From 280ef93cac6aa6dd24b981614157e7cf94b5fe77 Mon Sep 17 00:00:00 2001
From: Kevin Reid <kpreid@switchb.org>
Date: Wed, 25 Oct 2023 11:42:36 -0700
Subject: [PATCH] Execute lighting calculations in parallel.

This slightly changes the results due to batching (updates in a batch
don't see other updates in the same batch), but does not introduce
nondeterminism.

Note that this is a performance improvement, but what we should really
be doing is running light calculations in the background all the time,
which will require more work to make the data available for use.
---
 .../test-record-lighting-bench.png            | Bin 3662 -> 3670 bytes
 all-is-cubes/src/space/light/updater.rs       |  44 +++++++++++++++++-
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/all-is-cubes-desktop/tests/end-to-end/record-ok-png.out/test-record-lighting-bench.png b/all-is-cubes-desktop/tests/end-to-end/record-ok-png.out/test-record-lighting-bench.png
index ef58bbfcd507bca49f25dbe9e7fa0683a0b59529..9d3e938b2bd9d28ba46a68e763652187ddb15777 100644
GIT binary patch
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diff --git a/all-is-cubes/src/space/light/updater.rs b/all-is-cubes/src/space/light/updater.rs
index 19604daf5..a5cc369e9 100644
--- a/all-is-cubes/src/space/light/updater.rs
+++ b/all-is-cubes/src/space/light/updater.rs
@@ -9,6 +9,9 @@ use core::fmt;
 use euclid::{Point3D, Vector3D};
 use manyfmt::Fmt;
 
+#[cfg(feature = "threads")]
+use rayon::iter::{IntoParallelIterator as _, ParallelIterator as _};
+
 use super::debug::LightComputeOutput;
 use super::LightUpdateRequest;
 use crate::block::EvaluatedBlock;
@@ -79,6 +82,40 @@ impl Space {
                 None => usize::MAX,
             };
 
+            // TODO: More efficient threading. Instead of creating batches up front,
+            // run a continuous pipeline of calculations in the background and only apply
+            // them when we have this current opportunity.
+            // This will require making stored light data `Arc`ed and double-buffered or atomic,
+            // so it can be consulted by the calculation.
+            #[cfg(feature = "threads")]
+            while self.light_update_queue.len() > 0 {
+                // TODO: empirical tuning suggests that 128 is a good minimum batch size,
+                // but is too big for the amount of time we want to take
+                let some_updates: [Option<LightUpdateRequest>; 32] =
+                    [None::<LightUpdateRequest>; 32].map(|_| self.light_update_queue.pop());
+                let outputs = some_updates
+                    .as_slice()
+                    .into_par_iter()
+                    .flatten()
+                    .map(|&LightUpdateRequest { cube, .. }| self.compute_lighting(cube))
+                    .collect::<Vec<ComputedLight<()>>>();
+                for output in outputs {
+                    if false {
+                        // Log cubes that were updated for debug visualization.
+                        self.last_light_updates.push(output.cube);
+                    }
+                    light_update_count += 1;
+                    let (difference, cube_cost) = self.apply_lighting_update(output);
+                    max_difference = max_difference.max(difference);
+                    cost += cube_cost;
+                }
+
+                if cost >= max_cost {
+                    break;
+                }
+            }
+
+            #[cfg(not(feature = "threads"))]
             while let Some(LightUpdateRequest { cube, .. }) = self.light_update_queue.pop() {
                 if false {
                     // Log cubes that were updated for debug visualization.
@@ -88,7 +125,7 @@ impl Space {
 
                 let computation = self.compute_lighting(cube);
 
-                let (difference, cube_cost) = self.apply_lighting_update(cube, computation);
+                let (difference, cube_cost) = self.apply_lighting_update(computation);
                 max_difference = max_difference.max(difference);
                 cost += cube_cost;
                 if cost >= max_cost {
@@ -117,10 +154,10 @@ impl Space {
     #[inline]
     fn apply_lighting_update(
         &mut self,
-        cube: Cube,
         computation: ComputedLight<()>,
     ) -> (PackedLightScalar, usize) {
         let ComputedLight {
+            cube,
             light: new_light_value,
             dependencies,
             mut cost,
@@ -221,6 +258,7 @@ impl Space {
         let new_light_value = cube_buffer.finish(origin_is_opaque);
 
         ComputedLight {
+            cube,
             light: new_light_value,
             dependencies: cube_buffer.dependencies,
             cost: cube_buffer.cost,
@@ -531,6 +569,8 @@ impl LightBuffer {
 #[derive(Clone, Debug)]
 #[doc(hidden)] // used for debug rendering
 pub struct ComputedLight<D> {
+    pub cube: Cube,
+
     pub light: PackedLight,
     /// Cubes which the computed value depends on (imprecisely; empty cubes passed through
     /// are not listed).