ichigo/engine/drawlist.go

package engine

import (
	"errors"

	"github.com/hajimehoshi/ebiten/v2"
)

const commonDrawerComparisons = false

var _ Drawer = tombstone{}

type tombstone struct{}

func (tombstone) Draw(*ebiten.Image, *ebiten.DrawImageOptions) {}

func (tombstone) DrawAfter(x Drawer) bool { return x != tombstone{} }
func (tombstone) DrawBefore(Drawer) bool  { return false }

type drawList struct {
	list []Drawer
	rev  map[Drawer]int
}

func (d drawList) Less(i, j int) bool {
	// Deal with tombstones first, in case anything else thinks it
	// needs to go last.
	if d.list[i] == (tombstone{}) {
		return false
	}
	if d.list[j] == (tombstone{}) {
		return true
	}

	if commonDrawerComparisons {
		// Common logic for known interfaces (BoundingBoxer, ZPositioner), to
		// simplify Draw{Before,After} implementations.
		switch x := d.list[i].(type) {
		case BoundingBoxer:
			xb := x.BoundingBox()
			switch y := d.list[j].(type) {
			case BoundingBoxer:
				yb := y.BoundingBox()
				if xb.Min.Z >= yb.Max.Z { // x is in front of y
					return false
				}
				if xb.Max.Z <= yb.Min.Z { // x is behind y
					return true
				}
				if xb.Max.Y <= yb.Min.Y { // x is above y
					return false
				}
				if xb.Min.Y >= yb.Max.Y { // x is below y
					return true
				}
			case ZPositioner:
				return xb.Max.Z < y.ZPos() // x is before y
			}

		case ZPositioner:
			switch y := d.list[j].(type) {
			case BoundingBoxer:
				return x.ZPos() < y.BoundingBox().Min.Z
			case ZPositioner:
				return x.ZPos() < y.ZPos()
			}
		}
	}

	// Fallback case: ask the components themselves
	return d.list[i].DrawBefore(d.list[j]) || d.list[j].DrawAfter(d.list[i])
}

func (d drawList) Len() int { return len(d.list) }

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
}