4 files changed, 154 insertions, 0 deletions
diff --git a/internal/.gitkeep b/internal/.gitkeep
deleted file mode 100644
index e69de29..0000000
--- a/internal/.gitkeep
+++ /dev/null
diff --git a/internal/conn/conn.go b/internal/conn/conn.go
new file mode 100644
index 0000000..a1da6f1
--- /dev/null
+++ b/internal/conn/conn.go
@@ -0,0 +1,50 @@
+package conn
+
+import (
+	"context"
+	"io"
+	"net"
+	"time"
+)
+
+type Conn struct {
+	netConn net.Conn
+}
+
+// FromNetConn wraps an existing net.Conn. Useful for tests.
+func FromNetConn(n net.Conn) *Conn { return &Conn{netConn: n} }
+
+func Dial(ctx context.Context, addr string, timeout time.Duration) (*Conn, error) {
+	d := net.Dialer{Timeout: timeout}
+	c, err := d.DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return nil, err
+	}
+	return &Conn{netConn: c}, nil
+}
+
+func (c *Conn) Close() error {
+	if c.netConn == nil {
+		return nil
+	}
+	return c.netConn.Close()
+}
+
+func (c *Conn) Send(ctx context.Context, data []byte, timeout time.Duration) error {
+	if dl, ok := ctx.Deadline(); ok {
+		c.netConn.SetWriteDeadline(dl)
+	} else {
+		c.netConn.SetWriteDeadline(time.Now().Add(timeout))
+	}
+	_, err := c.netConn.Write(data)
+	return err
+}
+
+func (c *Conn) Receive(ctx context.Context, timeout time.Duration) ([]byte, error) {
+	if dl, ok := ctx.Deadline(); ok {
+		c.netConn.SetReadDeadline(dl)
+	} else {
+		c.netConn.SetReadDeadline(time.Now().Add(timeout))
+	}
+	return io.ReadAll(c.netConn)
+}
diff --git a/internal/proto/proto.go b/internal/proto/proto.go
new file mode 100644
index 0000000..d86ce75
--- /dev/null
+++ b/internal/proto/proto.go
@@ -0,0 +1,65 @@
+package proto
+
+import (
+	"encoding/base64"
+	"fmt"
+	"sort"
+	"strings"
+	"unicode"
+
+	"golang.org/x/text/encoding/charmap"
+)
+
+// EncodeParams converts params map into a sorted base64-encoded string using Windows-1251 encoding.
+func EncodeParams(params map[string]string) (string, error) {
+	keys := make([]string, 0, len(params))
+	for k := range params {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+
+	var sb strings.Builder
+	for i, k := range keys {
+		if i > 0 {
+			sb.WriteByte('|')
+		}
+		sb.WriteString(k)
+		sb.WriteByte('=')
+		sb.WriteString(params[k])
+	}
+	sb.WriteByte('|')
+
+	enc := charmap.Windows1251.NewEncoder()
+	encoded, err := enc.String(sb.String())
+	if err != nil {
+		return "", fmt.Errorf("encode params: %w", err)
+	}
+	return base64.StdEncoding.EncodeToString([]byte(encoded)), nil
+}
+
+// DecodeResponse decodes base64-encoded Windows-1251 text to UTF-8 and removes control characters.
+func DecodeResponse(data string) (string, error) {
+	raw, err := base64.StdEncoding.DecodeString(strings.TrimSpace(data))
+	if err != nil {
+		return "", fmt.Errorf("base64 decode: %w", err)
+	}
+	decoded, err := charmap.Windows1251.NewDecoder().Bytes(raw)
+	if err != nil {
+		return "", fmt.Errorf("decode charset: %w", err)
+	}
+	cleaned := strings.Map(func(r rune) rune {
+		if unicode.IsPrint(r) || r == '\n' || r == '\r' || r == '\t' {
+			return r
+		}
+		return -1
+	}, string(decoded))
+	return cleaned, nil
+}
+
+// BuildRequest returns byte slice representing the command and parameters.
+func BuildRequest(command, encodedParams string, quit bool) []byte {
+	if quit {
+		return []byte(fmt.Sprintf("%s %s\nQUIT\n", command, encodedParams))
+	}
+	return []byte(fmt.Sprintf("%s %s\n", command, encodedParams))
+}
diff --git a/internal/proto/proto_test.go b/internal/proto/proto_test.go
new file mode 100644
index 0000000..0e7e644
--- /dev/null
+++ b/internal/proto/proto_test.go
@@ -0,0 +1,39 @@
+package proto
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestEncodeParamsOrder(t *testing.T) {
+	params := map[string]string{"B": "2", "A": "1"}
+	encoded1, err := EncodeParams(params)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// encode again with different map order
+	encoded2, err := EncodeParams(map[string]string{"A": "1", "B": "2"})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if encoded1 != encoded2 {
+		t.Fatalf("expected deterministic encode, got %s vs %s", encoded1, encoded2)
+	}
+}
+
+func TestDecodeResponse(t *testing.T) {
+	// "привет" in Cyrillic
+	original := "привет"
+	params := map[string]string{"MSG": original}
+	enc, err := EncodeParams(params)
+	if err != nil {
+		t.Fatalf("encode params: %v", err)
+	}
+	dec, err := DecodeResponse(enc)
+	if err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if !strings.Contains(dec, original) {
+		t.Fatalf("expected to contain %q, got %q", original, dec)
+	}
+}