Store the used backgrounds in a bit array

agb/src/bitarray.rs

@@ -1,3 +1,4 @@
+#[derive(Debug)]
 pub struct Bitarray<const N: usize> {
     a: [u32; N],
 }
@@ -21,7 +22,26 @@ impl<const N: usize> Bitarray<N> {
         let value_mask = value << (index % 32);
         self.a[index / 32] = self.a[index / 32] & !mask | value_mask
     }
+
+    pub fn first_zero(&self) -> Option<usize> {
+        for index in 0..N * 32 {
+            if let Some(bit) = self.get(index) {
+                if !bit {
+                    return Some(index);
+                }
+            }
+        }
+
+        None
+    }
 }
+
+impl<const N: usize> Default for Bitarray<N> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

agb/src/display/background.rs

@@ -1,7 +1,10 @@
+use core::cell::RefCell;
+
 use alloc::vec::Vec;
 use alloc::{boxed::Box, vec};
 use hashbrown::HashMap;
 
+use crate::bitarray::Bitarray;
 use crate::{
     display,
     fixnum::{Rect, Vector2D},
@@ -611,8 +614,7 @@ fn div_ceil(x: i32, y: i32) -> i32 {
 }
 
 pub struct Tiled0<'a> {
-    num_regular: u8,
+    regular: RefCell<Bitarray<1>>,
-    next_screenblock: u8,
 
     pub vram: VRamManager<'a>,
 }
@@ -623,22 +625,22 @@ impl Tiled0<'_> {
         set_graphics_mode(DisplayMode::Tiled0);
 
         Self {
-            num_regular: 0,
+            regular: Default::default(),
-            next_screenblock: 16,
 
             vram: VRamManager::new(),
         }
     }
 
     pub fn background(&mut self, priority: Priority) -> RegularMap {
-        if self.num_regular == 4 {
+        let mut regular = self.regular.borrow_mut();
-            panic!("Can only create 4 backgrounds");
+        let new_background = regular.first_zero().unwrap();
+        if new_background >= 4 {
+            panic!("can only have 4 active backgrounds");
         }
 
-        let bg = RegularMap::new(self.num_regular, self.next_screenblock, priority);
+        let bg = RegularMap::new(new_background as u8, (new_background + 16) as u8, priority);
 
-        self.num_regular += 1;
+        regular.set(new_background, true);
-        self.next_screenblock += 1;
 
         bg
     }

examples/the-hat-chooses-the-wizard/Cargo.lock

@@ -24,6 +24,7 @@ dependencies = [
  "agb_sound_converter",
  "bare-metal",
  "bitflags",
+ "hashbrown",
 ]
 
 [[package]]
@@ -65,6 +66,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "ahash"
+version = "0.7.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fcb51a0695d8f838b1ee009b3fbf66bda078cd64590202a864a8f3e8c4315c47"
+dependencies = [
+ "getrandom",
+ "once_cell",
+ "version_check",
+]
+
 [[package]]
 name = "autocfg"
 version = "1.1.0"
@@ -77,6 +89,12 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8fe8f5a8a398345e52358e18ff07cc17a568fbca5c6f73873d3a62056309603"
 
+[[package]]
+name = "bare-metal"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8fe8f5a8a398345e52358e18ff07cc17a568fbca5c6f73873d3a62056309603"
+
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -136,6 +154,15 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8c21d40587b92fa6a6c6e3c1bdbf87d75511db5672f9c93175574b3a00df1758"
+dependencies = [
+ "ahash",
+]
+
 [[package]]
 name = "hound"
 version = "3.4.0"
@@ -219,6 +246,12 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "once_cell"
+version = "1.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "da32515d9f6e6e489d7bc9d84c71b060db7247dc035bbe44eac88cf87486d8d5"
+
 [[package]]
 name = "png"
 version = "0.17.4"
@@ -357,6 +390,12 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ccb82d61f80a663efe1f787a51b16b5a51e3314d6ac365b08639f52387b33f3"
 
+[[package]]
+name = "version_check"
+version = "0.9.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
+
 [[package]]
 name = "wasi"
 version = "0.10.2+wasi-snapshot-preview1"