1 files changed, 96 insertions, 0 deletions
diff --git a/src/boot/O2/rand.c b/src/boot/O2/rand.c
new file mode 100644
index 000000000..2db64e9d5
--- /dev/null
+++ b/src/boot/O2/rand.c
@@ -0,0 +1,96 @@
+#include "global.h"
+
+//! The latest generated random number, used to generate the next number in the sequence.
+static u32 sRandInt = 1;
+
+//! Space to store a value to be re-interpreted as a float.
+//! This can't be static because it is used in z_kankyo.
+u32 sRandFloat;
+
+//! These values are recommended by the algorithms book *Numerical Recipes in C. The Art of Scientific Computing*, 2nd
+//! Edition, 1992, ISBN 0-521-43108-5. (p. 284):
+//! > This is about as good as any 32-bit linear congruential generator, entirely adequate for many uses.
+#define RAND_MULTIPLIER 1664525
+#define RAND_INCREMENT 1013904223
+
+/**
+ * Generates the next pseudo-random integer.
+ */
+u32 Rand_Next(void) {
+    return sRandInt = (sRandInt * RAND_MULTIPLIER) + RAND_INCREMENT;
+}
+
+/**
+ * Seeds the internal pseudo-random number generator with a provided starting value.
+ */
+void Rand_Seed(u32 seed) {
+    sRandInt = seed;
+}
+
+/**
+ * Returns a pseudo-random float between 0.0f and 1.0f from the internal PRNG.
+ *
+ * @note Works by generating the next integer, masking it to an IEEE-754 compliant float between 1.0f and 2.0f, and
+ * subtracting 1.0f.
+ *
+ * @remark This is also recommended by Numerical Recipes, pp. 284-5.
+ */
+f32 Rand_ZeroOne(void) {
+    sRandInt = (sRandInt * RAND_MULTIPLIER) + RAND_INCREMENT;
+    sRandFloat = ((sRandInt >> 9) | 0x3F800000);
+    return *((f32*)&sRandFloat) - 1.0f;
+}
+
+/**
+ * Returns a pseudo-random float between -0.5f and 0.5f in the same way as Rand_ZeroOne().
+ */
+f32 Rand_Centered(void) {
+    sRandInt = (sRandInt * RAND_MULTIPLIER) + RAND_INCREMENT;
+    sRandFloat = ((sRandInt >> 9) | 0x3F800000);
+    return *((f32*)&sRandFloat) - 1.5f;
+}
+
+//! All functions below are unused variants of the above four, that use a provided random number variable instead of the
+//! internal `sRandInt`
+
+/**
+ * Seeds a provided pseudo-random number with a provided starting value.
+ *
+ * @see Rand_Seed
+ */
+void Rand_Seed_Variable(u32* rndNum, u32 seed) {
+    *rndNum = seed;
+}
+
+/**
+ * Generates the next pseudo-random number from the provided rndNum.
+ *
+ * @see Rand_Next
+ */
+u32 Rand_Next_Variable(u32* rndNum) {
+    return *rndNum = (*rndNum * RAND_MULTIPLIER) + RAND_INCREMENT;
+}
+
+/**
+ * Generates the next pseudo-random float between 0.0f and 1.0f from the provided rndNum.
+ *
+ * @see Rand_ZeroOne
+ */
+f32 Rand_ZeroOne_Variable(u32* rndNum) {
+    u32 next = (*rndNum * RAND_MULTIPLIER) + RAND_INCREMENT;
+
+    sRandFloat = ((*rndNum = next) >> 9) | 0x3F800000;
+    return *((f32*)&sRandFloat) - 1.0f;
+}
+
+/**
+ * Generates the next pseudo-random float between -0.5f and 0.5f from the provided rndNum.
+ *
+ * @see Rand_ZeroOne, Rand_Centered
+ */
+f32 Rand_Centered_Variable(u32* rndNum) {
+    u32 next = (*rndNum * RAND_MULTIPLIER) + RAND_INCREMENT;
+
+    sRandFloat = ((*rndNum = next) >> 9) | 0x3F800000;
+    return *((f32*)&sRandFloat) - 1.5f;
+}