Top command add

* invalid parameter tests added

pop3/transaction.go

@@ -1,6 +1,7 @@
 package pop3
 
 import (
+	"fmt"
 	"strconv"
 	"strings"
 )
@@ -395,3 +396,24 @@ func (c *Client) pass(password string) (string, error) {
 
 	return passResp, nil
 }
+
+func (c *Client) Top(msgNum, n int) ([]string, error) {
+	return c.top(msgNum, n)
+}
+
+func (c *Client) top(msgNum, n int) ([]string, error) {
+	if msgNum < 1 {
+		return nil, fmt.Errorf("%s message number should be greater than 0", e)
+	}
+	if n < 0 {
+		return nil, fmt.Errorf("%s line count cannot be negative", e)
+	}
+	cmd := "TOP"
+	arg := fmt.Sprintf("%d %d", msgNum, n)
+	err := c.sendCmdWithArg(cmd, arg)
+	if err != nil {
+		return nil, err
+	}
+
+	return c.readRespMultiLines()
+}

pop3/transaction_test.go

@@ -1,6 +1,7 @@
 package pop3
 
 import (
+	"log"
 	"math"
 	"os"
 	"strconv"
@@ -492,3 +493,59 @@ func TestRset(t *testing.T) {
 		t.Errorf("expected prefix: %s, got: %s", ok, r)
 	}
 }
+
+func TestTopNegativeMsgNum(t *testing.T) {
+	exp := "-ERR message number should be greater than 0"
+	pop, err := Connect(gmailTLSAddr, nil, true)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	log.Println("Connection established")
+
+	msgNum := -1
+	top, err := pop.Top(msgNum, 10)
+	if top != nil {
+		t.Errorf("need to be nil")
+	}
+	if !strings.HasPrefix(err.Error(), exp) {
+		t.Errorf(err.Error())
+	}
+	log.Println(err.Error())
+
+	quit, err := pop.Quit()
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !strings.HasPrefix(quit, ok) {
+		t.Errorf(quit)
+	}
+	log.Println("Connection closed")
+}
+
+func TestTopNegativeN(t *testing.T) {
+	exp := "-ERR line count cannot be negative"
+	pop, err := Connect(gmailTLSAddr, nil, true)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	log.Println("Connection established")
+
+	n := -1
+	top, err := pop.Top(1, n)
+	if top != nil {
+		t.Errorf("need to be nil")
+	}
+	if !strings.HasPrefix(err.Error(), exp) {
+		t.Errorf(err.Error())
+	}
+	log.Println(err.Error())
+
+	quit, err := pop.Quit()
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !strings.HasPrefix(quit, ok) {
+		t.Errorf(quit)
+	}
+	log.Println("Connection closed")
+}