed >> 11;
}
} // End of namespace Common

10
common/random.h
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@ -39,7 +39,7 @@ namespace Common {
class String;
/**
* Simple random number generator. Although it is definitely not suitable for
* Xorshift* random number generator. Although it is definitely not suitable for
* cryptographic purposes, it serves our purposes just fine.
*/
class RandomSource {
@ -90,6 +90,14 @@ public:
* @return a random number in the interval [min, max]
*/
int getRandomNumberRngSigned(int min, int max);
/**
* Scrambles the seed in order to get a new result.
* Code is shared between getRandomNumber and getRandomBit,
* so it is split off for clarity.
*/
private:
inline void scrambleSeed();
};
/** @} */