Include an unused list of DynamicTile16 useful for reducing vram usage (#1136)

We don't free vram until after vblank, so if we end up generating a
bunch of unused DynamicTiles, then even though we're going to free them
eventually, it's best to not use too much as it'll limit the amount of
rendering we can do in one go.

After:
<img width="290" height="180" alt="image"
src="https://github.com/user-attachments/assets/761f752c-fdb1-4a77-a960-e65c75d2ef5d"
/>

Before:
<img width="290" height="180" alt="image"
src="https://github.com/user-attachments/assets/6be10151-2bfb-4565-9870-5094816aeece"
/>

The actual number of used tiles is the same, but now we're using fewer
while doing the actual rendering.

- [x] no changelog update needed

Author: gwilymk

examples/dynamic-isometric/src/isometric_render.rs

@@ -65,7 +65,7 @@ use agb::{
     hash_map::{HashMap, HashSet},
 };
 use alloc::{rc::Rc, vec::Vec};
-use core::{array, hash::Hash, ops::Deref};
+use core::{hash::Hash, ops::Deref};
 
 use crate::tiles;
 
@@ -88,11 +88,23 @@ pub enum TileType {
 pub struct TileCache {
     cache: HashMap<TileSpec, [TileHolder; 2]>,
     tiles: HashSet<TileHolder>,
+    /// We hold on to `unused` tiles here which are ones we rendered onto but then didn't end up
+    /// needing because they are already in the cache. Because dynamic tiles keep using vram until
+    /// the next vblank, we end up using more vram then actually necessary. So if we don't actually
+    /// use the tile, we put it here and then render the next one on it. This doesn't reduce the size
+    /// of the `tiles` cache, but it _does_ reduce the amount of vram we're wasting between vblank.
+    unused: Vec<DynamicTile16>,
 }
 
 #[derive(Clone)]
 pub struct TileHolder(Rc<DynamicTile16>);
 
+impl TileHolder {
+    fn into_inner(self) -> Option<DynamicTile16> {
+        Rc::into_inner(self.0)
+    }
+}
+
 impl Hash for TileHolder {
     fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
         self.0.data().hash(state);
@@ -138,11 +150,20 @@ impl TileCache {
         let tile_spec = map.get_from_gba_tile(gba_tile_pos.x, gba_tile_pos.y);
 
         self.cache.entry(tile_spec).or_insert_with(|| {
-            let genned_tiles = build_combined_tile(tile_spec);
+            let render_targets = [
+                self.unused.pop().unwrap_or_default(),
+                self.unused.pop().unwrap_or_default(),
+            ];
+
+            let genned_tiles = build_combined_tile(tile_spec, render_targets);
 
             genned_tiles.map(|genned_tile| {
                 let tile_holder = TileHolder(Rc::new(genned_tile));
                 if let Some(existing_tile) = self.tiles.get(&tile_holder) {
+                    if let Some(unwrapped_tile) = tile_holder.into_inner() {
+                        self.unused.push(unwrapped_tile);
+                    }
+
                     existing_tile.clone()
                 } else {
                     self.tiles.insert(tile_holder.clone());
@@ -157,7 +178,10 @@ impl TileCache {
     }
 }
 
-fn build_combined_tile(tile_spec: TileSpec) -> [DynamicTile16; 2] {
+fn build_combined_tile(
+    tile_spec: TileSpec,
+    render_targets: [DynamicTile16; 2],
+) -> [DynamicTile16; 2] {
     let TileSpec {
         quadrant,
         me,
@@ -209,14 +233,13 @@ fn build_combined_tile(tile_spec: TileSpec) -> [DynamicTile16; 2] {
         }
     };
 
-    array::from_fn(|i| {
-        let i = i as u16;
-
+    let mut i = 0;
+    render_targets.map(|tile| {
         fn get_tile(offset: u16) -> &'static [u32] {
             tiles::ISOMETRIC.tiles.get_tile_data(offset)
         }
 
-        let mut tile = DynamicTile16::new().fill_with(0);
+        let mut tile = tile.fill_with(0);
 
         let me_tile = get_tile(get_tile_id(quadrant.offset(), me, false) + i);
         let them_tile = get_tile(get_tile_id(quadrant.reverse().offset(), them, false) + i);
@@ -240,6 +263,8 @@ fn build_combined_tile(tile_spec: TileSpec) -> [DynamicTile16; 2] {
         blit_16_colour(tile.data_mut(), first);
         blit_16_colour(tile.data_mut(), second);
 
+        i += 1;
+
         tile
     })
 }