From d45b96278a61b34b3df3e48bb2712de877bdefca Mon Sep 17 00:00:00 2001
From: Christian Rocha <christian@rocha.is>
Date: Fri, 23 Jul 2021 13:31:14 -0400
Subject: [PATCH] Copy values in Update method to avoid data races

---
 README.md               |  4 ++--
 examples/opengl/main.go |  6 +++---
 examples/tui/main.go    |  2 +-
 harmonica.go            | 24 +++++++++++++-----------
 4 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/README.md b/README.md
index 917d1ad..bacd53a 100644
--- a/README.md
+++ b/README.md
@@ -43,8 +43,8 @@ spring := harmonica.NewSpring(harmonica.FPS(60), 6.0, 0.5)
 
 // Animate!
 for {
-    spring.Update(&sprite.x, &sprite.xVelocity, targetX)
-    spring.Update(&sprite.y, &sprite.yVelocity, targetY)
+    sprite.x, sprite.xVelocity = spring.Update(&sprite.x, &sprite.xVelocity, targetX)
+    sprite.y, sprite.yVelocity = spring.Update(&sprite.y, &sprite.yVelocity, targetY)
     time.Sleep(time.Second/60)
 }
 ```
diff --git a/examples/opengl/main.go b/examples/opengl/main.go
index 09352ed..076a896 100644
--- a/examples/opengl/main.go
+++ b/examples/opengl/main.go
@@ -255,9 +255,9 @@ func (s *Sprite) Update() {
 	}
 
 	// Calculate positions based on our spring
-	s.spring.Update(&s.X, &s.xVel, s.TargetX)
-	s.spring.Update(&s.Y, &s.yVel, s.TargetY)
-	s.spring.Update(&s.radius, &s.radiusVel, s.TargetRadius)
+	s.X, s.xVel = s.spring.Update(s.X, s.xVel, s.TargetX)
+	s.Y, s.yVel = s.spring.Update(s.Y, s.yVel, s.TargetY)
+	s.radius, s.radiusVel = s.spring.Update(s.radius, s.radiusVel, s.TargetRadius)
 }
 
 func (s Sprite) Draw(ctx *gg.Context) {
diff --git a/examples/tui/main.go b/examples/tui/main.go
index 1ee9b17..955338d 100644
--- a/examples/tui/main.go
+++ b/examples/tui/main.go
@@ -66,7 +66,7 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		const targetX = 60
 
 		// Update x position (and velocity) with our spring.
-		m.spring.Update(&m.x, &m.xVel, targetX)
+		m.x, m.xVel = m.spring.Update(m.x, m.xVel, targetX)
 
 		// Quit when we're basically at the target position.
 		if math.Abs(m.x-targetX) < 0.01 {
diff --git a/harmonica.go b/harmonica.go
index cce5afc..da20b99 100644
--- a/harmonica.go
+++ b/harmonica.go
@@ -9,10 +9,10 @@
 //
 //     // Update on every frame.
 //     pos := 0.0
-//     targetPos := 100.0
 //     velocity := 0.0
+//     targetPos := 100.0
 //     someUpdateLoop(func() {
-//         spring.Update(&pos, &velocity, targetPos)
+//         pos, velocity = spring.Update(pos, velocity, targetPos)
 //     })
 //
 // For background on the algorithm see:
@@ -61,7 +61,7 @@ import (
 //
 // Example:
 //
-//     spring := NewSpring(FPS(60), 0.8, 0.98)
+//     spring := NewSpring(FPS(60), 5.0, 0.2)
 //
 func FPS(n int) float64 {
 	return (time.Second / time.Duration(n)).Seconds()
@@ -92,13 +92,13 @@ var epsilon = math.Nextafter(1, 2) - 1
 // Example:
 //
 //     // First precompute spring coefficients based on your settings:
-//     var x, xVel, y, yVel float
+//     var x, xVel, y, yVel float64
 //     deltaTime := FPS(60)
 //     s := NewSpring(deltaTime, 5.0, 0.2)
 //
 //     // Then, in your update loop:
-//     s.Update(&x, &xVel, 10)             // update the X position
-//     s.Update(&y, &yVel, 20)             // update the Y position
+//     x, xVel = s.Update(x, xVel, 10) // update the X position
+//     y, yVel = s.Update(y, yVel, 20) // update the Y position
 //
 type Spring struct {
 	posPosCoef, posVelCoef float64
@@ -212,10 +212,12 @@ func NewSpring(deltaTime, angularFrequency, dampingRatio float64) (s Spring) {
 
 // Update updates position and velocity values against a given target value.
 // Call this after calling NewSpring to update values.
-func (s Spring) Update(pos *float64, vel *float64, equilibriumPos float64) {
-	oldPos := *pos - equilibriumPos // update in equilibrium relative space
-	oldVel := *vel
+func (s Spring) Update(pos, vel float64, equilibriumPos float64) (newPos, newVel float64) {
+	oldPos := pos - equilibriumPos // update in equilibrium relative space
+	oldVel := vel
 
-	*pos = oldPos*s.posPosCoef + oldVel*s.posVelCoef + equilibriumPos
-	*vel = oldPos*s.velPosCoef + oldVel*s.velVelCoef
+	newPos = oldPos*s.posPosCoef + oldVel*s.posVelCoef + equilibriumPos
+	newVel = oldPos*s.velPosCoef + oldVel*s.velVelCoef
+
+	return newPos, newVel
 }