Feat:Add "copy" command

Add "copy" command,also fixes the out of range error of select when the number is negative.

Move execCopy to "keys.go" ,
Add test in "keys_test.go",
Add "copy.go" and "copy_test.go" file.

database/keys.go

@@ -11,6 +11,7 @@ import (
 	"github.com/hdt3213/godis/lib/wildcard"
 	"github.com/hdt3213/godis/redis/protocol"
 	"strconv"
+	"strings"
 	"time"
 )
 
@@ -308,6 +309,68 @@ func undoExpire(db *DB, args [][]byte) []CmdLine {
 	}
 }
 
+// execCopy usage: COPY source destination [DB destination-db] [REPLACE]
+// This command copies the value stored at the source key to the destination key.
+func execCopy(mdb *MultiDB, conn redis.Connection, args [][]byte) redis.Reply {
+	dbIndex := conn.GetDBIndex()
+	db := mdb.dbSet[dbIndex] // Current DB
+	replaceFlag := false
+	srcKey := string(args[0])
+	destKey := string(args[1])
+
+	// Parse options
+	if len(args) > 2 {
+		for i := 2; i < len(args); i++ {
+			arg := strings.ToLower(string(args[i]))
+			if arg == "db" {
+				if i+1 >= len(args) {
+					return &protocol.SyntaxErrReply{}
+				}
+				idx, err := strconv.Atoi(string(args[i+1]))
+				if err != nil {
+					return &protocol.SyntaxErrReply{}
+				}
+				if idx >= len(mdb.dbSet) || idx < 0 {
+					return protocol.MakeErrReply("ERR DB index is out of range")
+				}
+				dbIndex = idx
+				i++
+			} else if arg == "replace" {
+				replaceFlag = true
+			} else {
+				return &protocol.SyntaxErrReply{}
+			}
+		}
+	}
+
+	if srcKey == destKey && dbIndex == conn.GetDBIndex() {
+		return protocol.MakeErrReply("ERR source and destination objects are the same")
+	}
+
+	// source key does not exist
+	src, exists := db.GetEntity(srcKey)
+	if !exists {
+		return protocol.MakeIntReply(0)
+	}
+
+	destDB := mdb.dbSet[dbIndex]
+	if _, exists = destDB.GetEntity(destKey); exists != false {
+		// If destKey exists and there is no "replace" option
+		if replaceFlag == false {
+			return protocol.MakeIntReply(0)
+		}
+	}
+
+	destDB.PutEntity(destKey, src)
+	raw, exists := db.ttlMap.Get(srcKey)
+	if exists {
+		expire := raw.(time.Time)
+		destDB.Expire(destKey, expire)
+	}
+	mdb.aofHandler.AddAof(conn.GetDBIndex(), utils.ToCmdLine3("copy", args...))
+	return protocol.MakeIntReply(1)
+}
+
 func init() {
 	RegisterCommand("Del", execDel, writeAllKeys, undoDel, -2)
 	RegisterCommand("Expire", execExpire, writeFirstKey, undoExpire, 3)