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RNG doc (#1892)
* RNG doc * Add some missing note qualifiers in comments * code_800FD970 -> rand in Makefile and disasm CSVs
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57ae8ae1dd
commit
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5 changed files with 167 additions and 84 deletions
2
Makefile
2
Makefile
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@ -238,7 +238,7 @@ $(BUILD_DIR)/src/boot/stackcheck.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/__osMalloc.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/code_800FC620.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/code_800FCE80.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/code_800FD970.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/rand.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/gfxprint.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/jpegutils.o: OPTFLAGS := -O2
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$(BUILD_DIR)/src/code/jpegdecoder.o: OPTFLAGS := -O2
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2
spec
2
spec
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@ -482,7 +482,7 @@ beginseg
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include "$(BUILD_DIR)/src/code/code_800FCE80.o"
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include "$(BUILD_DIR)/src/code/fp.o"
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include "$(BUILD_DIR)/src/code/system_malloc.o"
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include "$(BUILD_DIR)/src/code/code_800FD970.o"
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include "$(BUILD_DIR)/src/code/rand.o"
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include "$(BUILD_DIR)/src/code/__osMalloc.o"
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#if !OOT_DEBUG
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include "$(BUILD_DIR)/src/libultra/libc/sprintf.o"
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@ -1,79 +0,0 @@
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#include "global.h"
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// The latest generated random number, used to generate the next number in the sequence.
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static u32 sRandInt = 1;
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// Space to store a value to be re-interpreted as a float.
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static u32 sRandFloat;
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/**
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* Gets the next integer in the sequence of pseudo-random numbers.
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*/
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u32 Rand_Next(void) {
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return sRandInt = (sRandInt * 1664525) + 1013904223;
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}
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/**
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* Seeds the pseudo-random number generator by providing a starting value.
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*/
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void Rand_Seed(u32 seed) {
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sRandInt = seed;
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}
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/**
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* Returns a pseudo-random floating-point number between 0.0f and 1.0f, by generating
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* the next integer and masking it to an IEEE-754 compliant floating-point number
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* between 1.0f and 2.0f, returning the result subtract 1.0f.
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*/
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f32 Rand_ZeroOne(void) {
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sRandInt = (sRandInt * 1664525) + 1013904223;
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sRandFloat = ((sRandInt >> 9) | 0x3F800000);
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return *((f32*)&sRandFloat) - 1.0f;
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}
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/**
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* Returns a pseudo-random floating-point number between -0.5f and 0.5f by the same
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* manner in which Rand_ZeroOne generates its result.
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*/
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f32 Rand_Centered(void) {
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sRandInt = (sRandInt * 1664525) + 1013904223;
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sRandFloat = ((sRandInt >> 9) | 0x3F800000);
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return *((f32*)&sRandFloat) - 1.5f;
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}
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/**
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* Seeds a pseudo-random number at rndNum with a provided seed.
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*/
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void Rand_Seed_Variable(u32* rndNum, u32 seed) {
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*rndNum = seed;
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}
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/**
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* Generates the next pseudo-random integer from the provided rndNum.
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*/
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u32 Rand_Next_Variable(u32* rndNum) {
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return *rndNum = (*rndNum * 1664525) + 1013904223;
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}
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/**
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* Generates the next pseudo-random floating-point number between 0.0f and
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* 1.0f from the provided rndNum.
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*/
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f32 Rand_ZeroOne_Variable(u32* rndNum) {
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u32 next = (*rndNum * 1664525) + 1013904223;
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sRandFloat = ((*rndNum = next) >> 9) | 0x3F800000;
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return *((f32*)&sRandFloat) - 1.0f;
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}
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/**
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* Generates the next pseudo-random floating-point number between -0.5f and
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* 0.5f from the provided rndNum.
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*/
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f32 Rand_Centered_Variable(u32* rndNum) {
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u32 next = (*rndNum * 1664525) + 1013904223;
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*rndNum = next;
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sRandFloat = (next >> 9) | 0x3F800000;
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return *((f32*)&sRandFloat) - 1.5f;
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}
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162
src/code/rand.c
Normal file
162
src/code/rand.c
Normal file
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@ -0,0 +1,162 @@
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/**
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* @file rand.c
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*
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* This file implements the primary random number generator the game relies on. The generator is pseudo-random and
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* implemented as a Linear Congruential Generator (LCG).
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*
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* A LCG computes random numbers sequentially via the relation
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* X(n+1) = (a * X(n) + c) mod m
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* where m is the modulus, a is the multiplier and c is the increment.
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*
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* These three parameters (a,c,m) completely specify the LCG and should be chosen such that
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* - m > 0
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* - 0 < a < m
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* - 0 <= c < m
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*
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* The period of the LCG (a, c, m) is the smallest period p such that X(n + p) = X(n), past n=p the sequence will repeat
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* itself in its outputs.
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* A good LCG should have the maximum possible period, which will be equal to m as there are at most m possible values
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* for X. This occurs when (Hull, T.E., & Dobell, A.R. (1962). Random Number Generators. Siam Review, 4, 230-254.):
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* - m,c are relatively prime, that is the only integer that divides both m and c with no remainder is 1.
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* - a - 1 is divisible by all prime factors of m.
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* - a - 1 is divisible by 4 if m is divisible by 4.
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*
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* Ideally m is chosen to be a large power of 2 so that the modulo operation is inexpensive to compute. In this case the
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* prime factors of m = 2^k are just k copies of 2. For k > 1 m is divisible by 4, so a - 1 must be divisible by 4. 2^k
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* and c can easily be made relatively prime by making c an odd number.
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* If we let k=32 to match the size of an integer, the modulo operation is made implicit by the width of the data type
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* and becomes free to compute.
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*
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* The parameter a should be selected such that a-1 is divisible by 4 (and hence divisible by 2) and c should be any odd
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* number. The precise values should fare well against the spectral test, a measure of "how random" a particular LCG is.
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* A pair (a,c) that satisfies these requirements is (1664525, 1013904223), recommended by "Numerical Recipes in C: The
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* Art of Scientific Computing" (p. 284).
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*
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* Therefore, the LCG with parameters (1664525, 1013904223, 2^32) that is implemented in this file has a maximal period
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* of 2^32 and produces high-quality pseudo-random numbers.
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*
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* @note If sampling the LCG for a n-bit number it is important to use the upper n bits instead of the lower n bits of
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* the LCG output. The lower n bits only have a period of 2^n which may significantly worsen the quality of the
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* resulting random numbers compared to the quality of the full 32-bit result.
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*
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* @note Original name: qrand.c
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*/
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#include "ultra64.h"
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#define RAND_MULTIPLIER 1664525
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#define RAND_INCREMENT 1013904223
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/**
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* The latest generated random number, used to generate the next number in the sequence.
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*
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* @note Original name: __qrand_idum
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*/
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static u32 sRandInt = 1;
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/**
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* Space to store a value to be re-interpreted as a float.
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*
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* @note Orignal name: __qrand_itemp
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*/
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static fu sRandFloat;
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/**
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* Gets the next integer in the sequence of pseudo-random numbers.
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*
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* @note Original name: qrand
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*/
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u32 Rand_Next(void) {
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return sRandInt = sRandInt * RAND_MULTIPLIER + RAND_INCREMENT;
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}
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/**
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* Seeds the pseudo-random number generator by providing a starting value.
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*
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* @note Original name: sqrand
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*/
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void Rand_Seed(u32 seed) {
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sRandInt = seed;
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}
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/**
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* Returns a pseudo-random floating-point number between 0.0f and 1.0f, by generating the next integer and masking it
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* to an IEEE-754 compliant floating-point number between 1.0f and 2.0f, returning the result subtract 1.0f.
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*
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* @note This technique for generating pseudo-random floats is recommended as a particularly fast but potentially
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* non-portable generator in "Numerical Recipes in C: The Art of Scientic Computing", pp. 284-5.
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*
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* @note Original name: fqrand
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*/
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f32 Rand_ZeroOne(void) {
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sRandInt = sRandInt * RAND_MULTIPLIER + RAND_INCREMENT;
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// Samples the upper 23 bits to avoid effectively reducing the LCG period.
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sRandFloat.i = (sRandInt >> 9) | 0x3F800000;
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return sRandFloat.f - 1.0f;
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}
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/**
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* Returns a pseudo-random floating-point number between -0.5f and 0.5f by the same manner in which Rand_ZeroOne
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* generates its result.
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*
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* @see Rand_ZeroOne
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*
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* @note Original name: fqrand2
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*/
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f32 Rand_Centered(void) {
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sRandInt = sRandInt * RAND_MULTIPLIER + RAND_INCREMENT;
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sRandFloat.i = (sRandInt >> 9) | 0x3F800000;
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return sRandFloat.f - 1.5f;
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}
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//! All functions below are unused variants of the above four, that use a provided random number variable instead of the
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//! internal `sRandInt`
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/**
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* Seeds a pseudo-random number at rndNum with a provided starting value.
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*
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* @see Rand_Seed
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*
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* @note Original name: sqrand_r
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*/
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void Rand_Seed_Variable(u32* rndNum, u32 seed) {
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*rndNum = seed;
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}
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/**
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* Generates the next pseudo-random integer from the provided rndNum.
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*
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* @see Rand_Next
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*
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* @note Original name: qrand_r
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*/
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u32 Rand_Next_Variable(u32* rndNum) {
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return *rndNum = (*rndNum) * RAND_MULTIPLIER + RAND_INCREMENT;
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}
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/**
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* Generates the next pseudo-random floating-point number between 0.0f and 1.0f from the provided rndNum.
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*
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* @see Rand_ZeroOne
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*
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* @note Original name: fqrand_r
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*/
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f32 Rand_ZeroOne_Variable(u32* rndNum) {
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u32 next = (*rndNum) * RAND_MULTIPLIER + RAND_INCREMENT;
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sRandFloat.i = ((*rndNum = next) >> 9) | 0x3F800000;
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return sRandFloat.f - 1.0f;
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}
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/**
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* Generates the next pseudo-random floating-point number between -0.5f and 0.5f from the provided rndNum.
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*
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* @see Rand_ZeroOne, Rand_Centered
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*
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* @note Original name: fqrand2_r
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*/
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f32 Rand_Centered_Variable(u32* rndNum) {
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u32 next = (*rndNum) * RAND_MULTIPLIER + RAND_INCREMENT;
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sRandFloat.i = ((*rndNum = next) >> 9) | 0x3F800000;
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return sRandFloat.f - 1.5f;
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}
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@ -132,7 +132,7 @@ BCBE0,800CDAE0,src/code/padsetup
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BCD20,800CDC20,src/code/code_800FCE80
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BD230,800CE130,src/code/fp
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BD390,800CE290,src/code/system_malloc
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BD560,800CE460,src/code/code_800FD970
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BD560,800CE460,src/code/rand
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BD720,800CE620,src/code/__osMalloc
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BE4D0,800CF3D0,src/libultra/libc/sprintf
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BE5A0,800CF4A0,src/code/printutils
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@ -295,7 +295,7 @@ F0640,80101540,src/code/logseverity
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F0650,80101550,src/code/gfxprint
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F0F00,80101E00,src/code/code_800FC620
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F0F20,80101E20,src/code/fp
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F0F30,80101E30,src/code/code_800FD970
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F0F30,80101E30,src/code/rand
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F0F40,80101E40,src/code/__osMalloc
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F0F50,80101E50,src/libultra/gu/sins
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F1750,80102650,src/libultra/io/siacs
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@ -451,7 +451,7 @@ offset,vram,.bss
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119530,8012A430,src/code/gfxprint
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119540,8012A440,src/code/code_800FCE80
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119550,8012A450,src/code/system_malloc
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119580,8012A480,src/code/code_800FD970
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119580,8012A480,src/code/rand
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119590,8012A490,src/code/__osMalloc
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1195A0,8012A4A0,src/code/jpegdecoder
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1195B0,8012A4B0,src/libultra/io/sptask
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