Merge pull request #214 from gwilymk/random-number-generator

Random number generator

agb/src/hash_map.rs

@@ -875,7 +875,7 @@ mod test {
     use core::cell::RefCell;
 
     use super::*;
-    use crate::Gba;
+    use crate::{rng::RandomNumberGenerator, Gba};
 
     #[test_case]
     fn can_store_and_retrieve_8_elements(_gba: &mut Gba) {
@@ -962,35 +962,6 @@ mod test {
         }
     }
 
-    struct RandomNumberGenerator {
-        state: [u32; 4],
-    }
-
-    impl RandomNumberGenerator {
-        const fn new() -> Self {
-            Self {
-                state: [1014776995, 476057059, 3301633994, 706340607],
-            }
-        }
-
-        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
-        }
-    }
-
     struct NoisyDrop {
         i: i32,
         dropped: bool,
@@ -1034,9 +1005,9 @@ mod test {
         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.gen().rem_euclid(2);
+            let key = rng.gen().rem_euclid(answers.len() as i32);
+            let value = rng.gen();
 
             match command {
                 0 => {

agb/src/lib.rs

@@ -155,6 +155,8 @@ pub mod mgba;
 pub use agb_fixnum as fixnum;
 /// Contains an implementation of a hashmap which suits the gameboy advance's hardware.
 pub mod hash_map;
+/// Simple random number generator
+pub mod rng;
 mod single;
 /// Implements sound output.
 pub mod sound;

agb/src/rng.rs

@@ -0,0 +1,102 @@
+use core::cell::RefCell;
+
+use bare_metal::Mutex;
+
+use crate::interrupt::free;
+
+/// A fast pseudo-random number generator. Note that the output of the
+/// random number generator for a given seed is guaranteed stable
+/// between minor releases, however could change in a major release.
+pub struct RandomNumberGenerator {
+    state: [u32; 4],
+}
+
+impl RandomNumberGenerator {
+    /// Create a new random number generator with a fixed seed
+    ///
+    /// Note that this seed is guaranteed to be the same between minor releases.
+    pub const fn new() -> Self {
+        Self::new_with_seed([1014776995, 476057059, 3301633994, 706340607])
+    }
+
+    /// Produces a random number generator with the given initial state / seed.
+    /// None of the values can be 0.
+    pub const fn new_with_seed(seed: [u32; 4]) -> Self {
+        // this can't be in a loop because const
+        assert!(seed[0] != 0, "seed must not be 0");
+        assert!(seed[1] != 0, "seed must not be 0");
+        assert!(seed[2] != 0, "seed must not be 0");
+        assert!(seed[3] != 0, "seed must not be 0");
+
+        Self { state: seed }
+    }
+
+    /// Returns the next value for the random number generator
+    pub fn gen(&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()));
+
+/// Using a global random number generator, provides the next random number
+pub fn gen() -> i32 {
+    free(|cs| GLOBAL_RNG.borrow(*cs).borrow_mut().gen())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::Gba;
+
+    #[test_case]
+    fn should_be_reasonably_distributed(_gba: &mut Gba) {
+        let mut values: [u32; 16] = Default::default();
+
+        let mut rng = RandomNumberGenerator::new();
+        for _ in 0..500 {
+            values[(rng.gen().rem_euclid(16)) as usize] += 1;
+        }
+
+        for (i, &value) in values.iter().enumerate() {
+            assert!(
+                value >= 500 / 10 / 3,
+                "{} came up less than expected {}",
+                i,
+                value
+            );
+        }
+    }
+
+    #[test_case]
+    fn global_rng_should_be_reasonably_distributed(_gba: &mut Gba) {
+        let mut values: [u32; 16] = Default::default();
+
+        for _ in 0..500 {
+            values[super::gen().rem_euclid(16) as usize] += 1;
+        }
+
+        for (i, &value) in values.iter().enumerate() {
+            assert!(
+                value >= 500 / 10 / 3,
+                "{} came up less than expected {}",
+                i,
+                value
+            );
+        }
+    }
+}

examples/the-purple-night/src/main.rs

@@ -3,15 +3,12 @@
 
 extern crate alloc;
 
-mod rng;
 mod sfx;
 
 use core::cmp::Ordering;
 
 use alloc::{boxed::Box, vec::Vec};
 
-use rng::get_random;
-
 use agb::{
     display::{
         object::{Graphics, Object, ObjectController, Sprite, Tag, TagMap},
@@ -21,6 +18,7 @@ use agb::{
     fixnum::{FixedNum, Rect, Vector2D},
     input::{Button, ButtonController, Tri},
     interrupt::VBlank,
+    rng,
 };
 use generational_arena::Arena;
 use sfx::Sfx;
@@ -1114,7 +1112,7 @@ impl MiniFlameData {
                     self.sprite_offset = 0;
                     self.state = MiniFlameState::Dead;
 
-                    if get_random() % 4 == 0 {
+                    if rng::gen() % 4 == 0 {
                         instruction = UpdateInstruction::CreateParticle(
                             ParticleData::new_health(),
                             entity.position,
@@ -1137,7 +1135,7 @@ impl MiniFlameData {
                     self.sprite_offset = 0;
                     self.state = MiniFlameState::Dead;
 
-                    if get_random() % 4 == 0 {
+                    if rng::gen() % 4 == 0 {
                         instruction = UpdateInstruction::CreateParticle(
                             ParticleData::new_health(),
                             entity.position,
@@ -1700,7 +1698,7 @@ impl<'a> Boss<'a> {
         Self {
             entity,
             health: 5,
-            target_location: get_random().rem_euclid(5) as u8,
+            target_location: rng::gen().rem_euclid(5) as u8,
             state: BossActiveState::Damaged(60),
             timer: 0,
             screen_coords,
@@ -1801,9 +1799,9 @@ impl<'a> Boss<'a> {
     fn commit(&mut self, offset: Vector2D<Number>) {
         let shake = if self.shake_magnitude != 0.into() {
             (
-                Number::from_raw(get_random()).rem_euclid(self.shake_magnitude)
+                Number::from_raw(rng::gen()).rem_euclid(self.shake_magnitude)
                     - self.shake_magnitude / 2,
-                Number::from_raw(get_random()).rem_euclid(self.shake_magnitude)
+                Number::from_raw(rng::gen()).rem_euclid(self.shake_magnitude)
                     - self.shake_magnitude / 2,
             )
                 .into()
@@ -1816,8 +1814,8 @@ impl<'a> Boss<'a> {
     }
     fn explode(&self, enemies: &mut Arena<Enemy<'a>>, object_controller: &'a ObjectController) {
         for _ in 0..(6 - self.health) {
-            let x_offset: Number = Number::from_raw(get_random()).rem_euclid(2.into()) - 1;
-            let y_offset: Number = Number::from_raw(get_random()).rem_euclid(2.into()) - 1;
+            let x_offset: Number = Number::from_raw(rng::gen()).rem_euclid(2.into()) - 1;
+            let y_offset: Number = Number::from_raw(rng::gen()).rem_euclid(2.into()) - 1;
             let mut flame = Enemy::new(
                 object_controller,
                 EnemyData::MiniFlame(MiniFlameData::new()),
@@ -1830,7 +1828,7 @@ impl<'a> Boss<'a> {
 
     fn get_next_target_location(&self) -> u8 {
         loop {
-            let a = get_random().rem_euclid(5) as u8;
+            let a = rng::gen().rem_euclid(5) as u8;
             if a != self.target_location {
                 break a;
             }
@@ -1975,8 +1973,8 @@ impl<'a> Game<'a> {
         if self.shake_time > 0 {
             let size = self.shake_time.min(4) as i32;
             let offset: Vector2D<Number> = (
-                Number::from_raw(get_random()) % size - Number::new(size) / 2,
-                Number::from_raw(get_random()) % size - Number::new(size) / 2,
+                Number::from_raw(rng::gen()) % size - Number::new(size) / 2,
+                Number::from_raw(rng::gen()) % size - Number::new(size) / 2,
             )
                 .into();
             this_frame_offset += offset;
@@ -2291,7 +2289,7 @@ fn game_with_level(gba: &mut agb::Gba) {
                 }
             }
 
-            get_random(); // advance RNG to make it less predictable between runs
+            rng::gen(); // advance RNG to make it less predictable between runs
         };
 
         game.clear(&mut vram);

examples/the-purple-night/src/rng.rs

@@ -1,34 +0,0 @@
-struct RandomNumberGenerator {
-    state: [u32; 4],
-}
-
-impl RandomNumberGenerator {
-    const fn new() -> Self {
-        Self {
-            state: [1014776995, 476057059, 3301633994, 706340607],
-        }
-    }
-
-    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 mut RANDOM_GENERATOR: RandomNumberGenerator = RandomNumberGenerator::new();
-
-pub fn get_random() -> i32 {
-    unsafe { &mut RANDOM_GENERATOR }.next()
-}

examples/the-purple-night/src/sfx.rs

@@ -1,5 +1,5 @@
-use super::rng::get_random;
 use agb::fixnum::Num;
+use agb::rng;
 use agb::sound::mixer::{ChannelId, Mixer, SoundChannel};
 
 const BAT_DEATH: &[u8] = agb::include_wav!("sfx/BatDeath.wav");
@@ -85,7 +85,7 @@ impl<'a> Sfx<'a> {
     }
 
     pub fn jump(&mut self) {
-        let r = get_random() % 3;
+        let r = rng::gen() % 3;
 
         let channel = match r {
             0 => SoundChannel::new(JUMP1),