WIP: topological sort

engine/drawlist.go

@@ -1,6 +1,10 @@
 package engine
 
-import "github.com/hajimehoshi/ebiten/v2"
+import (
+	"errors"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
 
 const commonDrawerComparisons = false
 
@@ -73,3 +77,51 @@ func (d drawList) Swap(i, j int) {
 	d.rev[d.list[i]], d.rev[d.list[j]] = j, i
 	d.list[i], d.list[j] = d.list[j], d.list[i]
 }
+
+// Bad, slow, topological sort
+func (d *drawList) topsort() error {
+	// Count indegrees
+	indegree := make(map[Drawer]int)
+	for _, u := range d.list {
+		for _, v := range d.list {
+			if u == v {
+				continue
+			}
+			if u.DrawBefore(v) || v.DrawAfter(u) {
+				indegree[v]++
+			}
+		}
+	}
+	// Sort into new list
+	list := make([]Drawer, 0, len(d.list))
+	for len(indegree) > 0 {
+		var bag []Drawer
+		for v, n := range indegree {
+			if n == 0 {
+				bag = append(bag, v)
+				break
+			}
+		}
+		if len(bag) == 0 {
+			return errors.New("no vertices with zero indegree")
+		}
+		list = append(list, bag...)
+		for _, u := range bag {
+			delete(indegree, u)
+		}
+		for _, u := range bag {
+			for v := range indegree {
+				if u.DrawBefore(v) || v.DrawAfter(u) {
+					indegree[v]--
+				}
+			}
+		}
+	}
+	// Replace list
+	d.list = list
+	// Update rev
+	for i, v := range list {
+		d.rev[v] = i
+	}
+	return nil
+}

engine/game.go

@@ -8,7 +8,6 @@ import (
 	"io/fs"
 	"log"
 	"reflect"
-	"sort"
 	"sync"
 	"time"
 
@@ -183,7 +182,10 @@ func (g *Game) Update() error {
 	}
 
 	// Sort the draw list (on every frame - this isn't as bad as it sounds)
-	sort.Stable(g.drawList)
+	//sort.Stable(g.drawList)
+	if err := g.drawList.topsort(); err != nil {
+		return fmt.Errorf("drawList.topsort: %v", err)
+	}
 	// Truncate tombstones from the end.
 	for i := g.drawList.Len() - 1; i >= 0; i-- {
 		if g.drawList.list[i] != (tombstone{}) {