put small field bitfields on the stack

battlesnake/src/types/simulation.rs

@@ -1,4 +1,9 @@
-use std::{collections::VecDeque, fmt::Display};
+use std::{
+    collections::VecDeque,
+    fmt::Display,
+    num::NonZeroUsize,
+    ops::{Deref, DerefMut},
+};
 
 use bitvec::prelude::*;
 use log::{error, warn};
@@ -6,6 +11,52 @@ use rand::prelude::*;
 
 use super::{wire::Request, Coord, Direction};
 
+#[derive(Debug, PartialEq, Eq, Clone)]
+enum SmallBitBox {
+    Stack {
+        storage: BitArr!(for Self::STACK_BITS),
+        len: NonZeroUsize,
+    },
+    Heap(BitBox),
+}
+
+impl SmallBitBox {
+    const STACK_BITS: usize = usize::BITS as usize * 3;
+
+    fn new(initial: bool, len: usize) -> Self {
+        if let len @ 1..Self::STACK_BITS = len {
+            let Some(len) = NonZeroUsize::new(len) else {
+                unreachable!()
+            };
+            let mut storage = BitArray::ZERO;
+            storage.fill(initial);
+            Self::Stack { storage, len }
+        } else {
+            Self::Heap(bitbox![u8::from(initial); len])
+        }
+    }
+}
+
+impl Deref for SmallBitBox {
+    type Target = BitSlice;
+
+    fn deref(&self) -> &Self::Target {
+        match self {
+            Self::Stack { storage, len } => &storage[..len.get()],
+            Self::Heap(bit_box) => bit_box,
+        }
+    }
+}
+
+impl DerefMut for SmallBitBox {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        match self {
+            Self::Stack { storage, len } => &mut storage[..len.get()],
+            Self::Heap(bit_box) => bit_box,
+        }
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Board {
     width: u8,
@@ -14,9 +65,9 @@ pub struct Board {
     food_spawn_chance: u8,
     min_food: u16,
     turn: u32,
-    food: BitBox,
-    hazard: BitBox,
-    free: BitBox,
+    food: SmallBitBox,
+    hazard: SmallBitBox,
+    free: SmallBitBox,
     snakes: Vec<Snake>,
     constrictor: bool,
 }
@@ -33,9 +84,9 @@ impl From<&Request> for Board {
             food_spawn_chance: value.game.ruleset.settings.food_spawn_chance,
             min_food: value.game.ruleset.settings.minimum_food,
             turn: value.turn,
-            food: bitbox![0; fields],
-            hazard: bitbox![0; fields],
-            free: bitbox![1; fields],
+            food: SmallBitBox::new(false, fields),
+            hazard: SmallBitBox::new(false, fields),
+            free: SmallBitBox::new(true, fields),
             snakes: Vec::with_capacity(value.board.snakes.len()),
             constrictor: value.game.ruleset.name == "constrictor",
         };