commit 70cee9e3f016bf5c8e7741887d73e0751d87993b
Author: iegod <diegob@gmail.com>
Date:   Thu Nov 27 22:50:36 2025 -0500

    Initial commit.

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e68436c
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+*.exe
+*.sum
+working/
+.vscode_build/
\ No newline at end of file
diff --git a/.vscode/launch.json b/.vscode/launch.json
new file mode 100644
index 0000000..7921202
--- /dev/null
+++ b/.vscode/launch.json
@@ -0,0 +1,16 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Launch Package",
+            "type": "go",
+            "request": "launch",
+            "mode": "auto",
+            "program": "${workspaceFolder}",
+            "output": "${workspaceFolder}/.vscode_build/debug_bin"
+        }
+    ]
+}
\ No newline at end of file
diff --git a/colliders/baserectcollider.go b/colliders/baserectcollider.go
new file mode 100644
index 0000000..9c89d8f
--- /dev/null
+++ b/colliders/baserectcollider.go
@@ -0,0 +1,50 @@
+package colliders
+
+import (
+	"fluids/gamedata"
+	"image"
+)
+
+type BaseRectCollider struct {
+	position    gamedata.Vector
+	dimensions  gamedata.Vector
+	iscontainer bool
+}
+
+func (b *BaseRectCollider) SetContainer(v bool) {
+	b.iscontainer = v
+}
+
+func (b *BaseRectCollider) SetPosition(p gamedata.Vector) {
+	b.position = p
+}
+
+func (b *BaseRectCollider) GetPosition() gamedata.Vector {
+	return b.position
+}
+
+func (b *BaseRectCollider) SetDimensions(d gamedata.Vector) {
+	b.dimensions = d
+}
+
+func (b *BaseRectCollider) GetDimensions() gamedata.Vector {
+	return b.dimensions
+}
+
+func (b *BaseRectCollider) GetBounds() image.Rectangle {
+	r := image.Rectangle{
+		Min: image.Point{
+			X: int(b.position.X - b.dimensions.X/2),
+			Y: int(b.position.Y - b.dimensions.Y/2),
+		},
+		Max: image.Point{
+			X: int(b.position.X + b.dimensions.X/2),
+			Y: int(b.position.Y + b.dimensions.Y/2),
+		},
+	}
+	return r
+}
+
+func (b *BaseRectCollider) IsContainer() bool {
+	return b.iscontainer
+}
diff --git a/colliders/collider.go b/colliders/collider.go
new file mode 100644
index 0000000..7958a90
--- /dev/null
+++ b/colliders/collider.go
@@ -0,0 +1,15 @@
+package colliders
+
+import (
+	"fluids/gamedata"
+)
+
+type Collider interface {
+	GetPosition() gamedata.Vector
+	GetDimensions() gamedata.Vector
+	SetPosition(gamedata.Vector)
+	//GetBounds() image.Rectangle
+
+	//indicates whether this collider is inverted, i.e. 'contains' elements
+	//IsContainer() bool
+}
diff --git a/elements/box.go b/elements/box.go
new file mode 100644
index 0000000..53b6dd4
--- /dev/null
+++ b/elements/box.go
@@ -0,0 +1,8 @@
+package elements
+
+import "fluids/gamedata"
+
+type Box struct {
+	Position   gamedata.Vector
+	Dimensions gamedata.Vector
+}
diff --git a/elements/particle.go b/elements/particle.go
new file mode 100644
index 0000000..e819134
--- /dev/null
+++ b/elements/particle.go
@@ -0,0 +1,25 @@
+package elements
+
+import "fluids/gamedata"
+
+type Particle struct {
+	Position gamedata.Vector
+	Velocity gamedata.Vector
+	Radius   float64
+}
+
+func (p Particle) GetDimensions() gamedata.Vector {
+	dim := gamedata.Vector{
+		X: p.Radius * 2,
+		Y: p.Radius * 2,
+	}
+	return dim
+}
+
+func (p Particle) GetPosition() gamedata.Vector {
+	return p.Position
+}
+
+func (p *Particle) SetPosition(position gamedata.Vector) {
+	p.Position = position
+}
diff --git a/game/game.go b/game/game.go
new file mode 100644
index 0000000..f289569
--- /dev/null
+++ b/game/game.go
@@ -0,0 +1,351 @@
+package game
+
+import (
+	"fluids/elements"
+	"fluids/gamedata"
+	"fluids/quadtree"
+	"fmt"
+	"image/color"
+	"math"
+	"math/rand"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+const (
+	GameWidth          = 640
+	GameHeight         = 360
+	GameParticleCount  = 1000
+	GameGravity        = 2
+	GameParticleRadius = 5
+	GameDamping        = .7
+	GameDeltaTimeStep  = 0.5
+
+	GameInfluenceRadius = 30
+)
+
+type Game struct {
+	particles         []*elements.Particle
+	cycle             int
+	particlebox       *gamedata.Vector
+	particlebuff      *ebiten.Image
+	quadtree          *quadtree.Quadtree
+	collisionquad     quadtree.Quadrant
+	paused            bool
+	renderquads       bool
+	resolvecollisions bool
+}
+
+func NewGame() *Game {
+	g := &Game{
+		particlebuff:      ebiten.NewImage(GameParticleRadius*2, GameParticleRadius*2),
+		paused:            false,
+		renderquads:       false,
+		resolvecollisions: false,
+	}
+
+	g.particlebox = &gamedata.Vector{
+		X: GameWidth - 50,
+		Y: GameHeight - 50,
+	}
+
+	quad := quadtree.Quadrant{
+		Position:   gamedata.Vector{X: GameWidth / 2, Y: GameHeight / 2},
+		Dimensions: gamedata.Vector{X: GameWidth, Y: GameHeight},
+	}
+	g.quadtree = quadtree.New(quad)
+
+	vector.FillCircle(g.particlebuff, GameParticleRadius, GameParticleRadius, GameParticleRadius, color.White, true)
+
+	g.collisionquad = quadtree.Quadrant{
+		Position: gamedata.Vector{
+			X: 0,
+			Y: 0,
+		},
+		Dimensions: gamedata.Vector{
+			X: GameParticleRadius * 2,
+			Y: GameParticleRadius * 2,
+		},
+	}
+
+	//g.InitializeColliders()
+	g.InitializeParticles()
+
+	return g
+}
+
+func (g *Game) Update() error {
+
+	g.ParseInputs()
+	g.RebuildQuadtree()
+
+	if !g.paused {
+		g.UpdateParticles()
+	}
+
+	g.cycle++
+
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	screen.Clear()
+	g.RenderParticles(screen)
+	g.RenderBox(screen)
+
+	if g.renderquads {
+		g.RenderQuadrants(screen)
+	}
+}
+
+func (g *Game) Layout(x, y int) (int, int) {
+	return GameWidth, GameHeight
+}
+
+func (g *Game) RenderParticles(img *ebiten.Image) {
+	// mx, my := ebiten.CursorPosition()
+	//clr := color.RGBA{R: 0xff, G: 0x00, B: 0x00, A: 0xff}
+
+	for _, particle := range g.particles {
+
+		x0 := particle.Position.X - GameParticleRadius
+		y0 := particle.Position.Y - GameParticleRadius
+
+		op := &ebiten.DrawImageOptions{}
+		op.GeoM.Translate(x0, y0)
+		img.DrawImage(g.particlebuff, op)
+
+		//vector.FillCircle(img, float32(particle.Position.X), float32(particle.Position.Y), float32(particle.Radius), color.White, true)
+		// vector.StrokeCircle(img, float32(particle.Position.X), float32(particle.Position.Y), GameInfluenceRadius, 1, clr, true)
+		// vector.StrokeLine(img, float32(mx), float32(my), float32(particle.Position.X), float32(particle.Position.Y), 2, clr, true)
+	}
+}
+
+func (g *Game) RenderBox(img *ebiten.Image) {
+	x0 := (GameWidth - g.particlebox.X) / 2
+	y0 := (GameHeight - g.particlebox.Y) / 2
+	vector.StrokeRect(img, float32(x0), float32(y0), float32(g.particlebox.X), float32(g.particlebox.Y), 2, color.White, true)
+}
+
+func (g *Game) InitializeColliders() {
+	/*
+		//box container
+		box := &colliders.BaseRectCollider{}
+		box.SetDimensions(gamedata.Vector{
+			X: GameWidth - 50,
+			Y: GameHeight - 50,
+		})
+		box.SetPosition(gamedata.Vector{
+			X: GameWidth / 2,
+			Y: GameHeight / 2,
+		})
+		box.SetContainer(true)
+		g.rectColliders = append(g.rectColliders, box)
+	*/
+}
+
+func (g *Game) InitializeParticles() {
+
+	g.particles = g.particles[:0]
+
+	xmin := (GameWidth-g.particlebox.X)/2 + GameParticleRadius
+	xmax := g.particlebox.X - GameParticleRadius*2
+	ymin := (GameHeight-g.particlebox.Y)/2 + GameParticleRadius
+	ymax := g.particlebox.Y - GameParticleRadius*2
+
+	for i := 0; i < GameParticleCount; i++ {
+
+		p := &elements.Particle{
+			Position: gamedata.Vector{
+				X: xmin + rand.Float64()*xmax,
+				Y: ymin + rand.Float64()*ymax,
+			},
+			Velocity: gamedata.Vector{
+				X: 0,
+				Y: 0,
+			},
+			Radius: GameParticleRadius,
+		}
+
+		g.particles = append(g.particles, p)
+	}
+
+}
+
+func (g *Game) UpdateParticles() {
+
+	dt := GameDeltaTimeStep
+
+	/*
+		mx, my := ebiten.CursorPosition()
+		mpos := gamedata.Vector{X: float64(mx), Y: float64(my)}
+		maxdeflect := 40 * GameDeltaTimeStep * GameGravity
+	*/
+
+	for _, particle := range g.particles {
+		particle.Velocity.Y += GameGravity * dt
+
+		//if inpututil.IsMouseButtonJustPressed(ebiten.MouseButtonLeft) {
+		/*
+			if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
+				delta := gamedata.Vector{
+					X: mpos.X - particle.Position.X,
+					Y: mpos.Y - particle.Position.Y,
+				}
+
+				dist := math.Sqrt(delta.X*delta.X + delta.Y*delta.Y)
+				theta := math.Atan2(delta.Y, delta.X)
+
+				if dist < GameInfluenceRadius {
+
+					dx := dist * math.Cos(theta)
+					dy := dist * math.Sin(theta)
+
+					if dx != 0 {
+						gainx := (-1./GameInfluenceRadius)*math.Abs(dx) + 1.
+						particle.Velocity.X += 20 * gainx * -1 * math.Copysign(1, dx)
+					}
+
+					if dy != 0 {
+						gainy := (-1./GameInfluenceRadius)*math.Abs(dy) + 1.
+						particle.Velocity.Y += maxdeflect * gainy * -1 * math.Copysign(1, dy)
+					}
+				}
+
+			}
+		*/
+
+		particle.Position.X += particle.Velocity.X * dt
+		particle.Position.Y += particle.Velocity.Y * dt
+
+		if g.resolvecollisions {
+			g.ResolveCollisions(particle)
+		}
+
+		g.BoundParticle(particle)
+	}
+
+}
+
+func (g *Game) BoundParticle(p *elements.Particle) {
+
+	xmin := (GameWidth-g.particlebox.X)/2 + p.Radius
+	xmax := xmin + g.particlebox.X - p.Radius*2
+
+	if p.Position.X > xmax {
+		p.Velocity.X *= -1 * GameDamping
+		p.Position.X = xmax
+	}
+
+	if p.Position.X < xmin {
+		p.Velocity.X *= -1 * GameDamping
+		p.Position.X = xmin
+	}
+
+	ymin := (GameHeight-g.particlebox.Y)/2 + p.Radius
+	ymax := ymin + g.particlebox.Y - p.Radius*2
+
+	if p.Position.Y > ymax {
+		p.Velocity.Y *= -1 * GameDamping
+		p.Position.Y = ymax
+	}
+
+	if p.Position.Y < ymin {
+		p.Velocity.Y *= -1 * GameDamping
+		p.Position.Y = ymin
+	}
+
+}
+
+func (g *Game) RenderQuadrants(img *ebiten.Image) {
+	clr := color.RGBA{R: 0xff, G: 0x00, B: 0x00, A: 0xff}
+
+	quadrants := g.quadtree.GetQuadrants()
+	for _, quad := range quadrants {
+		ox := float32(quad.Position.X - quad.Dimensions.X/2)
+		oy := float32(quad.Position.Y - quad.Dimensions.Y/2)
+		vector.StrokeRect(img, ox, oy, float32(quad.Dimensions.X), float32(quad.Dimensions.Y), 1, clr, true)
+	}
+
+}
+
+func (g *Game) ParseInputs() {
+	if inpututil.IsKeyJustPressed(ebiten.KeyR) {
+		g.InitializeParticles()
+	}
+
+	if inpututil.IsKeyJustPressed(ebiten.KeyP) {
+		g.paused = !g.paused
+	}
+
+	if inpututil.IsKeyJustPressed(ebiten.KeyQ) {
+		g.renderquads = !g.renderquads
+	}
+
+	if inpututil.IsKeyJustPressed(ebiten.KeyC) {
+		g.resolvecollisions = !g.resolvecollisions
+	}
+
+}
+
+func (g *Game) RebuildQuadtree() {
+	g.quadtree.Clear()
+
+	for _, p := range g.particles {
+		if !g.quadtree.Insert(p) {
+			fmt.Println("quadtree insertion failed")
+		}
+	}
+}
+
+func (g *Game) ResolveCollisions(particle *elements.Particle) {
+	//construct search quadrant from current particle
+	quadrant := quadtree.Quadrant{
+		Position:   particle.Position,
+		Dimensions: particle.GetDimensions(),
+	}
+
+	//find list of possible maybe collisions, we inspect those in more detail
+	maybes := g.quadtree.FindAll(quadrant)
+
+	sqdist := float64(GameParticleRadius*GameParticleRadius) * 4
+
+	for _, p := range maybes {
+		if p == particle {
+			continue
+		}
+
+		pos := p.GetPosition()
+		delta := gamedata.Vector{
+			X: pos.X - particle.Position.X,
+			Y: pos.Y - particle.Position.Y,
+		}
+		dist2 := delta.X*delta.X + delta.Y*delta.Y
+
+		if dist2 == 0 {
+			// Same position: pick a fallback direction to avoid NaN
+			delta.X = 1
+			delta.Y = 0
+			dist2 = 1
+		}
+
+		if dist2 < sqdist {
+			d := math.Sqrt(dist2)
+			nx, ny := delta.X/d, delta.Y/d
+			overlap := GameParticleRadius*2 - d
+
+			pos.X += nx * overlap
+			pos.Y += ny * overlap
+			p.SetPosition(pos)
+			/*
+				newpos := gamedata.Vector{
+					X: pos.X + delta.X,
+					Y: pos.Y + delta.Y,
+				}
+				p.SetPosition(newpos)
+			*/
+		}
+	}
+}
diff --git a/gamedata/vector.go b/gamedata/vector.go
new file mode 100644
index 0000000..becdff4
--- /dev/null
+++ b/gamedata/vector.go
@@ -0,0 +1,6 @@
+package gamedata
+
+type Vector struct {
+	X float64
+	Y float64
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..9a2c6df
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,14 @@
+module fluids
+
+go 1.24.1
+
+require github.com/hajimehoshi/ebiten/v2 v2.9.4
+
+require (
+	github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1 // indirect
+	github.com/ebitengine/hideconsole v1.0.0 // indirect
+	github.com/ebitengine/purego v0.9.0 // indirect
+	github.com/jezek/xgb v1.1.1 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+	golang.org/x/sys v0.36.0 // indirect
+)
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..168e6e5
--- /dev/null
+++ b/main.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fluids/game"
+	"fmt"
+	"log"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+const (
+	ScreenWidth  = 640
+	ScreenHeight = 360
+)
+
+func main() {
+	fmt.Println("fluid experiments")
+
+	g := game.NewGame()
+
+	ebiten.SetWindowTitle("fluids")
+	ebiten.SetWindowSize(ScreenWidth*2, ScreenHeight*2)
+
+	err := ebiten.RunGame(g)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+}
diff --git a/quadtree/quadtree.go b/quadtree/quadtree.go
new file mode 100644
index 0000000..c7e5d6f
--- /dev/null
+++ b/quadtree/quadtree.go
@@ -0,0 +1,192 @@
+package quadtree
+
+import (
+	"fluids/colliders"
+	"fluids/gamedata"
+)
+
+const (
+	QuadtreeMaxColliders = 10
+)
+
+type Quadrant struct {
+	Position   gamedata.Vector
+	Dimensions gamedata.Vector
+}
+
+type Quadtree struct {
+	quadrant  Quadrant
+	children  []*Quadtree
+	colliders []colliders.Collider
+}
+
+func New(quadrant Quadrant) *Quadtree {
+	qt := &Quadtree{
+		quadrant: quadrant,
+	}
+	return qt
+}
+
+func (q *Quadtree) Insert(obj colliders.Collider) bool {
+
+	var result bool = false
+	//check that object meets containment criteria
+	if q.ContainsPoint(obj.GetPosition()) {
+
+		//if we have children, we go deeper
+		if len(q.children) > 0 {
+			for _, node := range q.children {
+				if node.Insert(obj) {
+					result = true
+					break
+				}
+			}
+		} else if len(q.colliders) < QuadtreeMaxColliders {
+			//otherwise, if we have space, add
+			q.colliders = append(q.colliders, obj)
+			result = true
+		} else {
+			//otherwise we have to subdivide, reorganize, and add
+			result = q.SubdivideAndInsert(obj)
+		}
+	}
+	return result
+}
+
+func (q *Quadtree) ContainsPoint(p gamedata.Vector) bool {
+	var result bool = false
+	if (q.quadrant.Position.X-q.quadrant.Dimensions.X/2) <= p.X &&
+		p.X <= (q.quadrant.Position.X+q.quadrant.Dimensions.X/2) &&
+		(q.quadrant.Position.Y-q.quadrant.Dimensions.Y/2) <= p.Y &&
+		p.Y <= (q.quadrant.Position.Y+q.quadrant.Dimensions.Y/2) {
+		result = true
+	}
+	return result
+}
+
+func (q *Quadtree) Remove(obj colliders.Collider) bool {
+	var result bool = false
+
+	if q.ContainsPoint(obj.GetPosition()) {
+
+		if len(q.colliders) > 0 {
+			//examine existing colliders
+			var collection []colliders.Collider
+			for _, node := range q.colliders {
+				if node == obj {
+					result = true
+				} else {
+					collection = append(collection, node)
+				}
+			}
+			q.colliders = collection
+		} else {
+			//need to check children
+			for _, child := range q.children {
+				if child.Remove(obj) {
+					result = true
+					break
+				}
+			}
+		}
+	}
+
+	return result
+}
+
+func (q *Quadtree) SubdivideAndInsert(obj colliders.Collider) bool {
+
+	var result bool = false
+
+	//initialize up children
+	q.children = q.children[:0]
+	q.children = append(q.children, New(Quadrant{Position: gamedata.Vector{X: q.quadrant.Position.X - q.quadrant.Dimensions.X/4, Y: q.quadrant.Position.Y - q.quadrant.Dimensions.Y/4}, Dimensions: gamedata.Vector{X: q.quadrant.Dimensions.X / 2, Y: q.quadrant.Dimensions.Y / 2}}))
+	q.children = append(q.children, New(Quadrant{Position: gamedata.Vector{X: q.quadrant.Position.X + q.quadrant.Dimensions.X/4, Y: q.quadrant.Position.Y - q.quadrant.Dimensions.Y/4}, Dimensions: gamedata.Vector{X: q.quadrant.Dimensions.X / 2, Y: q.quadrant.Dimensions.Y / 2}}))
+	q.children = append(q.children, New(Quadrant{Position: gamedata.Vector{X: q.quadrant.Position.X - q.quadrant.Dimensions.X/4, Y: q.quadrant.Position.Y + q.quadrant.Dimensions.Y/4}, Dimensions: gamedata.Vector{X: q.quadrant.Dimensions.X / 2, Y: q.quadrant.Dimensions.Y / 2}}))
+	q.children = append(q.children, New(Quadrant{Position: gamedata.Vector{X: q.quadrant.Position.X + q.quadrant.Dimensions.X/4, Y: q.quadrant.Position.Y + q.quadrant.Dimensions.Y/4}, Dimensions: gamedata.Vector{X: q.quadrant.Dimensions.X / 2, Y: q.quadrant.Dimensions.Y / 2}}))
+
+	//move colliders into child nodes
+	var failed bool = false
+	for _, collider := range q.colliders {
+		var added bool = false
+		for _, node := range q.children {
+			if node.Insert(collider) {
+				added = true
+				break
+			}
+		}
+
+		//couldn't add collider into any subdivided node; failure
+		if !added {
+			failed = true
+			break
+		}
+	}
+
+	//if all went well:
+	// a. need to wipe colliders from current node
+	// b. need to now insert new collider into one of the children
+	if !failed {
+		q.colliders = q.colliders[:0]
+
+		for _, node := range q.children {
+			if node.Insert(obj) {
+				result = true
+				break
+			}
+		}
+	}
+
+	return result
+}
+
+func (q *Quadtree) Clear() {
+	q.colliders = q.colliders[:0]
+	q.children = q.children[:0]
+}
+
+func (q *Quadtree) GetQuadrants() []Quadrant {
+	var result []Quadrant
+
+	//if we have children, traverse and add their quadrants
+	if len(q.children) > 0 {
+		for _, child := range q.children {
+			result = append(result, child.GetQuadrants()...)
+		}
+	}
+
+	result = append(result, q.quadrant)
+
+	return result
+}
+
+// find and retrieve all colliders in given quadrant
+func (q *Quadtree) FindAll(quadrant Quadrant) []colliders.Collider {
+	var result []colliders.Collider
+
+	//search coordinates
+	sx0 := quadrant.Position.X - quadrant.Dimensions.X/2
+	sx1 := quadrant.Position.X + quadrant.Dimensions.X/2
+	sy0 := quadrant.Position.Y - quadrant.Dimensions.Y/2
+	sy1 := quadrant.Position.Y + quadrant.Dimensions.Y/2
+
+	//quadtree coordinates
+	qx0 := q.quadrant.Position.X - q.quadrant.Dimensions.X/2
+	qx1 := q.quadrant.Position.X + q.quadrant.Dimensions.X/2
+	qy0 := q.quadrant.Position.Y - q.quadrant.Dimensions.Y/2
+	qy1 := q.quadrant.Position.Y + q.quadrant.Dimensions.Y/2
+
+	//AABB check
+	if sx0 < qx1 && sx1 > qx0 &&
+		sy0 < qy1 && sy1 > qy0 {
+
+		//if we have children, check each of those
+		for _, node := range q.children {
+			result = append(result, node.FindAll(quadrant)...)
+		}
+
+		result = append(result, q.colliders...)
+	}
+
+	return result
+}