Add implementation of the SETNX command

This is an optimized "SET if not exists" command,
which basically does an EXISTS call before the SET.

README.md

@@ -34,6 +34,7 @@ Here's a full example of a Redis clone that accepts:
 
 - SET key value
 - GET key
+- SETNX key value
 - DEL key
 - PING
 - QUIT
@@ -96,6 +97,22 @@ func main() {
 				} else {
 					conn.WriteBulk(val)
 				}
+			case "setnx":
+				if len(cmd.Args) != 3 {
+					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")
+					return
+				}
+				mu.RLock()
+				_, ok := items[string(cmd.Args[1])]
+				mu.RUnlock()
+				if ok {
+					conn.WriteInt(0)
+					return
+				}
+				mu.Lock()
+				items[string(cmd.Args[1])] = cmd.Args[2]
+				mu.Unlock()
+				conn.WriteInt(1)
 			case "del":
 				if len(cmd.Args) != 2 {
 					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")

example/clone.go

@@ -82,6 +82,22 @@ func main() {
 				} else {
 					conn.WriteBulk(val)
 				}
+			case "setnx":
+				if len(cmd.Args) != 3 {
+					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")
+					return
+				}
+				mu.RLock()
+				_, ok := items[string(cmd.Args[1])]
+				mu.RUnlock()
+				if ok {
+					conn.WriteInt(0)
+					return
+				}
+				mu.Lock()
+				items[string(cmd.Args[1])] = cmd.Args[2]
+				mu.Unlock()
+				conn.WriteInt(1)
 			case "del":
 				if len(cmd.Args) != 2 {
 					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")

example/mux/clone.go

@@ -19,6 +19,7 @@ func main() {
 	mux.HandleFunc("quit", handler.quit)
 	mux.HandleFunc("set", handler.set)
 	mux.HandleFunc("get", handler.get)
+	mux.HandleFunc("setnx", handler.setnx)
 	mux.HandleFunc("del", handler.delete)
 
 	err := redcon.ListenAndServe(addr,

example/mux/handler.go

@@ -68,6 +68,28 @@ func (h *Handler) get(conn redcon.Conn, cmd redcon.Command) {
 	}
 }
 
+func (h *Handler) setnx(conn redcon.Conn, cmd redcon.Command) {
+	if len(cmd.Args) != 3 {
+		conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")
+		return
+	}
+
+	h.itemsMux.RLock()
+	_, ok := h.items[string(cmd.Args[1])]
+	h.itemsMux.RUnlock()
+
+	if ok {
+		conn.WriteInt(0)
+		return
+	}
+
+	h.itemsMux.Lock()
+	h.items[string(cmd.Args[1])] = cmd.Args[2]
+	h.itemsMux.Unlock()
+
+	conn.WriteInt(1)
+}
+
 func (h *Handler) delete(conn redcon.Conn, cmd redcon.Command) {
 	if len(cmd.Args) != 2 {
 		conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")

example/tls/clone.go

@@ -88,6 +88,22 @@ func main() {
 				} else {
 					conn.WriteBulk(val)
 				}
+			case "setnx":
+				if len(cmd.Args) != 3 {
+					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")
+					return
+				}
+				mu.RLock()
+				_, ok := items[string(cmd.Args[1])]
+				mu.RUnlock()
+				if ok {
+					conn.WriteInt(0)
+					return
+				}
+				mu.Lock()
+				items[string(cmd.Args[1])] = cmd.Args[2]
+				mu.Unlock()
+				conn.WriteInt(1)
 			case "del":
 				if len(cmd.Args) != 2 {
 					conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")