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love.math.setRandomSeed / RandomGenerator:setSeed now hashes the seed value before using it, which gives a much better distribution when comparing the results of different but similar seeds.

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Cleaned up RandomGenerator:setState/getState, now that love is using the C++11 standard library.

--HG--
branch : minor
This commit is contained in:
Alex Szpakowski
2015-01-26 14:13:42 -04:00
parent b4bacb38a6
commit a69f722f0f
1 changed files with 32 additions and 38 deletions
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src/modules/math/RandomGenerator.cpp
+32 -38
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@@ -21,11 +21,11 @@
#include "RandomGenerator.h"
// C++
#include <cmath>
#include <sstream>
#include <iomanip>
// C
#include <cmath>
#include <cstdlib>
namespace love
@@ -33,6 +33,20 @@ namespace love
namespace math
{
// Thomas Wang's 64-bit integer hashing function:
// https://web.archive.org/web/20110807030012/http://www.cris.com/%7ETtwang/tech/inthash.htm
static uint64 wangHash64(uint64 key)
{
key = (~key) + (key << 21); // key = (key << 21) - key - 1;
key = key ^ (key >> 24);
key = (key + (key << 3)) + (key << 8); // key * 265
key = key ^ (key >> 14);
key = (key + (key << 2)) + (key << 4); // key * 21
key = key ^ (key >> 28);
key = key + (key << 31);
return key;
}
// 64 bit Xorshift implementation taken from the end of Sec. 3 (page 4) in
// George Marsaglia, "Xorshift RNGs", Journal of Statistical Software, Vol.8 (Issue 14), 2003
@@ -75,17 +89,18 @@ double RandomGenerator::randomNormal(double stddev)
void RandomGenerator::setSeed(RandomGenerator::Seed newseed)
{
// 0 xor 0 is still 0, so Xorshift can't generate new numbers.
if (newseed.b64 == 0)
throw love::Exception("Random seed cannot be 0.");
seed = newseed;
rng_state = seed;
// Xorshift's first couple results after seeding will be similar to results
// from very similar seeds, so we immediately discard them here.
for (int i = 0; i < 2; i++)
rand();
// Xorshift isn't designed to give a good distribution of values across many
// similar seeds, so we hash the state integer before using it.
// http://www.reedbeta.com/blog/2013/01/12/quick-and-easy-gpu-random-numbers-in-d3d11/
// Xorshift also can't handle a state value of 0, so we avoid that.
do
{
newseed.b64 = wangHash64(newseed.b64);
} while (newseed.b64 == 0);
rng_state = newseed;
}
RandomGenerator::Seed RandomGenerator::getSeed() const
@@ -98,32 +113,17 @@ void RandomGenerator::setState(const std::string &statestr)
// For this implementation we'll accept a hex string representing the
// 64-bit state integer xorshift uses.
Seed state = {};
// Hex string must start with 0x.
if (statestr.find("0x") != 0 || statestr.size() < 3)
throw love::Exception("Invalid random state.");
throw love::Exception("Invalid random state: %s", statestr.c_str());
// standardized strtoull (or 64 bit integer support for stringstream)
// requires C++11's standard library, which we can't use yet.
// I use strtol like this not because it's the best solution, but because
// it's "good enough".
Seed state = {};
// Convert the hex string to the state integer character-by-character.
for (size_t i = 2; i < statestr.size(); i++)
{
char hex[2] = {statestr[i], 0};
char *end = nullptr;
char *end = nullptr;
state.b64 = strtoull(statestr.c_str(), &end, 16);
// Convert the current hex character to a number.
int nibble = strtol(hex, &end, 16);
// Check if strtol failed to convert it.
if (end != nullptr && *end != 0)
throw love::Exception("Invalid random state.");
state.b64 = (state.b64 << 4) + nibble;
}
if (end != nullptr && *end != 0)
throw love::Exception("Invalid random state: %s", statestr.c_str());
rng_state = state;
}
@@ -132,14 +132,8 @@ std::string RandomGenerator::getState() const
{
// For this implementation we'll return a hex string representing the 64-bit
// state integer xorshift uses.
std::stringstream ss;
ss << "0x";
// Again with the stringstream not dealing with 64 bit integers...
ss << std::setfill('0') << std::setw(8) << std::hex << rng_state.b32.high;
ss << std::setfill('0') << std::setw(8) << std::hex << rng_state.b32.low;
ss << "0x" << std::setfill('0') << std::setw(16) << std::hex << rng_state.b64;
return ss.str();
}