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#include <math.h>
#include "libs/pcg/pcg.c"
#define CountLeadingZeroes64(Value) __builtin_clzll(Value)
u64
RandomU64(pcg64_random_t *RNG)
{
u64 Result = pcg64_random_r(RNG);
return Result;
}
//~ Random 64 bit float
// From: https://mumble.net/~campbell/tmp/random_real.c
/*
* Copyright (c) 2014, Taylor R Campbell
*
* Verbatim copying and distribution of this entire article are
* permitted worldwide, without royalty, in any medium, provided
* this notice, and the copyright notice, are preserved.
*
*/
/*
* random_real: Generate a stream of bits uniformly at random and
* interpret it as the fractional part of the binary expansion of a
* number in [0, 1], 0.00001010011111010100...; then round it.
*/
f64
RandomF64(pcg64_random_t *RNG)
{
s32 Exponent = -64;
u64 Significand;
umm Shift;
/*
* Read zeros into the exponent until we hit a one; the rest
* will go into the significand.
*/
while((Significand = RandomU64(RNG)) == 0)
{
Exponent -= 64;
/*
* If the exponent falls below -1074 = emin + 1 - p,
* the exponent of the smallest subnormal, we are
* guaranteed the result will be rounded to zero. This
* case is so unlikely it will happen in realistic
* terms only if RandomU64 is broken.
*/
if ((Exponent < -1074))
return 0;
}
/*
* There is a 1 somewhere in significand, not necessarily in
* the most significant position. If there are leading zeros,
* shift them into the exponent and refill the less-significant
* bits of the significand. Can't predict one way or another
* whether there are leading zeros: there's a fifty-fifty
* chance, if RandomU64() is uniformly distributed.
*/
Shift = CountLeadingZeroes64(Significand);
if (Shift != 0) {
Exponent -= Shift;
Significand <<= Shift;
Significand |= (RandomU64(RNG) >> (64 - Shift));
}
/*
* Set the sticky bit, since there is almost surely another 1
* in the bit stream. Otherwise, we might round what looks
* like a tie to even when, almost surely, were we to look
* further in the bit stream, there would be a 1 breaking the
* tie.
*/
Significand |= 1;
/*
* Finally, convert to f64 (rounding) and scale by
* 2^exponent.
*/
return ldexp((f64)Significand, Exponent);
}
f64
RandomUnilateral(pcg64_random_t *RNG)
{
return RandomF64(RNG);
}
f64
RandomBilateral(pcg64_random_t *RNG)
{
f64 Result = 2.0*RandomUnilateral(RNG) - 1.0;
return Result;
}
f64
RandomBetween(pcg64_random_t *RNG, f64 Min, f64 Max)
{
f64 Range = Max - Min;
f64 Result = Min + RandomUnilateral(RNG)*Range;
return Result;
}
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