# cartographer/sensor/internal/collator.h

## Overview

`Collator` synchronizes sensor data from multiple sensors using an `OrderedMultiQueue`, then dispatches to a `TrajectoryBuilderInterface`. Provides higher-level interface than `OrderedMultiQueue`.

## Class: `cartographer::sensor::Collator`

### Construction

```cpp
Collator(
    const std::set<TrajectoryBuilderInterface::SensorId>& expected_sensor_ids,
    const std::function<void(int trajectory_id, 
                             std::unique_ptr<Data>)>& callback);
```

### Methods

```cpp
// Add sensor data for a trajectory
void AddSensorData(int trajectory_id, 
                   const std::string& sensor_id,
                   std::unique_ptr<Data> data);

// Mark trajectory as finished
void FinishTrajectory(int trajectory_id);

// Flush all pending data
void Flush();
```

### Sensor IDs

`expected_sensor_ids` defines valid sensor IDs for this collator:
```cpp
std::set<SensorId> sensors = {
  {"rangefinder", "laser"},
  {"imu", "imu"},
  {"odometer", "odom"}
};
```

## TrajectoryCollator

Manages multiple `Collator` instances (one per trajectory):

```cpp
class TrajectoryCollator {
 public:
  TrajectoryCollator(
      const std::function<void(int trajectory_id,
                               std::unique_ptr<Data>)>& callback);

  void AddTrajectory(int trajectory_id,
                     const std::set<SensorId>& expected_sensor_ids);
  
  void AddSensorData(int trajectory_id,
                     const std::string& sensor_id,
                     std::unique_ptr<Data> data);
  
  void FinishTrajectory(int trajectory_id);
  void Flush();
};
```

## Usage

```cpp
// In MapBuilder
auto collator = std::make_unique<Collator>(expected_sensors, 
    [this](int traj_id, std::unique_ptr<Data> data) {
      trajectory_builders_[traj_id]->AddSensorData(data->GetSensorId(), 
                                                   std::move(data));
    });

// From sensor driver
collator->AddSensorData(traj_id, "laser", 
    MakeDispatchable("laser", time, range_data));
collator->AddSensorData(traj_id, "imu", 
    MakeDispatchable("imu", time, imu_data));
```

## Configuration

Controlled by `MapBuilder` option:
```lua
MAP_BUILDER = {
  collate_by_trajectory = false,  -- Single collator for all trajectories
}
```

- `false` — Single `Collator` for all trajectories
- `true` — `TrajectoryCollator` with per-trajectory `Collator`

## See Also

- [OrderedMultiQueue](ordered_multi_queue.md) — Underlying queue
- [TrajectoryBuilderInterface](../../mapping/trajectory_builder_interface.md) — Receives collated data
- [MapBuilder](../../mapping/map_builder.md) — Creates collator