ADR 004: Split Signal Path Optimization (Branchless Signal Path)

Status

Accepted

Context

The historical Enigma Machine's signal path flows forward through rotors, hits a reflector, and returns in reverse. Initially, the system used a single transform(position, reverse) virtual method. Every character transformation required checking the reverse flag, introducing a "branch" in the CPU's execution pipeline millions of times per message.

Decision

We will use Dedicated Forward and Reverse Signal Paths by splitting the transformation into transformForward(position) and transformReverse(position) methods.

1. CPU Performance: Eliminates the conditional branch (if (reverse)) inside the tightest loop of the engine, improving CPU instruction pipelining and branch prediction.
2. Explicit Intent: Code reading the signal flow (in RotorBox::keyTransform) becomes more readable and explicit about which direction the signal is traveling.
3. Reflector Symmetry: Reflectors, which only have a "forward" entry in a physical machine, can explicitly return an error or a placeholder in the transformReverse path, making the implementation more faithful to the physical hardware.
4. Static Polymorphism Readiness: This split is a prerequisite for future C++20 template-based optimizations, where the compiler can inline the specific forward or reverse path logic at compile-time.

Consequences

• Virtual Table Size: Increases the size of the vtable for Transformer by adding two more virtual functions.
• Code Duplication: Some logic is duplicated between transformForward and transformReverse (coordinate shifts), but the performance gain in the hot path outweighs the maintenance cost.
• Method Maintenance: Any future change to the coordinate system or signal mapping must be applied consistently to both forward and reverse methods.