message_id.hpp

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#pragma once
 
#include <cstdint>
#include <type_traits>
 
namespace commrat {
 
// ============================================================================
// Message ID Structure: 0xPSMM
// P  = Prefix (1 byte)      - System(0x00) or UserDefined(0x01+)
// S  = SubPrefix (1 byte)   - Category within prefix
// MM = Message ID (2 bytes) - Specific message within category
// ============================================================================
 
enum class MessagePrefix : uint8_t {
    System = 0x00,       
    UserDefined = 0x01   
};
 
enum class SystemSubPrefix : uint8_t {
    Subscription = 0x00,  
    Control = 0x01,       
    Reserved = 0xFF       
};
 
enum class UserSubPrefix : uint8_t {
    Data = 0x00,          
    Commands = 0x01,      
    Events = 0x02,        
    Custom = 0x03         
};
 
constexpr uint32_t make_message_id(uint8_t prefix, uint8_t subprefix, uint16_t id) {
    return (static_cast<uint32_t>(prefix) << 24) |
           (static_cast<uint32_t>(subprefix) << 16) |
           static_cast<uint32_t>(id);
}
 
constexpr uint32_t system_message_id(SystemSubPrefix subprefix, uint16_t id) {
    return make_message_id(
        static_cast<uint8_t>(MessagePrefix::System),
        static_cast<uint8_t>(subprefix),
        id
    );
}
 
constexpr uint32_t user_message_id(UserSubPrefix subprefix, uint16_t id) {
    return make_message_id(
        static_cast<uint8_t>(MessagePrefix::UserDefined),
        static_cast<uint8_t>(subprefix),
        id
    );
}
 
// ============================================================================
// Message Definition - Compile-time message metadata
// ============================================================================
 
struct DefaultMessageDef {
    static constexpr MessagePrefix prefix = MessagePrefix::UserDefined;
    static constexpr UserSubPrefix user_subprefix = UserSubPrefix::Data;
    static constexpr SystemSubPrefix system_subprefix = SystemSubPrefix::Reserved;
    static constexpr uint16_t id = 0xFFFF;  // Auto-assign marker
};
 
template<
    typename PayloadT,
    MessagePrefix Prefix_ = DefaultMessageDef::prefix,
    auto SubPrefix_ = DefaultMessageDef::user_subprefix,  // auto to accept both enum types
    uint16_t ID_ = DefaultMessageDef::id
>
struct MessageDefinition {
    using Payload = PayloadT;
    static constexpr MessagePrefix prefix = Prefix_;
    static constexpr uint16_t local_id = ID_;
    
    // Extract subprefix value based on prefix type
    static constexpr uint8_t subprefix = []() constexpr {
        if constexpr (Prefix_ == MessagePrefix::System) {
            if constexpr (std::is_same_v<decltype(SubPrefix_), SystemSubPrefix>) {
                return static_cast<uint8_t>(SubPrefix_);
            } else {
                return static_cast<uint8_t>(DefaultMessageDef::system_subprefix);
            }
        } else {
            if constexpr (std::is_same_v<decltype(SubPrefix_), UserSubPrefix>) {
                return static_cast<uint8_t>(SubPrefix_);
            } else {
                return static_cast<uint8_t>(DefaultMessageDef::user_subprefix);
            }
        }
    }();
    
    // Full message ID (set during registry construction with auto-increment)
    static constexpr bool needs_auto_id = (ID_ == 0xFFFF);
};
 
// ============================================================================
// Request-Reply Pairing (RACK-style: reply = -request_id)
// ============================================================================
 
template<typename MessageDef>
struct Request : MessageDef {
    static constexpr bool is_request = true;
};
 
template<typename RequestMessageDef>
struct Reply {
    using Payload = typename RequestMessageDef::Payload;  // Can be different, override if needed
    static constexpr MessagePrefix prefix = RequestMessageDef::prefix;
    static constexpr uint8_t subprefix = RequestMessageDef::subprefix;
    
    // Reply ID is negative of request ID (in int16_t space)
    static constexpr uint16_t local_id = []() constexpr {
        int16_t signed_id = static_cast<int16_t>(RequestMessageDef::local_id);
        return static_cast<uint16_t>(-signed_id);
    }();
    
    static constexpr bool is_reply = true;
    static constexpr bool needs_auto_id = false;  // Always explicit from request
};
 
// ============================================================================
// Empty message support (commands with no payload)
// ============================================================================
 
struct EmptyPayload {};
 
} // namespace commrat