1 files changed, 134 insertions, 0 deletions
diff --git a/internal/server/ping.go b/internal/server/ping.go
new file mode 100644
index 0000000..4783ec7
--- /dev/null
+++ b/internal/server/ping.go
@@ -0,0 +1,134 @@
+package server
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"time"
+)
+
+type pingResponse struct {
+	Version struct {
+		Name string `json:"name"`
+	} `json:"version"`
+	Players struct {
+		Online int `json:"online"`
+		Max    int `json:"max"`
+	} `json:"players"`
+	Description json.RawMessage `json:"description"`
+}
+
+func (p pingResponse) motdText() string {
+	var text string
+	if json.Unmarshal(p.Description, &text) == nil {
+		return text
+	}
+
+	var obj struct {
+		Text string `json:"text"`
+	}
+	if json.Unmarshal(p.Description, &obj) == nil {
+		return obj.Text
+	}
+	return ""
+}
+
+func pingServer(host string, port int, timeout time.Duration) (*pingResponse, error) {
+	conn, err := net.DialTimeout("tcp", fmt.Sprintf("%s:%d", host, port), timeout)
+	if err != nil {
+		return nil, err
+	}
+	defer conn.Close()
+	conn.SetDeadline(time.Now().Add(timeout))
+
+	var handshake bytes.Buffer
+	writeVarInt(&handshake, 0x00)
+	writeVarInt(&handshake, -1)
+	writeString(&handshake, host)
+	binary.Write(&handshake, binary.BigEndian, uint16(port))
+	writeVarInt(&handshake, 1)
+	if err := writePacket(conn, handshake.Bytes()); err != nil {
+		return nil, err
+	}
+
+	if err := writePacket(conn, []byte{0x00}); err != nil {
+		return nil, err
+	}
+
+	reader := bufio.NewReader(conn)
+	if _, err := readVarInt(reader); err != nil {
+		return nil, err
+	}
+	packetID, err := readVarInt(reader)
+	if err != nil {
+		return nil, err
+	}
+	if packetID != 0x00 {
+		return nil, fmt.Errorf("unexpected packet id %d", packetID)
+	}
+
+	jsonLen, err := readVarInt(reader)
+	if err != nil {
+		return nil, err
+	}
+	payload := make([]byte, jsonLen)
+	if _, err := io.ReadFull(reader, payload); err != nil {
+		return nil, err
+	}
+
+	var resp pingResponse
+	if err := json.Unmarshal(payload, &resp); err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}
+
+func writePacket(conn net.Conn, data []byte) error {
+	var buf bytes.Buffer
+	writeVarInt(&buf, len(data))
+	buf.Write(data)
+	_, err := conn.Write(buf.Bytes())
+	return err
+}
+
+func writeString(buf *bytes.Buffer, s string) {
+	writeVarInt(buf, len(s))
+	buf.WriteString(s)
+}
+
+func writeVarInt(buf *bytes.Buffer, value int) {
+	uv := uint32(value)
+	for {
+		if uv&^uint32(0x7F) == 0 {
+			buf.WriteByte(byte(uv))
+			return
+		}
+		buf.WriteByte(byte(uv&0x7F | 0x80))
+		uv >>= 7
+	}
+}
+
+func readVarInt(r io.ByteReader) (int, error) {
+	var result uint32
+	var shift uint
+	for {
+		b, err := r.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		result |= uint32(b&0x7F) << shift
+		if b&0x80 == 0 {
+			break
+		}
+		shift += 7
+		if shift >= 32 {
+			return 0, errors.New("varint too long")
+		}
+	}
+	return int(result), nil
+}