Add RetrieveOffset Method

transfer.go

@@ -13,10 +13,10 @@ import (
 
 // Retrieve file "path" from server and write bytes to "dest". If the
 // server supports resuming stream transfers, Retrieve will continue
-// resuming a failed download as long as it continues making progress.
-// Retrieve will also verify the file's size after the transfer if the
+// at the specified offset.
+// RetrieveOffset will also verify the file's size after the transfer if the
 // server supports the SIZE command.
-func (c *Client) Retrieve(path string, dest io.Writer) error {
+func (c *Client) RetrieveOffset(path string, dest io.Writer, bytesSoFar int64) error {
 	// fetch file size to check against how much we transferred
 	size, err := c.size(path)
 	if err != nil {
@@ -25,7 +25,6 @@ func (c *Client) Retrieve(path string, dest io.Writer) error {
 
 	canResume := c.canResume()
 
-	var bytesSoFar int64
 	for {
 		n, err := c.transferFromOffset(path, dest, nil, bytesSoFar)
 
@@ -53,6 +52,15 @@ func (c *Client) Retrieve(path string, dest io.Writer) error {
 	return nil
 }
 
+// Retrieve file "path" from server and write bytes to "dest". If the
+// server supports resuming stream transfers, Retrieve will continue
+// resuming a failed download as long as it continues making progress.
+// Retrieve will also verify the file's size after the transfer if the
+// server supports the SIZE command.
+func (c *Client) Retrieve(path string, dest io.Writer) error {
+	return c.RetrieveOffset(path, dest, 0)
+}
+
 // Store bytes read from "src" into file "path" on the server. If the
 // server supports resuming stream transfers and "src" is an io.Seeker
 // (*os.File is an io.Seeker), Store will continue resuming a failed upload

transfer_test.go

@@ -16,6 +16,35 @@ import (
 	"time"
 )
 
+func TestRetrieveOffset(t *testing.T) {
+	for _, addr := range ftpdAddrs {
+		c, err := DialConfig(goftpConfig, addr)
+
+		if err != nil {
+			t.Fatal(err)
+		}
+
+
+		buf := new(bytes.Buffer)
+
+		err = c.RetrieveOffset("subdir/1234.bin", buf, 1)
+
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if !bytes.Equal([]byte{2, 3, 4}, buf.Bytes()) {
+			t.Errorf("Got %v", buf.Bytes())
+		}
+
+		err = c.RetrieveOffset("subdir/1234.bin", buf, 5)
+
+		if err == nil {
+			t.Error("BytesSoFar is greater than the source File and should throw an error")
+		}
+	}
+}
+
 func TestRetrieve(t *testing.T) {
 	for _, addr := range ftpdAddrs {
 		c, err := DialConfig(goftpConfig, addr)