From 533da47f2cc03a8b60a7c6c0bb5a938a7e523a03 Mon Sep 17 00:00:00 2001
From: Kai Stevenson <kai@kaistevenson.com>
Date: Thu, 20 Aug 2026 23:20:21 -0700
Subject: particles

---
 src/sim/cell_sim/chunk.rs        |   5 +-
 src/sim/mod.rs                   |   6 +--
 src/sim/particle_sim/mod.rs      | 101 +++++++++++++++++++++++++++++++++++++++
 src/sim/particle_sim/particle.rs |  21 ++++++++
 4 files changed, 125 insertions(+), 8 deletions(-)
 create mode 100644 src/sim/particle_sim/mod.rs
 create mode 100644 src/sim/particle_sim/particle.rs

(limited to 'src/sim')

diff --git a/src/sim/cell_sim/chunk.rs b/src/sim/cell_sim/chunk.rs
index 85d2c09..3fe5056 100644
--- a/src/sim/cell_sim/chunk.rs
+++ b/src/sim/cell_sim/chunk.rs
@@ -1,10 +1,7 @@
 use crate::{
     config::{CELLS_IN_CHUNK, CHUNK_SIZE},
     sim::{
-        cell::{
-            cell::Cell,
-            materials::{MaterialForm, MaterialId},
-        },
+        cell::{cell::Cell, materials::MaterialForm},
         lib::marching_squares::Marchable,
     },
 };
diff --git a/src/sim/mod.rs b/src/sim/mod.rs
index c73a373..d9e358b 100644
--- a/src/sim/mod.rs
+++ b/src/sim/mod.rs
@@ -1,11 +1,9 @@
-use crate::{
-    config::CHUNK_SIZE,
-    sim::{cell::materials::MaterialId, cell_sim::world::World, rb_sim::RbSimManager},
-};
+use crate::sim::{cell::materials::MaterialId, cell_sim::world::World, rb_sim::RbSimManager};
 
 pub mod cell;
 pub mod cell_sim;
 pub mod lib;
+pub mod particle_sim;
 pub mod rb_sim;
 
 pub fn write_rb_entity_to_world(
diff --git a/src/sim/particle_sim/mod.rs b/src/sim/particle_sim/mod.rs
new file mode 100644
index 0000000..8bad61d
--- /dev/null
+++ b/src/sim/particle_sim/mod.rs
@@ -0,0 +1,101 @@
+use glam::{IVec2, Vec2};
+
+use crate::{
+    config::PIXELS_TO_METRES,
+    sim::{
+        cell::{cell::Cell, materials::MaterialId},
+        cell_sim::world::World,
+        particle_sim::particle::Particle,
+    },
+};
+
+pub mod particle;
+
+pub struct ParticleManager {
+    pub particles: Vec<Particle>,
+}
+
+const PARTICLE_GRAVITY: f32 = 9.81 * PIXELS_TO_METRES;
+
+impl ParticleManager {
+    pub fn particle_tick(&mut self, world: &mut World, delta_time: f32) {
+        let mut i = 0;
+        'outer: while i < self.particles.len() {
+            let p = &mut self.particles[i];
+
+            p.life -= delta_time;
+            if p.life <= 0.0 {
+                self.particles.swap_remove(i);
+                continue 'outer;
+            }
+
+            p.velocity.y += PARTICLE_GRAVITY * delta_time;
+            let dt_velocity = p.velocity * delta_time;
+
+            // Amanatides and Woo's fast Voxel Traversal
+            {
+                let mut cur = IVec2::new(p.position.x.round() as i32, p.position.y.round() as i32);
+                // direction we step for each component on each iteration
+                let step_sign =
+                    IVec2::new(dt_velocity.x.signum() as i32, dt_velocity.y.signum() as i32);
+                let delta = Vec2::new(1.0 / dt_velocity.x.abs(), 1.0 / dt_velocity.y.abs());
+
+                let frac = Vec2::new(
+                    if step_sign.x > 0 {
+                        (cur.x as f32 + 0.5) - p.position.x
+                    } else {
+                        p.position.x - (cur.x as f32 - 0.5)
+                    },
+                    if step_sign.y > 0 {
+                        (cur.y as f32 + 0.5) - p.position.y
+                    } else {
+                        p.position.y - (cur.y as f32 - 0.5)
+                    },
+                );
+                let mut t_max = frac * delta;
+
+                let mut prev = cur;
+                if let Some(cell) = world.get_cell_from_game_position(prev.x, prev.y)
+                    && cell.material != MaterialId::Void
+                {
+                    // already occupied, die
+                    self.particles.swap_remove(i);
+                    continue 'outer;
+                }
+                while t_max.min_element() <= 1.0 {
+                    if t_max.x < t_max.y {
+                        cur.x += step_sign.x;
+                        t_max.x += delta.x;
+                    } else {
+                        cur.y += step_sign.y;
+                        t_max.y += delta.y;
+                    }
+                    if let Some(cell) = world.get_cell_from_game_position(cur.x, cur.y)
+                        && cell.material != MaterialId::Void
+                    {
+                        // write ourselves to the board
+                        world.set_cell_from_game_position(
+                            prev.x,
+                            prev.y,
+                            Cell::from_material(p.material),
+                            false,
+                        );
+                        self.particles.swap_remove(i);
+                        continue 'outer;
+                    }
+                    prev = cur;
+                }
+            }
+
+            // no collision, just move
+            p.position = p.position + dt_velocity;
+            i += 1;
+        }
+    }
+
+    pub fn new() -> Self {
+        ParticleManager {
+            particles: Vec::new(),
+        }
+    }
+}
diff --git a/src/sim/particle_sim/particle.rs b/src/sim/particle_sim/particle.rs
new file mode 100644
index 0000000..cc7dd28
--- /dev/null
+++ b/src/sim/particle_sim/particle.rs
@@ -0,0 +1,21 @@
+use glam::Vec2;
+
+use crate::sim::cell::materials::MaterialId;
+
+pub struct Particle {
+    pub position: Vec2,
+    pub velocity: Vec2,
+    pub material: MaterialId,
+    pub life: f32,
+}
+
+impl Particle {
+    pub fn new(position: Vec2, velocity: Vec2, material: MaterialId, life: f32) -> Self {
+        Particle {
+            position,
+            velocity,
+            material,
+            life,
+        }
+    }
+}
-- 
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