add a silly test example

taskgroup_test.go

@@ -3,6 +3,7 @@ package taskgroup_test
 import (
 	"context"
 	"errors"
+	"fmt"
 	"math/rand/v2"
 	"reflect"
 	"sync"
@@ -413,3 +414,47 @@ func (p *peakValue) dec() {
 	p.cur--
 	p.μ.Unlock()
 }
+
+func TestTree(t *testing.T) {
+	defer leaktest.Check(t)()
+
+	vs := rand.Perm(1000)
+
+	g, run := taskgroup.New(nil).Limit(5)
+
+	type result [3]int
+	r := taskgroup.NewRunner[result](run)
+	r.Collect(func(v result) { t.Logf("+ %d at %d: %d", v[0], v[1], v[2]) })
+
+	for i := range vs {
+		r.Run(func() result {
+			// Find the location of i in the permutation.
+			for j, v := range vs {
+				if v != i {
+					continue
+				}
+
+				// Count the number of things less than i earlier in vs than i.
+				// Do this in the most inefficient possible way.
+				g, run := taskgroup.New(nil).Limit(5)
+				var countLess int
+				r := taskgroup.NewRunner[int](run).Collect(func(int) {
+					countLess++
+				})
+
+				for k := range j {
+					r.Call(func() (int, error) {
+						if vs[k] < v {
+							return k, nil
+						}
+						return -1, errors.New("no")
+					})
+				}
+				g.Wait()
+				return result{i, j, countLess}
+			}
+			panic(fmt.Sprintf("%d not found", i))
+		})
+	}
+	g.Wait()
+}