Handle game state immutably

game/ai.go

@@ -2,8 +2,9 @@ package game
 
 import "log"
 
-func chooseCommand(t Team, s State) command {
-	h := t.Baton.Holder
+func chooseCommand(s State, teamID int) command {
+	t := s.Teams[teamID]
+	h := t.BatonHolder()
 	if collide(h.Pos+1, h.Lane, s) {
 		if h.Lane <= t.Lane {
 			return left
@@ -13,7 +14,7 @@ func chooseCommand(t Team, s State) command {
 
 	var nextBot *Bot
 	for i, b := range t.Bots {
-		if b.id == h.id+1 {
+		if b.ID == h.ID+1 {
 			nextBot = &t.Bots[i]
 			break
 		}

game/commands.go

@@ -1,24 +1,8 @@
 package game
 
-import "math/rand"
-
-func doCommand(cmd command, b *Bot) {
-	da := 1
-	da += rand.Intn(3) - 1
-
-	switch cmd {
-	case speedUp:
-		b.a += da
-		accelerate(b)
-	case slowDown:
-		b.a -= da
-		accelerate(b)
-	case left:
-		b.Lane++
-	case right:
-		b.Lane--
-	}
-}
+import (
+	"math/rand"
+)
 
 type command int
 
@@ -28,3 +12,28 @@ const (
 	left
 	right
 )
+
+func doCommand(cmd command, s State, teamID int) State {
+	da := 1
+	da += rand.Intn(3) - 1
+
+	b := activeBot(s.Teams[teamID])
+	if b == nil {
+		return s
+	}
+
+	switch cmd {
+	case speedUp:
+		b.a += da
+		*b = accelerate(*b)
+	case slowDown:
+		b.a -= da
+		*b = accelerate(*b)
+	case left:
+		b.Lane++
+	case right:
+		b.Lane--
+	}
+
+	return updateBot(s, teamID, *b)
+}

game/game.go

@@ -16,7 +16,7 @@ func NewState() State {
 		var bots []Bot
 		for j := 0; j < numBots; j++ {
 			b := Bot{
-				id:   i*NumTeams + j,
+				ID:   i*NumTeams + j,
 				Lane: i,
 				Pos:  j * (Steps / numBots),
 			}
@@ -25,7 +25,7 @@ func NewState() State {
 		teams = append(teams, Team{
 			id:    i,
 			Bots:  bots,
-			Baton: Baton{Holder: &bots[0]},
+			Baton: Baton{HolderID: i * NumTeams},
 			Lane:  i,
 		})
 	}
@@ -61,8 +61,17 @@ type Team struct {
 	Lane  int
 }
 
+func (t Team) BatonHolder() *Bot {
+	for _, b := range t.Bots {
+		if b.ID == t.Baton.HolderID {
+			return &b
+		}
+	}
+	return nil
+}
+
 type Bot struct {
-	id   int
+	ID   int
 	Lane int
 	Pos  int
 	v    int
@@ -70,7 +79,7 @@ type Bot struct {
 }
 
 type Baton struct {
-	Holder *Bot
+	HolderID int
 }
 
 type Obstacle struct {
@@ -78,17 +87,17 @@ type Obstacle struct {
 	Pos  int
 }
 
-func UpdateState(sOld State) State {
-	s := sOld
-
+func UpdateState(s State) State {
 	for i, t := range s.Teams {
-		doCommand(chooseCommand(t, sOld), t.Baton.Holder)
-		moveBot(t.Baton.Holder, sOld)
-		maybePassBaton(&s.Teams[i])
+		s = doCommand(chooseCommand(s, i), s, i)
+		if b := activeBot(t); b != nil {
+			s = moveBot(s, i, *b)
+		}
+		s = maybePassBaton(s, i)
 	}
 
 	for _, t := range s.Teams {
-		if won(*t.Baton.Holder, s) {
+		if b := activeBot(t); b != nil && won(*b, s) {
 			s.GameOver = true
 		}
 	}
@@ -96,21 +105,57 @@ func UpdateState(sOld State) State {
 	return s
 }
 
-func maybePassBaton(t *Team) {
+func maybePassBaton(s State, teamID int) State {
+	t := s.Teams[teamID]
+	h := activeBot(t)
+	if h == nil {
+		return s
+	}
+
 	for i, b := range t.Bots {
-		h := t.Baton.Holder
-		if h.id >= b.id || h.Lane != b.Lane {
+		if h.ID >= b.ID || h.Lane != b.Lane {
 			continue
 		}
 		if abs(b.Pos-h.Pos) <= passDistance {
-			log.Printf("team %v pass from %v to %v!", t.id, h.id, b.id)
-			t.Baton.Holder.v = 0
-			t.Baton.Holder.a = 0
-			t.Baton.Holder = &t.Bots[i]
-			t.Bots[i].a = baseAccel
-			return
+			log.Printf("team %v pass from %v to %v!", teamID, h.ID, b.ID)
+			h.v = 0
+			h.a = 0
+			s = updateBot(s, teamID, *h)
+			newH := t.Bots[i]
+			newH.a = baseAccel
+			t.Baton.HolderID = newH.ID
+			s = updateTeam(s, t)
+			return updateBot(s, teamID, newH)
 		}
 	}
+
+	return s
+}
+
+func activeBot(t Team) *Bot {
+	for _, b := range t.Bots {
+		if b.ID == t.Baton.HolderID {
+			return &b
+		}
+	}
+	return nil
+}
+
+func updateBot(s State, teamID int, b Bot) State {
+	t := s.Teams[teamID]
+	for i, bb := range t.Bots {
+		if bb.ID == b.ID {
+			t.Bots = append(t.Bots[:i], append([]Bot{b}, t.Bots[i+1:]...)...)
+			break
+		}
+	}
+
+	return updateTeam(s, t)
+}
+
+func updateTeam(s State, t Team) State {
+	s.Teams = append(s.Teams[:t.id], append([]Team{t}, s.Teams[t.id+1:]...)...)
+	return s
 }
 
 func won(b Bot, s State) bool {

game/physics.go

@@ -1,6 +1,6 @@
 package game
 
-func accelerate(b *Bot) {
+func accelerate(b Bot) Bot {
 	if b.a < -maxA {
 		b.a = -maxA
 	}
@@ -15,14 +15,18 @@ func accelerate(b *Bot) {
 	if b.v < -maxV {
 		b.v = -maxV
 	}
+
+	return b
 }
 
-func moveBot(b *Bot, s State) {
+func moveBot(s State, teamID int, b Bot) State {
 	for i := 0; i < b.v; i++ {
 		if !collide(b.Pos+1, b.Lane, s) {
 			b.Pos++
 		}
 	}
+
+	return updateBot(s, teamID, b)
 }
 
 func collide(pos, lane int, s State) bool {

gfx/gfx.go

@@ -50,7 +50,7 @@ func renderBots(s game.State, w *pixelgl.Window, d time.Duration, colors map[*ga
 			im.Circle(botWidth, 0)
 
 			im.Draw(w)
-			if &t.Bots[j] == t.Baton.Holder {
+			if t.Bots[j].ID == t.Baton.HolderID {
 				renderBaton(pos, w)
 			}
 		}