From 26d3ce9a0ed25ed3f3cb38beb8cbd6f0774f4a2a Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Wed, 7 Dec 2022 00:08:03 +0000
Subject: [PATCH] add basic heat transfer #4

---
 half-earth/materials/tile.tres   |  4 +-
 half-earth/nodes/game.tscn       |  1 +
 half-earth/scripts/GridCursor.cs |  6 +--
 half-earth/scripts/WorldGrid.cs  | 69 +++++++++++++++++++++++++++-----
 4 files changed, 66 insertions(+), 14 deletions(-)

diff --git a/half-earth/materials/tile.tres b/half-earth/materials/tile.tres
index 9f921a3..5d91ee6 100644
--- a/half-earth/materials/tile.tres
+++ b/half-earth/materials/tile.tres
@@ -5,6 +5,6 @@
 [resource]
 resource_local_to_scene = true
 shader = ExtResource( 1 )
-shader_param/lowColor = Color( 0.0313726, 0, 1, 1 )
-shader_param/highColor = Color( 1, 0, 0, 1 )
+shader_param/lowColor = Color( 0, 0, 0, 1 )
+shader_param/highColor = Color( 1, 0.560784, 0, 1 )
 shader_param/t = null
diff --git a/half-earth/nodes/game.tscn b/half-earth/nodes/game.tscn
index 1cf51a8..a23d717 100644
--- a/half-earth/nodes/game.tscn
+++ b/half-earth/nodes/game.tscn
@@ -16,4 +16,5 @@ Grid = NodePath("../Grid")
 
 [node name="Clock" parent="." instance=ExtResource( 4 )]
 
+[connection signal="OnTick" from="Clock" to="Grid" method="_on_Clock_OnTick"]
 [connection signal="OnTick" from="Clock" to="Debug UI" method="_on_Clock_OnTick"]
diff --git a/half-earth/scripts/GridCursor.cs b/half-earth/scripts/GridCursor.cs
index 6ff61a0..c2f1631 100644
--- a/half-earth/scripts/GridCursor.cs
+++ b/half-earth/scripts/GridCursor.cs
@@ -15,11 +15,11 @@ public class GridCursor : Sprite
         _grid = GetNode<WorldGrid>(Grid);
         _material = (ShaderMaterial)Material;
     }
-    
+
     public override void _Input(InputEvent @event)
     {
         base._Input(@event);
-        
+
         if (@event is InputEventMouseMotion mouseMoveEvent)
         {
             var pos = mouseMoveEvent.Position;
@@ -37,7 +37,7 @@ public class GridCursor : Sprite
                     {
                         var pos = mouseButtonEvent.Position;
                         _grid.GetGridPos(pos, out var x, out var y);
-                        _grid.ToggleTileHighlight(x, y);
+                        _grid.SetTileValue(x, y, 1.0f);
                         _material.SetShaderParam(T, 1f);
                     }
                     else
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index 8a69681..d5a8755 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -8,11 +8,15 @@ public class WorldGrid : Node2D
 
     [Export]
     public int Size { get; set; }
-    
+
     [Export]
     public int CellSize { get; set; }
-    
+
+    [Export]
+    public float DiffusionCoefficient { get; set; } = 0.01f;
+
     private Tile[,] _tiles;
+    private float[,] _nextValues = null;
 
     // Called when the node enters the scene tree for the first time.
     public override void _Ready()
@@ -23,18 +27,18 @@ public class WorldGrid : Node2D
     public override void _Process(float delta)
     {
         base._Process(delta);
-        
+
         for (int x = 0; x < Size; x++)
         {
             for (int y = 0; y < Size; y++)
             {
                 var tile = _tiles[x, y];
                 var material = tile.material;
-                material.SetShaderParam("t", tile.isHighlighted ? 1f : 0f);
+                material.SetShaderParam("t", tile.value);
             }
         }
     }
-    
+
     public bool IsInBounds(int x, int y)
     {
         return x >= 0 && x < Size && y >= 0 && y < Size;
@@ -47,7 +51,7 @@ public class WorldGrid : Node2D
 
         _tiles[x, y].isHighlighted ^= true;
     }
-    
+
     public void SetTileValue(int x, int y, float value)
     {
         _tiles[x, y].value = value;
@@ -75,8 +79,9 @@ public class WorldGrid : Node2D
     private void GenerateGrid(int size)
     {
         this.Clear();
-        
+
         _tiles = new Tile[size, size];
+        _nextValues = new float[size, size];
 
         for (int x = 0; x < size; x++)
         {
@@ -85,7 +90,7 @@ public class WorldGrid : Node2D
                 var tile = new Tile();
                 tile.isHighlighted = false;
                 tile.value = 0.0f;
-                
+
                 var node = TileScene.Instance<Node2D>();
                 this.AddChild(node);
                 var position = new Vector2
@@ -96,9 +101,55 @@ public class WorldGrid : Node2D
                 node.Position = position;
                 var canvasItem = (CanvasItem)node;
                 tile.material = (ShaderMaterial)canvasItem.Material;
-                
+
                 _tiles[x, y] = tile;
             }
         }
     }
+
+    public void _on_Clock_OnTick(int ticks)
+    {
+        Diffuse();
+    }
+
+    private float TransferHeat(float t0, float alpha, float nx, float ny, float px, float py)
+    {
+        float d2tdx2 = nx - 2 * t0 + px;
+        float d2tdy2 = ny - 2 * t0 + py;
+        return t0 + alpha * (d2tdx2 + d2tdy2);
+    }
+
+    private void GetNeighbourTemperatures(int x, int y, out float nx, out float ny, out float px, out float py)
+    {
+        // default value
+        var t = _tiles[x, y].value;
+
+        nx = x > 0 ? _tiles[x - 1, y].value : t;
+        px = x < Size - 1 ? _tiles[x + 1, y].value : t;
+
+        ny = y > 0 ? _tiles[x, y - 1].value : t;
+        py = y < Size - 1 ? _tiles[x, y + 1].value : t;
+    }
+
+    private void Diffuse()
+    {
+        for (int x = 0; x < Size; x++)
+        {
+            for (int y = 0; y < Size; y++)
+            {
+                float t = _tiles[x, y].value;
+                var D = DiffusionCoefficient;
+
+                GetNeighbourTemperatures(
+                    x, y,
+                    out var nx,
+                    out var ny,
+                    out var px,
+                    out var py);
+
+                // current value
+                _nextValues[x, y] = TransferHeat(t, D, nx, ny, px, py);
+            }
+        }
+    }
 }