feat: add protocol and reconciliation decisions

internal/domain/protocol.go

@@ -0,0 +1,28 @@
+package domain
+
+import (
+	"errors"
+
+	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
+)
+
+var ErrNoCompatibleProtocol = errors.New("no compatible protocol version")
+
+// SelectProtocol chooses the highest mutually supported minor version.
+func SelectProtocol(server, client *rvboxv1.ProtocolRange) (*rvboxv1.ProtocolVersion, error) {
+	if server == nil || client == nil || server.Major == 0 || client.Major == 0 || server.Major != client.Major || server.MinMinor > server.MaxMinor || client.MinMinor > client.MaxMinor {
+		return nil, ErrNoCompatibleProtocol
+	}
+	minimum := server.MinMinor
+	if client.MinMinor > minimum {
+		minimum = client.MinMinor
+	}
+	maximum := server.MaxMinor
+	if client.MaxMinor < maximum {
+		maximum = client.MaxMinor
+	}
+	if minimum > maximum {
+		return nil, ErrNoCompatibleProtocol
+	}
+	return &rvboxv1.ProtocolVersion{Major: server.Major, Minor: maximum}, nil
+}