jce: optimize JceWriter_WriteMap.

name                  old time/op    new time/op    delta
JceWriter_WriteMap-8    4.09µs ± 3%    2.44µs ± 1%  -40.39%  (p=0.000 n=10+8)

name                  old speed      new speed      delta
JceWriter_WriteMap-8  22.8MB/s ± 3%  38.2MB/s ± 1%  +67.73%  (p=0.000 n=10+8)

name                  old alloc/op   new alloc/op   delta
JceWriter_WriteMap-8    2.34kB ± 0%    1.30kB ± 0%  -44.37%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
JceWriter_WriteMap-8      52.0 ± 0%      30.0 ± 0%  -42.31%  (p=0.000 n=10+10)

binary/jce/writer.go

@@ -160,11 +160,11 @@ func (w *JceWriter) WriteMap(m interface{}, tag int) {
 		return
 	}
 	w.writeHead(8, tag)
-	w.WriteInt32(int32(len(va.MapKeys())), 0)
-	for _, k := range va.MapKeys() {
-		v := va.MapIndex(k)
-		w.WriteObject(k.Interface(), 0)
-		w.WriteObject(v.Interface(), 1)
+	w.WriteInt32(int32(va.Len()), 0)
+	iter := va.MapRange()
+	for iter.Next() {
+		w.WriteObject(iter.Key().Interface(), 0)
+		w.WriteObject(iter.Value().Interface(), 1)
 	}
 }
 

binary/jce/writer_test.go

@@ -0,0 +1,16 @@
+package jce
+
+import "testing"
+
+var globalBytes []byte
+
+func BenchmarkJceWriter_WriteMap(b *testing.B) {
+	var x = globalBytes
+	for i := 0; i < b.N; i++ {
+		w := NewJceWriter()
+		w.WriteMap(req.Map, 0)
+		x = w.Bytes()
+	}
+	globalBytes = x
+	b.SetBytes(int64(len(globalBytes)))
+}