From 610722a1bfd91e0ef527d5f593a60228d31cf2ed Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Wed, 23 Mar 2022 21:52:37 +0000
Subject: [PATCH] Extract the random number generator

---
 agb/src/hash_map.rs |  9 ++++-----
 agb/src/rng.rs      | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 5 deletions(-)
 create mode 100644 agb/src/rng.rs

diff --git a/agb/src/hash_map.rs b/agb/src/hash_map.rs
index fe9ee3a..9009478 100644
--- a/agb/src/hash_map.rs
+++ b/agb/src/hash_map.rs
@@ -831,7 +831,7 @@ mod test {
     use core::cell::RefCell;
 
     use super::*;
-    use crate::Gba;
+    use crate::{rng, Gba};
 
     #[test_case]
     fn can_store_and_retrieve_8_elements(_gba: &mut Gba) {
@@ -956,14 +956,13 @@ mod test {
     #[test_case]
     fn extreme_case(_gba: &mut Gba) {
         let mut map = HashMap::new();
-        let mut rng = crate::rng::RandomNumberGenerator::new();
 
         let mut answers: [Option<i32>; 128] = [None; 128];
 
         for _ in 0..5_000 {
-            let command = rng.next().rem_euclid(2);
-            let key = rng.next().rem_euclid(answers.len() as i32);
-            let value = rng.next();
+            let command = rng::next().rem_euclid(2);
+            let key = rng::next().rem_euclid(answers.len() as i32);
+            let value = rng::next();
 
             match command {
                 0 => {
diff --git a/agb/src/rng.rs b/agb/src/rng.rs
new file mode 100644
index 0000000..57bc440
--- /dev/null
+++ b/agb/src/rng.rs
@@ -0,0 +1,41 @@
+use core::cell::RefCell;
+
+use bare_metal::Mutex;
+
+use crate::interrupt::free;
+
+pub struct RandomNumberGenerator {
+    state: [u32; 4],
+}
+
+impl RandomNumberGenerator {
+    pub const fn new() -> Self {
+        Self {
+            state: [1014776995, 476057059, 3301633994, 706340607],
+        }
+    }
+
+    pub fn next(&mut self) -> i32 {
+        let result = (self.state[0].wrapping_add(self.state[3]))
+            .rotate_left(7)
+            .wrapping_mul(9);
+        let t = self.state[1].wrapping_shr(9);
+
+        self.state[2] ^= self.state[0];
+        self.state[3] ^= self.state[1];
+        self.state[1] ^= self.state[2];
+        self.state[0] ^= self.state[3];
+
+        self.state[2] ^= t;
+        self.state[3] = self.state[3].rotate_left(11);
+
+        result as i32
+    }
+}
+
+static GLOBAL_RNG: Mutex<RefCell<RandomNumberGenerator>> =
+    Mutex::new(RefCell::new(RandomNumberGenerator::new()));
+
+pub fn next() -> i32 {
+    free(|cs| GLOBAL_RNG.borrow(*cs).borrow_mut().next())
+}