From d7eccba2055cd7784a0db6d9ee8692a4f3510931 Mon Sep 17 00:00:00 2001
From: Kai Stevenson <kai@kaistevenson.com>
Date: Sat, 15 Aug 2026 22:46:53 -0700
Subject: fire, optimize viewport rendering

---
 src/sim/sim.rs | 71 +++++++++++++++++++++++++++++++---------------------------
 1 file changed, 38 insertions(+), 33 deletions(-)

(limited to 'src/sim/sim.rs')

diff --git a/src/sim/sim.rs b/src/sim/sim.rs
index 4c6d908..84c3126 100644
--- a/src/sim/sim.rs
+++ b/src/sim/sim.rs
@@ -1,6 +1,7 @@
 use std::marker::PhantomData;
 
 use fxhash::FxHashMap;
+use rand::{Rng, SeedableRng, rngs::SmallRng};
 use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
 
 use crate::{
@@ -30,17 +31,6 @@ impl<'a> ChunkAccess<'a> {
 
 unsafe impl Sync for ChunkAccess<'_> {}
 
-pub struct UpdateCtx<'a, 'b, 'c> {
-    pub chunks: &'a mut [Option<&'b mut Chunk>; 9],
-    pub seqno: u64,
-    pub seqno_parity: u8,
-
-    pub x: i32,
-    pub y: i32,
-    pub cell: &'c mut Cell,
-    pub material: &'c MaterialDef,
-}
-
 fn get_cell(chunks: &[Option<&mut Chunk>; 9], x: i32, y: i32) -> Option<Cell> {
     let dcx = x.div_euclid(CHUNK_SIZE);
     let dcy = y.div_euclid(CHUNK_SIZE);
@@ -74,17 +64,28 @@ pub fn set_cell(chunks: &mut [Option<&mut Chunk>; 9], x: i32, y: i32, cell: Cell
         if let Some(chunk) = &mut chunks[(dcx + 1 + (dcy + 1) * 3) as usize] {
             chunk.set_cell_at_local_position(nc_x, nc_y, cell);
             chunk.needs_texture_update = true;
-            chunk.sleeping = false;
         }
     } else {
         if let Some(target) = &mut chunks[4] {
             target.set_cell_at_local_position(x as u8, y as u8, cell);
             target.needs_texture_update = true;
-            target.sleeping = false;
         }
     }
 }
 
+pub struct UpdateCtx<'a, 'b, 'c> {
+    pub chunks: &'a mut [Option<&'b mut Chunk>; 9],
+    pub seqno: u64,
+    pub seqno_parity: u8,
+
+    pub x: i32,
+    pub y: i32,
+    pub cell: &'c mut Cell,
+    pub material: &'c MaterialDef,
+
+    pub rng: &'c mut dyn Rng,
+}
+
 impl UpdateCtx<'_, '_, '_> {
     pub fn get_cell(&self, dx: i32, dy: i32) -> Option<Cell> {
         let x = self.x + dx;
@@ -92,7 +93,7 @@ impl UpdateCtx<'_, '_, '_> {
         get_cell(self.chunks, x, y)
     }
 
-    fn set_cell(&mut self, dx: i32, dy: i32, cell: Cell) {
+    pub fn set_cell(&mut self, dx: i32, dy: i32, cell: Cell) {
         // cannot move out of the neighbourhood, but also cannot move to the edge of the neighbourhood
         // as this would wake a chunk outside of the neighbourhood
         debug_assert!(dx > -15 && dx < 15);
@@ -122,8 +123,8 @@ impl UpdateCtx<'_, '_, '_> {
 
 pub fn sim_tick_chunk(chunks: &mut [Option<&mut Chunk>; 9], seqno: u64) {
     puffin::profile_function!();
-    // scan bottom to top to enable contiguous falling
     let seqno_parity = (seqno as u8) & 0b1;
+    let mut rng = SmallRng::seed_from_u64(seqno);
 
     if chunks[4].is_some() {
         for y in (0..CHUNK_SIZE).rev() {
@@ -135,24 +136,28 @@ pub fn sim_tick_chunk(chunks: &mut [Option<&mut Chunk>; 9], seqno: u64) {
                 };
 
                 let mut cell = get_cell(chunks, x, y).unwrap();
-                if cell.flags & 0b1 == seqno_parity {
-                    // flip the parity bit
-                    // TODO if the cell doesn't move this doesn't stay
-                    cell.flags = cell.flags ^ 0b1;
-                    let material = cell.material.def();
-
-                    let mut update_ctx = UpdateCtx {
-                        chunks,
-                        seqno,
-                        seqno_parity,
-
-                        x,
-                        y,
-                        cell: &mut cell,
-                        material,
-                    };
-
-                    if let Some(update) = material.sim_update {
+                let material = cell.material.def();
+
+                if let Some(update) = material.sim_update {
+                    if cell.parity() == seqno_parity {
+                        cell.flip_parity();
+                        // apply the flipped parity in case the sim target doesn't
+                        set_cell(chunks, x, y, cell);
+
+                        let mut update_ctx = UpdateCtx {
+                            chunks,
+                            seqno,
+                            seqno_parity,
+
+                            x,
+                            y,
+                            cell: &mut cell,
+                            material,
+
+                            // TODO this is platform-dependent, will break for multiplayer
+                            rng: &mut rng,
+                        };
+
                         update(&mut update_ctx);
                     }
                 }
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