From d1e361f74e0d424b4d6a99adea1463982d9f1475 Mon Sep 17 00:00:00 2001
From: Martin Kagamino Lehoux <martin@lehoux.net>
Date: Thu, 15 Jan 2026 23:03:12 +0100
Subject: First integration test

---
 README.md           |  6 +++++
 integration_test.go | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 main.go             |  4 ++++
 3 files changed, 74 insertions(+)
 create mode 100644 integration_test.go

diff --git a/README.md b/README.md
index e3ba5a5..e4cca5c 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,11 @@
 
 - Restart after kernel update until `glxinfo | grep "OpenGL version"` works
 
+## Testing
+
+- **Integration tests** Use `Update()` to run N ticks.
+- **Performance benchmarks**
+
 ## TODO
 
 - see the current action (debug?)
@@ -19,6 +24,7 @@
 - perf: measure frame budget
 - rename Selection to Selectable
 - extract todos in todo list
+- abstraction between ebiten and update to handle inputs in a simulation?
 
 ## Bugs
 
diff --git a/integration_test.go b/integration_test.go
new file mode 100644
index 0000000..8305719
--- /dev/null
+++ b/integration_test.go
@@ -0,0 +1,64 @@
+package main
+
+import (
+	"testing"
+	"time"
+
+	"age_of_empires/physics"
+)
+
+func TestGatherLoop(t *testing.T) {
+	// Initialize Game
+	game := &Game{}
+
+	// Town Center (Storage) at -100,0 (different grid cell from unit at 0,0)
+	tcBuilder := NewEntityBuilder().
+		WithPosition(physics.Point{X: -100, Y: 0}).
+		WithResourceStorage()
+	_ = game.Append(tcBuilder.Build())
+
+	// Mine (Source) at 0, 200
+	mineBuilder := NewEntityBuilder().
+		WithPosition(physics.Point{X: 0, Y: 200}).
+		WithResourceSource(100)
+	mineID := game.Append(mineBuilder.Build())
+
+	// Unit (Gatherer) at 0, 0
+	unitBuilder := NewEntityBuilder().
+		WithPosition(physics.Point{X: 0, Y: 0}).
+		WithMove().
+		WithOrder().
+		WithResourceGatherer(10)
+	unitID := game.Append(unitBuilder.Build())
+
+	// Set up simulated time
+	simTime := time.Now()
+	game.NowFunc = func() time.Time { return simTime }
+
+	// Simulate user selection: select the unit
+	game.GlobalSelection.Selected = append(game.GlobalSelection.Selected, unitID)
+
+	// Simulate right-click on mine position to trigger Gather via MainAction
+	minePosition := game.Position[mineID]
+	MainAction(game, unitID, minePosition, game.getMoveMap())
+
+	initialResources := game.ResourceAmount
+	dt := 16 * time.Millisecond
+	maxTicks := 100_000
+
+	deposited := false
+
+	for i := 0; i < maxTicks; i++ {
+		simTime = simTime.Add(dt)
+		game.Update()
+
+		if game.ResourceAmount > initialResources {
+			deposited = true
+			break
+		}
+	}
+
+	if !deposited {
+		t.Errorf("Unit failed to gather resources within %d ticks. ResourceAmount: %d", maxTicks, game.ResourceAmount)
+	}
+}
diff --git a/main.go b/main.go
index e67e50b..80ee064 100644
--- a/main.go
+++ b/main.go
@@ -36,6 +36,7 @@ type Game struct {
 	ResourceAmount  int
 	FaceSource      *text.GoTextFaceSource
 	UnitBuilder     EntityBuilder
+	NowFunc         func() time.Time
 	// Components
 	Mask []ecs.Mask
 	// Entities         []*Entity
@@ -53,6 +54,9 @@ type Game struct {
 }
 
 func (g Game) Now() time.Time {
+	if g.NowFunc != nil {
+		return g.NowFunc()
+	}
 	return time.Now()
 }
 
-- 
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