Add GoogleTest harness with 22 unit tests

.gitignore

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+build/

README.md

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+# MakerPlane CAN-FIX Arduino Library
+
+**Status:** Open Source — Experimental Amateur-Built Category  
+**License:** See COPYING  
+**Protocol:** CAN-FIX (CAN bus implementation of the Flight Information eXchange protocol)
+
+---
+
+## What This Is
+
+This is an Arduino library that enables Arduino-based devices to participate in a **CAN-FIX avionics network**. CAN-FIX is an open-source protocol designed specifically for Experimental Amateur-Built (E-AB) aircraft to exchange flight data between avionics nodes in a vendor-neutral way.
+
+With this library, Arduino hardware can:
+- Read and write named flight data parameters (airspeed, altitude, heading, engine data, etc.) on the CAN bus
+- Act as a sensor node, sending data from connected hardware into the avionics network
+- Act as an actuator or display node, receiving and responding to parameter updates
+- Interoperate with other CAN-FIX devices including FIX-Gateway, pyEfis, and custom electronics
+
+## Protocol Background
+
+**FIX** (Flight Information eXchange) is a family of open, Creative Commons-licensed specifications for aircraft avionics communication. **CAN-FIX** is the CAN bus implementation of FIX. It is designed to:
+
+- Allow any builder to construct devices that communicate with other FIX-compatible hardware without licensing fees
+- Provide a standard parameter namespace covering the full range of aircraft state (position, attitude, airspeed, engine data, control surfaces, systems status)
+- Enable redundancy through multiple nodes publishing the same parameter type on separate identifiers
+
+See the [canfix-spec](../canfix-spec) repository for the full protocol specification.
+
+## Repository Contents
+
+| File | Description |
+|---|---|
+| `canfix.h` | Library header — parameter definitions, message structures, node API |
+| `canfix.cpp` | Library implementation |
+| `examples/` | Example Arduino sketches demonstrating common use patterns |
+| `keywords.txt` | Arduino IDE syntax highlighting keywords |
+
+## Installation
+
+Follow standard Arduino library installation:
+1. Download or clone this repository
+2. Copy the folder into your Arduino `libraries/` directory
+3. Restart the Arduino IDE
+4. Access the library under **Sketch → Include Library → CAN-FIX**
+
+For detailed installation guidance see: http://arduino.cc/en/Guide/Libraries
+
+## Hardware Requirements
+
+- Any Arduino board with a CAN controller, or an Arduino paired with a CAN transceiver module (MCP2515-based shields are common in the MakerPlane ecosystem)
+- CAN bus wiring per the CAN-FIX physical layer spec (120Ω termination at each end)
+
+## Important Disclaimer
+
+> **This library is experimental and is not suited for primary or backup flight/engine instrumentation or navigation. Use at your own risk.**  
+> For Experimental Amateur-Built aircraft use only. Not FAA-approved avionics software.

tests/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+project(canfix_tests CXX)
+
+set(CMAKE_CXX_STANDARD 14)
+
+# ── GoogleTest via FetchContent ───────────────────────────────────────────────
+include(FetchContent)
+FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG        release-1.12.1
+    GIT_SHALLOW    TRUE
+)
+# Prevent overriding the parent project's compiler/linker settings on Windows
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_GetProperties(googletest)
+if(NOT googletest_POPULATED)
+    FetchContent_Populate(googletest)
+    add_subdirectory(${googletest_SOURCE_DIR} ${googletest_BINARY_DIR})
+endif()
+
+# ── canfix library (compiled with mock headers) ───────────────────────────────
+add_library(canfix_lib STATIC
+    ../canfix.cpp
+    mock_impl.cpp
+)
+target_include_directories(canfix_lib PUBLIC
+    ${CMAKE_CURRENT_SOURCE_DIR}/mocks   # provides can.h, EEPROM.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/..      # provides canfix.h
+)
+
+# ── Test executable ───────────────────────────────────────────────────────────
+add_executable(canfix_tests
+    test_canfix.cpp
+)
+target_include_directories(canfix_tests PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}/mocks
+    ${CMAKE_CURRENT_SOURCE_DIR}/..
+)
+target_link_libraries(canfix_tests
+    canfix_lib
+    GTest::gtest_main
+)
+
+# ── CTest integration ─────────────────────────────────────────────────────────
+include(GoogleTest)
+gtest_discover_tests(canfix_tests)

tests/mock_impl.cpp

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+/*
+ * Global mock object instances — compiled once, linked with all test objects.
+ */
+#include "mocks/can.h"
+#include "mocks/EEPROM.h"
+
+MockCanState g_can;
+EEPROMClass  EEPROM;

tests/mocks/EEPROM.h

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+/*
+ * Mock EEPROM.h for unit testing canfix.cpp outside of the Arduino environment.
+ * Simulates a 512-byte EEPROM with read/write and the bitRead/bitSet/bitClear macros.
+ */
+#pragma once
+#include <cstdint>
+#include <cstring>
+
+#define EEPROM_SIZE 512
+
+struct EEPROMClass {
+    uint8_t data[EEPROM_SIZE];
+
+    EEPROMClass() { memset(data, 0xFF, sizeof(data)); }
+
+    uint8_t read(int addr)             { return data[addr]; }
+    void    write(int addr, uint8_t v) { data[addr] = v; }
+
+    void reset() { memset(data, 0xFF, sizeof(data)); }
+};
+
+extern EEPROMClass EEPROM;
+
+// Arduino bit manipulation macros
+#define bitRead(value, bit)       (((value) >> (bit)) & 0x01)
+#define bitSet(value, bit)        ((value) |=  (1UL << (bit)))
+#define bitClear(value, bit)      ((value) &= ~(1UL << (bit)))
+#define bitWrite(value, bit, bv)  ((bv) ? bitSet(value, bit) : bitClear(value, bit))

tests/mocks/can.h

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+/*
+ * Mock can.h for unit testing canfix.cpp outside of the Arduino environment.
+ * Records last frame written so tests can inspect it.
+ */
+#pragma once
+#include <cstdint>
+
+typedef uint8_t  byte;
+typedef uint16_t word;
+
+struct CanFrame {
+    uint16_t id;
+    uint8_t  eid;
+    uint8_t  data[8];
+    uint8_t  length;
+};
+
+// Commands and modes used by canfix.cpp
+#define CMD_RESET     0
+#define MODE_CONFIG   1
+#define MODE_NORMAL   2
+
+// Recorded state for test inspection
+struct MockCanState {
+    CanFrame last_written;
+    int      write_count;
+    uint8_t  rx_status;       // returned by getRxStatus()
+    CanFrame rx_frame[2];     // frame available for reading
+
+    MockCanState() : write_count(0), rx_status(0) {
+        last_written = {};
+        rx_frame[0] = {};
+        rx_frame[1] = {};
+    }
+};
+
+extern MockCanState g_can;
+
+class CAN {
+public:
+    explicit CAN(uint8_t pin) { (void)pin; }
+    void sendCommand(uint8_t cmd) { (void)cmd; }
+    void setBitRate(int rate) { (void)rate; }
+    void setMode(uint8_t mode) { (void)mode; }
+
+    uint8_t writeFrame(CanFrame frame) {
+        g_can.last_written = frame;
+        g_can.write_count++;
+        return 0; // success
+    }
+
+    uint8_t getRxStatus() { return g_can.rx_status; }
+
+    CanFrame readFrame(uint8_t buffer) {
+        return g_can.rx_frame[buffer];
+    }
+};