From 944ca8c3f21a1eda669a88ce2c25ebcf71276c36 Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Tue, 6 Dec 2022 19:29:01 +0000
Subject: [PATCH 1/6] extract struct to file

---
 half-earth/scripts/Tile.cs      |  8 ++++++++
 half-earth/scripts/WorldGrid.cs | 11 ++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)
 create mode 100644 half-earth/scripts/Tile.cs

diff --git a/half-earth/scripts/Tile.cs b/half-earth/scripts/Tile.cs
new file mode 100644
index 0000000..f46e5ee
--- /dev/null
+++ b/half-earth/scripts/Tile.cs
@@ -0,0 +1,8 @@
+using Godot;
+
+public struct Tile
+{
+    public bool isHighlighted;
+    public float value;
+    public ShaderMaterial material;
+}
\ No newline at end of file
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index a59855d..7f55d6c 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -12,11 +12,6 @@ public class WorldGrid : Node2D
     [Export]
     public int CellSize { get; set; }
     
-    private struct Tile
-    {
-        public bool isHighlighted;
-        public ShaderMaterial material;
-    }
     private Tile[,] _tiles;
 
     // Called when the node enters the scene tree for the first time.
@@ -52,6 +47,11 @@ public class WorldGrid : Node2D
 
         _tiles[x, y].isHighlighted ^= true;
     }
+    
+    public void SetTileValue(int x, int y, float value)
+    {
+        _tiles[x, y].value = value;
+    }
 
     public void GetGridPos(Vector2 position, out int x, out int y)
     {
@@ -84,6 +84,7 @@ public class WorldGrid : Node2D
             {
                 var tile = new Tile();
                 tile.isHighlighted = false;
+                tile.value = 0.0f;
                 
                 var node = TileScene.Instance<Node2D>();
                 this.AddChild(node);
-- 
2.34.1


From e32cd257fd193ae6bc17131984e05da61c5004dc Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Tue, 6 Dec 2022 20:11:42 +0000
Subject: [PATCH 2/6] add tick clock and tick counter UI #4

---
 half-earth/nodes/clock.tscn    |  6 ++++++
 half-earth/nodes/debug_ui.tscn | 22 ++++++++++++++++++++++
 half-earth/nodes/game.tscn     | 12 ++++++++++--
 half-earth/scripts/Clock.cs    | 31 +++++++++++++++++++++++++++++++
 half-earth/scripts/DebugUI.cs  | 22 ++++++++++++++++++++++
 5 files changed, 91 insertions(+), 2 deletions(-)
 create mode 100644 half-earth/nodes/clock.tscn
 create mode 100644 half-earth/nodes/debug_ui.tscn
 create mode 100644 half-earth/scripts/Clock.cs
 create mode 100644 half-earth/scripts/DebugUI.cs

diff --git a/half-earth/nodes/clock.tscn b/half-earth/nodes/clock.tscn
new file mode 100644
index 0000000..7d4c9fe
--- /dev/null
+++ b/half-earth/nodes/clock.tscn
@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/Clock.cs" type="Script" id=1]
+
+[node name="Clock" type="Node"]
+script = ExtResource( 1 )
diff --git a/half-earth/nodes/debug_ui.tscn b/half-earth/nodes/debug_ui.tscn
new file mode 100644
index 0000000..1cf603b
--- /dev/null
+++ b/half-earth/nodes/debug_ui.tscn
@@ -0,0 +1,22 @@
+[gd_scene load_steps=2 format=2]
+
+[ext_resource path="res://scripts/DebugUI.cs" type="Script" id=1]
+
+[node name="Debug UI" type="Control"]
+anchor_right = 1.0
+anchor_bottom = 1.0
+script = ExtResource( 1 )
+TicksLabelPath = NodePath("Ticks")
+
+[node name="Ticks" type="Label" parent="."]
+anchor_left = 1.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+margin_right = -10.0
+margin_bottom = -10.0
+grow_horizontal = 0
+grow_vertical = 0
+text = "ticks: 0"
+align = 2
+valign = 2
diff --git a/half-earth/nodes/game.tscn b/half-earth/nodes/game.tscn
index c7bf9d9..1cf51a8 100644
--- a/half-earth/nodes/game.tscn
+++ b/half-earth/nodes/game.tscn
@@ -1,11 +1,19 @@
-[gd_scene load_steps=3 format=2]
+[gd_scene load_steps=5 format=2]
 
 [ext_resource path="res://nodes/grid.tscn" type="PackedScene" id=1]
 [ext_resource path="res://nodes/grid_cursor.tscn" type="PackedScene" id=2]
+[ext_resource path="res://nodes/debug_ui.tscn" type="PackedScene" id=3]
+[ext_resource path="res://nodes/clock.tscn" type="PackedScene" id=4]
 
-[node name="Root" type="Node2D"]
+[node name="Root" type="Node"]
 
 [node name="Grid" parent="." instance=ExtResource( 1 )]
 
 [node name="Cursor" parent="." instance=ExtResource( 2 )]
 Grid = NodePath("../Grid")
+
+[node name="Debug UI" parent="." instance=ExtResource( 3 )]
+
+[node name="Clock" parent="." instance=ExtResource( 4 )]
+
+[connection signal="OnTick" from="Clock" to="Debug UI" method="_on_Clock_OnTick"]
diff --git a/half-earth/scripts/Clock.cs b/half-earth/scripts/Clock.cs
new file mode 100644
index 0000000..9050073
--- /dev/null
+++ b/half-earth/scripts/Clock.cs
@@ -0,0 +1,31 @@
+using Godot;
+using System;
+
+public class Clock : Node
+{
+    [Signal]
+    delegate void OnTick(int ticks);
+    
+    private int _ticks = 0;
+
+    public override void _Input(InputEvent @event)
+    {
+        base._Input(@event);
+
+        if (@event is InputEventKey keyEvent && keyEvent.Pressed)
+        {
+            switch ((KeyList)keyEvent.Scancode)
+            {
+                case KeyList.Space:
+                    Tick();
+                    break;
+            }
+        }
+    }
+    
+    private void Tick()
+    {
+        _ticks++;
+        EmitSignal(nameof(OnTick), _ticks);
+    }
+}
diff --git a/half-earth/scripts/DebugUI.cs b/half-earth/scripts/DebugUI.cs
new file mode 100644
index 0000000..f8a8bdb
--- /dev/null
+++ b/half-earth/scripts/DebugUI.cs
@@ -0,0 +1,22 @@
+using Godot;
+using System;
+
+public class DebugUI : Control
+{
+    [Export]
+    public NodePath TicksLabelPath { private get; set; }
+    private Label _ticksLabel;
+    
+    public override void _Ready()
+    {
+        _ticksLabel = GetNode<Label>(TicksLabelPath);
+        SetTicksText(0);
+    }
+    
+    public void _on_Clock_OnTick(int ticks) => SetTicksText(ticks);
+    
+    private void SetTicksText(int ticks)
+    {
+        _ticksLabel.Text = $"ticks: {ticks}";
+    }
+}
-- 
2.34.1


From c013ab00571f1395974e826759bd6f5e2ca3f08a Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Tue, 6 Dec 2022 20:16:24 +0000
Subject: [PATCH 3/6] generalise shader #4

---
 half-earth/materials/cursor.tres        | 6 +++---
 half-earth/materials/tile.tres          | 6 +++---
 half-earth/scripts/GridCursor.cs        | 6 +++---
 half-earth/scripts/WorldGrid.cs         | 2 +-
 half-earth/shaders/tile_shader.gdshader | 8 ++++----
 5 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/half-earth/materials/cursor.tres b/half-earth/materials/cursor.tres
index a3218a5..91a714d 100644
--- a/half-earth/materials/cursor.tres
+++ b/half-earth/materials/cursor.tres
@@ -4,6 +4,6 @@
 
 [resource]
 shader = ExtResource( 1 )
-shader_param/baseColor = Color( 0.992157, 1, 0, 1 )
-shader_param/highlightColor = Color( 1, 0, 0.984314, 1 )
-shader_param/isHighlighted = 0
+shader_param/lowColor = Color( 1, 0.960784, 0, 1 )
+shader_param/highColor = Color( 0.921569, 0, 1, 1 )
+shader_param/t = null
diff --git a/half-earth/materials/tile.tres b/half-earth/materials/tile.tres
index 327c5eb..9f921a3 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/baseColor = Color( 0.0823529, 0, 1, 1 )
-shader_param/highlightColor = Color( 1, 0, 0, 1 )
-shader_param/isHighlighted = null
+shader_param/lowColor = Color( 0.0313726, 0, 1, 1 )
+shader_param/highColor = Color( 1, 0, 0, 1 )
+shader_param/t = null
diff --git a/half-earth/scripts/GridCursor.cs b/half-earth/scripts/GridCursor.cs
index fd97c42..6ff61a0 100644
--- a/half-earth/scripts/GridCursor.cs
+++ b/half-earth/scripts/GridCursor.cs
@@ -8,7 +8,7 @@ public class GridCursor : Sprite
     private WorldGrid _grid;
     private ShaderMaterial _material;
 
-    private const string IS_HIGHLIGHTED = "isHighlighted";
+    private const string T = "t";
 
     public override void _Ready()
     {
@@ -38,11 +38,11 @@ public class GridCursor : Sprite
                         var pos = mouseButtonEvent.Position;
                         _grid.GetGridPos(pos, out var x, out var y);
                         _grid.ToggleTileHighlight(x, y);
-                        _material.SetShaderParam(IS_HIGHLIGHTED, 1);
+                        _material.SetShaderParam(T, 1f);
                     }
                     else
                     {
-                        _material.SetShaderParam(IS_HIGHLIGHTED, 0);
+                        _material.SetShaderParam(T, 0f);
                     }
                     break;
             }
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index 7f55d6c..8a69681 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -30,7 +30,7 @@ public class WorldGrid : Node2D
             {
                 var tile = _tiles[x, y];
                 var material = tile.material;
-                material.SetShaderParam("isHighlighted", tile.isHighlighted ? 1 : 0);
+                material.SetShaderParam("t", tile.isHighlighted ? 1f : 0f);
             }
         }
     }
diff --git a/half-earth/shaders/tile_shader.gdshader b/half-earth/shaders/tile_shader.gdshader
index 98f8fe0..537a2b6 100644
--- a/half-earth/shaders/tile_shader.gdshader
+++ b/half-earth/shaders/tile_shader.gdshader
@@ -1,9 +1,9 @@
 shader_type canvas_item;
 
-uniform vec4 baseColor : hint_color;
-uniform vec4 highlightColor : hint_color;
-uniform int isHighlighted;
+uniform vec4 lowColor : hint_color;
+uniform vec4 highColor : hint_color;
+uniform float t;
 
 void fragment() {
-	COLOR = mix(baseColor, highlightColor, float(isHighlighted));
+	COLOR = mix(lowColor, highColor, t);
 }
-- 
2.34.1


From fa1449b412a1ac847a9ce4dd3c39d707c408bba5 Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Wed, 7 Dec 2022 00:08:03 +0000
Subject: [PATCH 4/6] 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  | 77 ++++++++++++++++++++++++++++----
 4 files changed, 74 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..45e7e2b 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 +101,63 @@ 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);
+            }
+        }
+
+        for (int x = 0; x < Size; x++)
+        {
+            for (int y = 0; y < Size; y++)
+            {
+                _tiles[x, y].value = _nextValues[x, y];
+            }
+        }
+    }
 }
-- 
2.34.1


From 2b02ccfe33fe8d02db1229c9b8c6e00c59684254 Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Thu, 8 Dec 2022 18:43:21 +0000
Subject: [PATCH 5/6] add diffusion explanation to gdd

---
 gdd.md | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/gdd.md b/gdd.md
index 8ad3e5d..3c72c77 100644
--- a/gdd.md
+++ b/gdd.md
@@ -28,6 +28,27 @@ Add a tick counter as a UI element.
 Diffusing should use 'pull' operations; tiles should pull values from surrounding tiles.
 This is an attempt to think forwards to parallelising computation of diffusiong - ideally using the GPU.
 
+#### Diffusion Algorithm
+
+Diffusion is modelled as two-dimensional steady-state heat conduction.
+This means that the amount of a value does not change between the start and end of the computation of the diffusion step.
+The value at one 2D location is calculated based on the values of itself and its neighbours in the previous step.
+A diffusion coefficient controls the rate of diffusion.
+
+[Video explanation](https://www.youtube.com/watch?v=RGbV7T_iWT8)
+[Derivation](https://www.tec-science.com/thermodynamics/heat/heat-equation-diffusion-equation/)
+
+Implementation:
+
+```cs
+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);
+}
+```
+
 ### Tile Types
 
 Tiles start as wild tiles.
-- 
2.34.1


From beb59aebbb8c7f4e5e77b78a29410d2776dfaad0 Mon Sep 17 00:00:00 2001
From: Cat Flynn <me@ktyl.dev>
Date: Thu, 8 Dec 2022 19:37:10 +0000
Subject: [PATCH 6/6] auto-tick, pausable clock #4

---
 half-earth/nodes/clock.tscn    |  1 +
 half-earth/nodes/debug_ui.tscn | 16 +++++++++++++-
 half-earth/nodes/game.tscn     |  3 +++
 half-earth/scripts/Clock.cs    | 40 ++++++++++++++++++++++++++++++++--
 half-earth/scripts/DebugUI.cs  | 21 +++++++++++++++---
 5 files changed, 75 insertions(+), 6 deletions(-)

diff --git a/half-earth/nodes/clock.tscn b/half-earth/nodes/clock.tscn
index 7d4c9fe..2c5bc42 100644
--- a/half-earth/nodes/clock.tscn
+++ b/half-earth/nodes/clock.tscn
@@ -4,3 +4,4 @@
 
 [node name="Clock" type="Node"]
 script = ExtResource( 1 )
+_tickRate = 10.0
diff --git a/half-earth/nodes/debug_ui.tscn b/half-earth/nodes/debug_ui.tscn
index 1cf603b..4f2ab7f 100644
--- a/half-earth/nodes/debug_ui.tscn
+++ b/half-earth/nodes/debug_ui.tscn
@@ -7,6 +7,7 @@ anchor_right = 1.0
 anchor_bottom = 1.0
 script = ExtResource( 1 )
 TicksLabelPath = NodePath("Ticks")
+PauseLabelPath = NodePath("Paused")
 
 [node name="Ticks" type="Label" parent="."]
 anchor_left = 1.0
@@ -14,9 +15,22 @@ anchor_top = 1.0
 anchor_right = 1.0
 anchor_bottom = 1.0
 margin_right = -10.0
+margin_bottom = -30.0
+grow_horizontal = 0
+grow_vertical = 0
+text = "ticks: 0"
+align = 2
+valign = 2
+
+[node name="Paused" type="Label" parent="."]
+anchor_left = 1.0
+anchor_top = 1.0
+anchor_right = 1.0
+anchor_bottom = 1.0
+margin_right = -10.0
 margin_bottom = -10.0
 grow_horizontal = 0
 grow_vertical = 0
-text = "ticks: 0"
+text = "pause: false"
 align = 2
 valign = 2
diff --git a/half-earth/nodes/game.tscn b/half-earth/nodes/game.tscn
index a23d717..9b75583 100644
--- a/half-earth/nodes/game.tscn
+++ b/half-earth/nodes/game.tscn
@@ -8,6 +8,7 @@
 [node name="Root" type="Node"]
 
 [node name="Grid" parent="." instance=ExtResource( 1 )]
+DiffusionCoefficient = 0.1
 
 [node name="Cursor" parent="." instance=ExtResource( 2 )]
 Grid = NodePath("../Grid")
@@ -15,6 +16,8 @@ Grid = NodePath("../Grid")
 [node name="Debug UI" parent="." instance=ExtResource( 3 )]
 
 [node name="Clock" parent="." instance=ExtResource( 4 )]
+_autoTick = true
 
+[connection signal="OnPauseChanged" from="Clock" to="Debug UI" method="_on_Clock_OnPauseChanged"]
 [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/Clock.cs b/half-earth/scripts/Clock.cs
index 9050073..3fb7456 100644
--- a/half-earth/scripts/Clock.cs
+++ b/half-earth/scripts/Clock.cs
@@ -5,8 +5,18 @@ public class Clock : Node
 {
     [Signal]
     delegate void OnTick(int ticks);
-    
+    [Signal]
+    delegate void OnPauseChanged(bool paused);
+
+    [Export(PropertyHint.None, "Number of ticks per second")]
+    private float _tickRate;
+    private float TickPeriod => 1.0f / _tickRate;
+
+    [Export]
+    private bool _autoTick;
+
     private int _ticks = 0;
+    private bool _paused = false;
 
     public override void _Input(InputEvent @event)
     {
@@ -17,12 +27,38 @@ public class Clock : Node
             switch ((KeyList)keyEvent.Scancode)
             {
                 case KeyList.Space:
+                    if (_autoTick)
+                    {
+                        _paused = !_paused;
+                        EmitSignal(nameof(OnPauseChanged), _paused);
+                        return;
+                    }
                     Tick();
                     break;
             }
         }
     }
-    
+
+    private float _secondsSinceLastTick;
+
+    public override void _Process(float delta)
+    {
+        base._Process(delta);
+
+        if (!_autoTick)
+            return;
+
+        if (_paused)
+            return;
+
+        _secondsSinceLastTick += delta;
+        if (_secondsSinceLastTick < TickPeriod)
+            return;
+
+        _secondsSinceLastTick -= TickPeriod;
+        Tick();
+    }
+
     private void Tick()
     {
         _ticks++;
diff --git a/half-earth/scripts/DebugUI.cs b/half-earth/scripts/DebugUI.cs
index f8a8bdb..8adc836 100644
--- a/half-earth/scripts/DebugUI.cs
+++ b/half-earth/scripts/DebugUI.cs
@@ -6,17 +6,32 @@ public class DebugUI : Control
     [Export]
     public NodePath TicksLabelPath { private get; set; }
     private Label _ticksLabel;
-    
+
+    [Export]
+    public NodePath PauseLabelPath { private get; set; }
+    private Label _pauseLabel;
+
     public override void _Ready()
     {
         _ticksLabel = GetNode<Label>(TicksLabelPath);
+        _pauseLabel = GetNode<Label>(PauseLabelPath);
+
         SetTicksText(0);
+        SetPauseText(false);
     }
-    
+
+    #region Signals
+    public void _on_Clock_OnPauseChanged(bool paused) => SetPauseText(paused);
     public void _on_Clock_OnTick(int ticks) => SetTicksText(ticks);
-    
+    #endregion
+
     private void SetTicksText(int ticks)
     {
         _ticksLabel.Text = $"ticks: {ticks}";
     }
+
+    private void SetPauseText(bool paused)
+    {
+        _pauseLabel.Text = $"pause: {paused}";
+    }
 }
-- 
2.34.1