1 files changed, 78 insertions, 0 deletions
diff --git a/src/sim/materials/water.rs b/src/sim/materials/water.rs
new file mode 100644
index 0000000..d875aee
--- /dev/null
+++ b/src/sim/materials/water.rs
@@ -0,0 +1,78 @@
+use crate::sim::sim::UpdateCtx;
+
+#[inline]
+pub fn sim_update(ctx: &mut UpdateCtx) {
+    // if the water can fall, do so
+    if ctx.candidates_swap(&[
+        (ctx.self_x, ctx.self_y + 1),
+        (ctx.self_x - 1 + 2 * ctx.seqno_parity as i32, ctx.self_y + 1),
+        (ctx.self_x + 1 - 2 * ctx.seqno_parity as i32, ctx.self_y + 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.board.cell_at_position(ctx.self_x - 1, ctx.self_y);
+    let can_move_left = left_target
+        .is_some_and(|c| c.material.def().density < ctx.self_cell.material.def().density);
+    let right_target = ctx.board.cell_at_position(ctx.self_x + 1, ctx.self_y);
+    let can_move_right = right_target
+        .is_some_and(|c| c.material.def().density < ctx.self_cell.material.def().density);
+
+    // we can't move down or to other side, so we're stuck
+    if !can_move_left && !can_move_right {
+        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
+        };
+        if (side == 1 && !can_move_right) || (side == -1 && !can_move_left) {
+            continue;
+        }
+
+        let offset = side * (1 + i / 2);
+        let hole_target = ctx
+            .board
+            .cell_at_position(ctx.self_x + offset, ctx.self_y + 1);
+        if let Some(target) = hole_target
+            && target.material.def().density < ctx.self_cell.material.def().density
+        {
+            // we identified a hole and we know that the space on this side is open
+            // move toward the hole
+            let move_target = if side == 1 { right_target } else { left_target }.clone();
+            // new_target.flags = new_target.flags ^ 0b1;
+            // safe to unwrap
+            ctx.board
+                .set_cell_at_position(ctx.self_x, ctx.self_y, move_target.unwrap());
+            ctx.board
+                .set_cell_at_position(ctx.self_x + side, ctx.self_y, ctx.self_cell);
+            return;
+        }
+    }
+
+    // we didn't find a hole, so just move "randomly" on the same surface
+    // TODO when to settle?
+    let (target, target_x) = if !can_move_left {
+        (right_target, 1)
+    } else if !can_move_right {
+        (left_target, -1)
+    } else if ctx.seqno_parity % 2 == 1 {
+        (right_target, 1)
+    } else {
+        (left_target, -1)
+    };
+
+    ctx.board
+        .set_cell_at_position(ctx.self_x, ctx.self_y, target.unwrap());
+    ctx.board
+        .set_cell_at_position(ctx.self_x + target_x, ctx.self_y, ctx.self_cell);
+}