# Step 2 — LDF Loading & Display (C++)

## What Was Built

Custom LDF file parser, GUI table population, baud rate detection, schedule table loading, and auto-reload on file change — all in C++, matching the Python implementation feature-for-feature.

## Architecture

```
User clicks Browse → QFileDialog → filePath
                                      │
                                      ▼
                             loadLdfFile(path)
                                      │
                         ┌────────────┼────────────┐
                         ▼            ▼            ▼
                   parseLdf()    On error:    Setup file
                   (ldf_parser)  show dialog   watcher
                         │
                         ▼
                      LdfData
                    ┌────┼────────────┬──────────────┐
                    ▼    ▼            ▼              ▼
                 Baud  Tx table    Rx table      Schedule
                 rate  (master    (slave         dropdown
                 label  frames)    frames)
```

## Key Module: ldf_parser.h / ldf_parser.cpp

Custom parser (no third-party library). Parses LDF sections using QRegularExpression.

| C++ Struct | Python Equivalent | Purpose |
|-----------|-------------------|---------|
| `LdfData` | `LdfData` | Complete parsed result |
| `FrameInfo` | `FrameInfo` | One frame with signals |
| `SignalInfo` | `SignalInfo` | One signal's metadata |
| `ScheduleTableInfo` | `ScheduleTableInfo` | One schedule table |

### Python vs C++ Parser Differences

| Aspect | Python | C++ |
|--------|--------|-----|
| Library | `ldfparser` (3rd party) | Custom regex parser |
| Baud rate | `ldf.baudrate / 1000` (lib returns *1000) | Read raw value from file (already correct) |
| Frame classification | `isinstance(frame.publisher, LinMaster)` | `frame.publisher == masterName` (string compare) |
| Schedule delay | `entry.delay * 1000` (lib returns seconds) | Read raw ms value from file |
| Signal field name | `frame.signals` | `frame.signal_list` (Qt `signals` macro conflict) |

## C++ Concepts Introduced

### struct vs @dataclass
Python `@dataclass` → C++ `struct`. Both are data containers, but C++ structs have compile-time type enforcement.

### std::optional
`self._ldf_data: LdfData | None` → `std::optional<LdfData>`. Holds a value or is empty. Access with `->` and `*`.

### Raw string delimiters
When a regex contains `"` characters: `R"re(pattern with "quotes")re"` uses custom delimiter `re` so `"` inside is safe.

### Qt macro collision
`signals` is a Qt macro — can't use as a variable name. Renamed to `signal_list`.

### QTimer for test modal dialogs
Python uses `monkeypatch` to suppress QMessageBox. C++ uses `QTimer::singleShot(0, ...)` to auto-dismiss the dialog.

## Files

| File | Purpose |
|------|---------|
| `cpp/src/ldf_parser.h` | Data structs + parseLdf() declaration |
| `cpp/src/ldf_parser.cpp` | Custom LDF parser (regex-based) |
| `cpp/src/main_window.h` | Updated: LdfData, file watcher, new methods |
| `cpp/src/main_window.cpp` | Updated: loadLdfFile(), table population, auto-reload |
| `cpp/tests/test_ldf_parser.cpp` | 28 parser unit tests |
| `cpp/tests/test_ldf_loading.cpp` | 22 GUI integration tests |
| `cpp/CMakeLists.txt` | Updated: new sources + 2 new test targets |

## Test Results
- **84 total C++ tests passing** (Step 1: 34 + Step 2: 50)
- Parser tests: valid/invalid files, frame classification, signal extraction, schedules
- GUI tests: table population, baud rate, schedule combo, error handling, auto-reload