add download method

internal/rand.go

@@ -0,0 +1,28 @@
+/**
+ * @Author: fuxiao
+ * @Email: 576101059@qq.com
+ * @Date: 2021/8/26 4:51 下午
+ * @Desc: TODO
+ */
+
+package internal
+
+import (
+	"math/rand"
+	"time"
+)
+
+var seedStr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+
+// RandStr generate a string of specified length.
+func RandStr(length int) (lastStr string) {
+	rand.Seed(time.Now().UnixNano())
+	
+	pos, seedLen := 0, len(seedStr)
+	for i := 0; i < length; i++ {
+		pos = rand.Intn(seedLen)
+		lastStr += seedStr[pos : pos+1]
+	}
+	
+	return lastStr
+}