ftp: cosmetic fixes

Use single quotes for strings.
Use string interpolation instead of concatenation.
Don't print messages in non-debug mode.
Fix typos in error messages.
Add space after comma, add spaces between operators.
extract code to separate functions

vlib/ftp/ftp.v

@@ -75,8 +75,8 @@ fn (ftp FTP) write(data string) ?int {
 	$if debug {
 		println('FTP.v >>> $data')
 	}
-	n := ftp.sock.send_string(data + '\n') or {
-		return error('cannot send data')
+	n := ftp.sock.send_string('$data\n') or {
+		return error('Cannot send data')
 	}
 	return n
 }
@@ -119,9 +119,10 @@ pub fn (ftp mut FTP) connect(ip string) bool {
 }
 
 pub fn (ftp FTP) login(user, passwd string) bool {
-
-	ftp.write('USER '+user) or {
+	ftp.write('USER $user') or {
+		$if debug {
 			println('ERROR sending user')
+		}
 		return false
 	}
 
@@ -137,8 +138,10 @@ pub fn (ftp FTP) login(user, passwd string) bool {
 		return false
 	}
 
-	ftp.write('PASS '+passwd) or {
+	ftp.write('PASS $passwd') or {
+		$if debug {
 			println('ERROR sending password')
+		}
 		return false
 	}
 
@@ -174,7 +177,9 @@ pub fn (ftp FTP) cd(dir string) {
 	mut code, mut data := ftp.read()
 	match code {
 		Denied {
-			println("CD $dir denied!")
+			$if debug {
+				println('CD $dir denied!')
+			}
 		}
 		Complete {
 			code, data = ftp.read()
@@ -182,23 +187,20 @@ pub fn (ftp FTP) cd(dir string) {
 		else {}
 	}
 
-	println('cd $data')
+	$if debug {
+		println('CD $data')
+	}
 }
 
 fn new_dtp(msg string) ?DTP {
-	// it receives a control message 227 like: 
-	// '227 Entering Passive Mode (209,132,183,61,48,218)'
-
-	if !msg.contains('(') || !msg.contains(')') || !msg.contains(',') {
-		return error('bad message')
+	if !is_dtp_message_valid(msg) {
+		return error('Bad message')
 	}
 
-	t := msg.split('(')[1].split(')')[0].split(',')
-	ip := t[0]+'.'+t[1]+'.'+t[2]+'.'+t[3]
-	port := t[4].int()*256+t[5].int()
+	ip, port := get_host_ip_from_dtp_message(msg)
 
 	sock := net.dial(ip, port) or {
-		return error('Cant connect to the data channel')
+		return error('Cannot connect to the data channel')
 	}
 
 	dtp := DTP {
@@ -212,7 +214,9 @@ fn new_dtp(msg string) ?DTP {
 fn (ftp FTP) pasv() ?DTP {
 	ftp.write('PASV') or {}
 	code, data := ftp.read()
-	println("pass: $data")
+	$if debug {
+		println('pass: $data')
+	}
 
 	if code != PassiveMode {
 		return error('pasive mode not allowed')
@@ -231,7 +235,7 @@ pub fn (ftp FTP) dir() ?[]string {
 	ftp.write('LIST') or {}
 	code, _ := ftp.read()
 	if code == Denied {
-		return error('list denied')
+		return error('LIST denied')
 	}
 	if code != OpenDataConnection {
 		return error('data channel empty')
@@ -258,18 +262,18 @@ pub fn (ftp FTP) dir() ?[]string {
 
 pub fn (ftp FTP) get(file string) ?[]byte {
 	dtp := ftp.pasv() or {
-		return error('cant stablish data connection')
+		return error('Cannot stablish data connection')
 	}
 
 	ftp.write('RETR $file') or {}
 	code, _ := ftp.read()
 
 	if code == Denied {
-		return error('permission denied')
+		return error('Permission denied')
 	}
 
 	if code != OpenDataConnection {
-		return error('data connection not ready')
+		return error('Data connection not ready')
 	}
 
 	blob := dtp.read()
@@ -277,3 +281,28 @@ pub fn (ftp FTP) get(file string) ?[]byte {
 
 	return blob
 }
+
+fn is_dtp_message_valid(msg string) bool {
+	// An example of message:
+	// '227 Entering Passive Mode (209,132,183,61,48,218)'
+	return msg.contains('(') && msg.contains(')') && msg.contains(',')
+}
+
+fn get_host_ip_from_dtp_message(msg string) (string, int) {
+	mut par_start_idx := -1
+	mut par_end_idx := -1
+
+	for i, c in msg {
+		if c == `(` {
+			par_start_idx = i + 1
+		} else if c == `)` {
+			par_end_idx = i
+		}
+	}
+	data := msg[par_start_idx..par_end_idx].split(',')
+
+	ip := data[0..4].join('.')
+	port := data[4].int() * 256 + data[5].int()
+
+	return ip, port
+}