add subset algorithm

rpcx/internal/resolver/resolver.go

@@ -12,6 +12,7 @@ import (
 const (
 	DiscovScheme = "discov"
 	EndpointSep  = ","
+	subsetSize   = 36
 )
 
 var builder discovBuilder
@@ -36,7 +37,7 @@ func (b *discovBuilder) Build(target resolver.Target, cc resolver.ClientConn, op
 
 	update := func() {
 		var addrs []resolver.Address
-		for _, val := range sub.Values() {
+		for _, val := range subset(sub.Values(), subsetSize) {
 			addrs = append(addrs, resolver.Address{
 				Addr: val,
 			})

rpcx/internal/resolver/subset.go

@@ -0,0 +1,14 @@
+package resolver
+
+import "math/rand"
+
+func subset(set []string, sub int) []string {
+	rand.Shuffle(len(set), func(i, j int) {
+		set[i], set[j] = set[j], set[i]
+	})
+	if len(set) <= sub {
+		return set
+	} else {
+		return set[:sub]
+	}
+}

rpcx/internal/resolver/subset_test.go

@@ -0,0 +1,65 @@
+package resolver
+
+import (
+	"strconv"
+	"testing"
+
+	"zero/core/mathx"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSubset(t *testing.T) {
+	tests := []struct {
+		name string
+		set  int
+		sub  int
+	}{
+		{
+			name: "more vals to subset",
+			set:  100,
+			sub:  36,
+		},
+		{
+			name: "less vals to subset",
+			set:  100,
+			sub:  200,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var vals []string
+			for i := 0; i < test.set; i++ {
+				vals = append(vals, strconv.Itoa(i))
+			}
+
+			m := make(map[interface{}]int)
+			for i := 0; i < 1000; i++ {
+				set := subset(append([]string(nil), vals...), test.sub)
+				for _, val := range set {
+					m[val]++
+				}
+			}
+
+			assert.True(t, mathx.CalcEntropy(m) > 0.95)
+		})
+	}
+}
+
+func TestSubsetLess(t *testing.T) {
+	var vals []string
+	for i := 0; i < 100; i++ {
+		vals = append(vals, strconv.Itoa(i))
+	}
+
+	m := make(map[interface{}]int)
+	for i := 0; i < 1000; i++ {
+		set := subset(append([]string(nil), vals...), 200)
+		for _, val := range set {
+			m[val]++
+		}
+	}
+
+	assert.True(t, mathx.CalcEntropy(m) > 0.95)
+}