feat: Implement protocol handshake

server.cob

@@ -3,12 +3,17 @@ PROGRAM-ID. server.
 
 DATA DIVISION.
 WORKING-STORAGE SECTION.
-01 PORT             PIC X(5) VALUE "25565".
-01 LISTEN           PIC X(4).
-01 HNDL             PIC X(4).
-01 ERRNO            PIC 9(3) VALUE 0.
-*> State of the player (0 = handshake, 1 = status, 2 = login, 3 = play, 255 = disconnect)
-01 CLIENT-STATE     PIC 9(3) VALUE 0.
+    01 PORT             PIC X(5) VALUE "25565".
+    01 LISTEN           PIC X(4).
+    01 HNDL             PIC X(4).
+    01 ERRNO            PIC 9(3) VALUE 0.
+    *> State of the player (0 = handshake, 1 = status, 2 = login, 3 = play, 255 = disconnect)
+    01 CLIENT-STATE     PIC 9(3) VALUE 0.
+    *> Incoming packet data
+    01 BYTE-COUNT       PIC 9(5).
+    01 PACKET-LENGTH    PIC S9(10).
+    01 PACKET-ID        PIC S9(10).
+    01 BUFFER           PIC X(64000).
 
 PROCEDURE DIVISION.
 
@@ -34,15 +39,59 @@ AcceptConnection.
     STOP RUN.
 
 ReceivePacket SECTION.
-    *> TODO: Implement packet handling
+    *> Read packet length
+    CALL "Read-VarInt" USING HNDL ERRNO BYTE-COUNT PACKET-LENGTH.
+    PERFORM HandleError.
+
+    *> Read packet ID
+    CALL "Read-VarInt" USING HNDL ERRNO BYTE-COUNT PACKET-ID.
+    PERFORM HandleError.
+    SUBTRACT BYTE-COUNT FROM PACKET-LENGTH GIVING PACKET-LENGTH.
+
+    DISPLAY "[state=" CLIENT-STATE "] Received packet ID: " PACKET-ID " with length " PACKET-LENGTH " bytes.".
+
+    *> Handshake
+    IF CLIENT-STATE = 0 THEN
+        PERFORM HandleHandshake
+        EXIT SECTION
+    END-IF
+
+    *> TODO: Implement login state, play state, etc.
+    DISPLAY "Login state not implemented."
     MOVE 255 TO CLIENT-STATE.
 
     EXIT SECTION.
 
+HandleHandshake SECTION.
+    IF PACKET-ID NOT = 0 THEN
+        DISPLAY "  Unexpected packet ID: " PACKET-ID
+        MOVE 255 TO CLIENT-STATE
+        EXIT SECTION
+    END-IF
+
+    *> Read payload. The final byte encodes the target state.
+    MOVE PACKET-LENGTH TO BYTE-COUNT
+    CALL "Read-Raw" USING HNDL BYTE-COUNT ERRNO BUFFER
+    PERFORM HandleError
+    MOVE FUNCTION ORD(BUFFER(BYTE-COUNT:1)) TO CLIENT-STATE
+    SUBTRACT 1 FROM CLIENT-STATE
+
+    *> Validate target state
+    IF CLIENT-STATE NOT = 1 AND CLIENT-STATE NOT = 2 THEN
+        DISPLAY "  Invalid target state: " CLIENT-STATE
+        MOVE 255 TO CLIENT-STATE
+    ELSE
+        DISPLAY "  Target state: " CLIENT-STATE
+    END-IF
+
+    EXIT SECTION.
+
 HandleError SECTION.
     IF ERRNO NOT = 0 THEN
         DISPLAY "Error: " ERRNO
         STOP RUN
     END-IF.
 
+    EXIT SECTION.
+
 END PROGRAM server.

src/read.cob

@@ -0,0 +1,67 @@
+*> --- Read-Raw ---
+*> Read a raw byte array from the socket.
+IDENTIFICATION DIVISION.
+PROGRAM-ID. Read-Raw.
+
+DATA DIVISION.
+LINKAGE SECTION.
+01 LK-HNDL              PIC X(4).
+01 LK-READ-COUNT        PIC 9(5).
+01 LK-ERRNO             PIC 9(3).
+01 LK-VALUE             PIC X(64000).
+
+PROCEDURE DIVISION USING BY REFERENCE LK-HNDL LK-READ-COUNT LK-ERRNO LK-VALUE.
+    IF LK-READ-COUNT < 1
+        MOVE 0 TO LK-ERRNO
+        EXIT PROGRAM
+    END-IF
+    CALL "CBL_GC_SOCKET" USING "04" LK-HNDL LK-READ-COUNT LK-VALUE GIVING LK-ERRNO.
+
+END PROGRAM Read-Raw.
+
+*> --- Read-VarInt ---
+*> Read a VarInt from the socket into an S9(10) field.
+IDENTIFICATION DIVISION.
+PROGRAM-ID. Read-VarInt.
+
+DATA DIVISION.
+WORKING-STORAGE SECTION.
+    01 BUFFER               PIC X.
+    01 BYTE-COUNT            PIC 9(5).
+LOCAL-STORAGE SECTION.
+    01 VARINT-BYTE          PIC 9(3) COMP   VALUE 0.
+    01 VARINT-BYTE-VALUE    PIC 9(3) COMP   VALUE 0.
+    01 VARINT-MULTIPLIER    PIC 9(10) COMP  VALUE 1.
+    01 VARINT-CONTINUE      PIC 9 COMP      VALUE 1.
+LINKAGE SECTION.
+    01 LK-HNDL              PIC X(4).
+    01 LK-ERRNO             PIC 9(3).
+    01 LK-READ-COUNT        PIC 9(5).
+    01 LK-VALUE             PIC S9(10).
+
+PROCEDURE DIVISION USING BY REFERENCE LK-HNDL LK-ERRNO LK-READ-COUNT LK-VALUE.
+    MOVE 0 TO LK-VALUE.
+    MOVE 0 TO LK-READ-COUNT.
+    PERFORM UNTIL VARINT-CONTINUE = 0
+        *> Receive the next byte
+        MOVE 1 TO BYTE-COUNT
+        CALL "CBL_GC_SOCKET" USING "04" LK-HNDL BYTE-COUNT BUFFER GIVING LK-ERRNO
+        IF LK-ERRNO NOT = 0
+            EXIT PROGRAM
+        END-IF
+        ADD 1 TO LK-READ-COUNT
+        MOVE FUNCTION ORD(BUFFER(1:1)) TO VARINT-BYTE
+        SUBTRACT 1 FROM VARINT-BYTE
+        *> Extract the lower 7 bits
+        MOVE FUNCTION MOD(VARINT-BYTE, 128) TO VARINT-BYTE-VALUE
+        *> This yields the value when multiplied by the position multiplier
+        MULTIPLY VARINT-BYTE-VALUE BY VARINT-MULTIPLIER GIVING VARINT-BYTE-VALUE
+        ADD VARINT-BYTE-VALUE TO LK-VALUE
+        MULTIPLY VARINT-MULTIPLIER BY 128 GIVING VARINT-MULTIPLIER
+        *> Check if we need to continue (if the high bit is set and the maximum number of bytes has not been reached)
+        IF VARINT-BYTE < 128 OR LK-READ-COUNT >= 5
+            MOVE 0 TO VARINT-CONTINUE
+        END-IF
+    END-PERFORM.
+
+END PROGRAM Read-VarInt.

src/socket.cob

@@ -4,9 +4,9 @@ PROGRAM-ID. Socket-Listen.
 
 DATA DIVISION.
 LINKAGE SECTION.
-01 LK-PORT          PIC X(5).
-01 LK-LISTEN        PIC X(4).
-01 LK-ERRNO         PIC 9(3).
+    01 LK-PORT          PIC X(5).
+    01 LK-LISTEN        PIC X(4).
+    01 LK-ERRNO         PIC 9(3).
 
 PROCEDURE DIVISION USING BY REFERENCE LK-PORT LK-LISTEN LK-ERRNO.
     CALL "CBL_GC_SOCKET" USING "00" LK-PORT LK-LISTEN GIVING LK-ERRNO.
@@ -19,9 +19,9 @@ PROGRAM-ID. Socket-Accept.
 
 DATA DIVISION.
 LINKAGE SECTION.
-01 LK-LISTEN        PIC X(4).
-01 LK-HNDL          PIC X(4).
-01 LK-ERRNO         PIC 9(3).
+    01 LK-LISTEN        PIC X(4).
+    01 LK-HNDL          PIC X(4).
+    01 LK-ERRNO         PIC 9(3).
 
 PROCEDURE DIVISION USING BY REFERENCE LK-LISTEN LK-HNDL LK-ERRNO.
     CALL "CBL_GC_SOCKET" USING "07" LK-LISTEN LK-HNDL GIVING LK-ERRNO.
@@ -34,8 +34,8 @@ PROGRAM-ID. Socket-Close.
 
 DATA DIVISION.
 LINKAGE SECTION.
-01 LK-HNDL          PIC X(4).
-01 LK-ERRNO         PIC 9(3).
+    01 LK-HNDL          PIC X(4).
+    01 LK-ERRNO         PIC 9(3).
 
 PROCEDURE DIVISION USING BY REFERENCE LK-HNDL LK-ERRNO.
     CALL "CBL_GC_SOCKET" USING "06" LK-HNDL GIVING LK-ERRNO.