# ADR 001: C++ for the Embedded Agents

- HTML version: https://robbiepalmer.me/projects/autonomic-satellite-swarm/adrs/001-c-plus-plus
- Project: Autonomic Satellite Swarm (https://robbiepalmer.me/projects/autonomic-satellite-swarm.md)
- Status: Accepted
- Date: 2016-07-01

# Context

Choosing the Arduino Uno constrains the software choice. Arduino sketches and the board's sensor,
infrared, timing, and actuator libraries are built for its C and C++ toolchain. The implementation
needs direct hardware access and predictable use of the Uno's 2 KB of SRAM, but the coordination
logic is complex enough to benefit from types and classes for agents, messages, and swarm state.

C would run on the target but would give up useful language support for those domain boundaries.
Python and other interpreted languages require a different class of board or an interpreter that
does not fit the Uno. Rust is young, has no mature Arduino or AVR workflow for this target, and
cannot use the required Arduino libraries without substantial integration work.

# Decision

Write the agent and swarm software in C++. Use the Arduino framework for board access and existing
device libraries. Keep the domain logic in ordinary C++ where practical so it can later compile on a
development computer as well as the Uno.

# Consequences

C++ gives the prototype direct access to the supported Arduino ecosystem and enough structure for
the state machine and message types. It also leaves memory ownership, integer width, and allocation
discipline to the programmer. The small target makes mistakes in those areas expensive.

# Status in 2026

Rust is still not a replacement for this target's supported workflow. The current Rust compiler
lists AVR as a [Tier 3 target](https://doc.rust-lang.org/rustc/platform-support/avr-none.html). It
requires an external AVR GCC toolchain and the unstable `build-std` feature, and the Rust project
does not build or test Tier 3 targets automatically. A future board can justify another language,
but the Uno reference target continues to drive a C++ implementation.

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