2 files changed, 87 insertions, 2 deletions

diff --git a/src/sim/materials/gas.rs b/src/sim/materials/gas.rs
new file mode 100644
index 0000000..8d9f42b
--- /dev/null
+++ b/src/sim/materials/gas.rs
@@ -0,0 +1,76 @@
+use crate::sim::sim::UpdateCtx;
+
+#[inline]
+pub fn sim_update(ctx: &mut UpdateCtx) {
+    // if the water can fall, do so
+    if ctx.candidates_swap(&[
+        (0, -1),
+        (-1 + 2 * ctx.seqno_parity as i32, -1),
+        (1 - 2 * ctx.seqno_parity as i32, -1),
+    ]) {
+        return;
+    }
+
+    // if the water can't fall, check if we can move left or right
+    // these are inverted on parity so that we don't preference a direction
+    let left_target = ctx.get_cell(-1, 0);
+    let can_move_left =
+        left_target.is_some_and(|c| c.material.def().density < ctx.material.density);
+    let right_target = ctx.get_cell(1, 0);
+    let can_move_right =
+        right_target.is_some_and(|c| c.material.def().density < ctx.material.density);
+
+    // we can't move down or to other side, so we're stuck
+    if !can_move_left && !can_move_right {
+        return;
+    }
+
+    // if we can't move left, just move right
+    if !can_move_left {
+        ctx.candidates_swap(&[(1, 0)]);
+        return;
+    }
+    // and vice versa
+    if !can_move_right {
+        ctx.candidates_swap(&[(-1, 0)]);
+        return;
+    }
+
+    // find the closest hole within 20 pixels (TODO optimize)
+    // a hole is any space below us with a lesser density
+    // prevents equidistance stuck state
+    let starting_side = if ctx.seqno_parity == 0 { 1 } else { -1 };
+    for i in 0..20 {
+        let side = if i % 2 == 0 {
+            starting_side
+        } else {
+            -starting_side
+        };
+
+        let offset = side * (1 + i / 2);
+        let hole_target = ctx.get_cell(offset, -1);
+        if let Some(target) = hole_target
+            && target.material.def().density < ctx.material.density
+        {
+            // we identified a hole and we know that the space on this side is open
+            // move toward the hole
+            // new_target.flags = new_target.flags ^ 0b1;
+            ctx.candidates_swap(&[(side, 0)]);
+            return;
+        }
+    }
+
+    // we didn't find a hole, so just move "randomly" on the same surface
+    // TODO when to settle?
+    let target_x = if !can_move_left {
+        1
+    } else if !can_move_right {
+        -1
+    } else if ctx.seqno_parity % 2 == 1 {
+        1
+    } else {
+        -1
+    };
+
+    ctx.candidates_swap(&[(target_x, 0)]);
+}
diff --git a/src/sim/materials/mod.rs b/src/sim/materials/mod.rs
index 86ae7d5..0cba5ff 100644
--- a/src/sim/materials/mod.rs
+++ b/src/sim/materials/mod.rs
@@ -1,5 +1,6 @@
 use crate::sim::sim::UpdateCtx;
 
+mod gas;
 mod sand;
 mod water;
 
@@ -10,6 +11,7 @@ pub enum MaterialId {
     Sand,
     Wood,
     Water,
+    Gas,
 }
 
 pub struct MaterialDef {
@@ -19,7 +21,7 @@ pub struct MaterialDef {
     pub sim_update: Option<fn(ctx: &mut UpdateCtx) -> ()>,
 }
 
-static MATERIALS: [MaterialDef; 4] = [
+static MATERIALS: [MaterialDef; 5] = [
     MaterialDef {
         name: "Void",
         color: (0x00, 0x00, 0x00, 0xFF),
@@ -44,14 +46,21 @@ static MATERIALS: [MaterialDef; 4] = [
         density: 40,
         sim_update: Some(water::sim_update),
     },
+    MaterialDef {
+        name: "Gas",
+        color: (0xBD, 0xFF, 0xE4, 0xAA),
+        density: 10,
+        sim_update: Some(gas::sim_update),
+    },
 ];
 
 impl MaterialId {
-    pub const ALL: [MaterialId; 4] = [
+    pub const ALL: [MaterialId; 5] = [
         MaterialId::Void,
         MaterialId::Sand,
         MaterialId::Wood,
         MaterialId::Water,
+        MaterialId::Gas,
     ];
     #[inline]
     pub fn def(self) -> &'static MaterialDef {