Terminology
===========

General
-------
**State**: The state of the vehicle. We define this to be x, y, yaw and speed.

**Control Action**: The control action to be executed by the vehicle. We define this to be steering wheel angle and wheel throttle.

**Action Trajectory**: An array of control actions, representing a temporal sequence of control actions to be executed every ``controller_period``.

**Dynamics Model**: A model that future state after a given time, given the current state and control action.

**Cost Function**: A function that evaluates the quality of a given state. The goal is to minimize this function.

**Cost-to-go**: The total cost of being in a given state and all the states that follow it.

**Controller Period**: The time interval between control actions.

**Perturbation**: Brownian noise centered at 0 that we add to a base action trajectory to generate a sample.

.. _twist:
**twist**: 6-dimensional velocity, 3 for linear velocity and 3 for angular velocity.

.. _pose:
**pose**: 6-dimensional position, 3 for translation and 3 for orientation.

**spline**: The line through the middle of the track boundaries, generated by |Path Planning|'s SVM algorithm.
Represented as an array of (x,y,z) points. Currently, the car is always fixed to be at point (0,0,0).

**callback** : A function that is called when an event occurs.

**Coordinate frames**:

- *inertial*: Standard coordinate frame, translated and rotated such that the car is at point (0,0,0) with yaw 0 being
the direction the car is facing. Otherwise known as *path planning* frame or erroneously *world* frame.
- *curvilinear*: Coordinate frame in relation to the spline. Coordinates represent distance along spline,
perpendicular distance from spline, angle with tangent of spline, etc. Needed for cost calculation.

**rollout**: In the context of MPPI, refers to state rollout. A sequence of predicted states calculated by repeated
application of the model on a given starting state and control action trajectory.

CUDA
----
**host**: The CPU

**device**: The GPU, or one of many.

``__device__``: Tag for function subroutines to be run exclusively on the GPU, or symbols to be statically allocated
in GPU global memory.

``__global__``: Tag for kernels: functions that are called by the host and run on the device.

``__constant__``: Tag for symbols in device memory that are unmodified by kernels . Can still copy to and from the host.
Read-only in CUDA code. Allows for better caching. Equivalent of ``const``. Constants for both the host and device
should be marked ``__constant__ const``.

ROS
---

**Node**: A process that performs computation. Nodes can communicate with each other by publishing messages to topics.

**Publisher**: A node that sends messages to a topic.

**Subscriber**: A node that receives messages from a topic.

**Topic**: A channel for sending messages. Nodes can publish messages to a topic or subscribe to a topic to receive messages.

Acronyms
--------
**MPPI**: Model Predictive Path Integral. See :doc:`here </source/explainers/mppi_algorithm>` for more information.

**ROS**: Robot Operating System. We are currently using ROS2 Humble_.

**CUDA**: Compute Unified Device Architecture. A toolkit provided by NVIDIA for parallel computing on GPUs.

**LIDAR**: Light Detection and Ranging. A sensor that uses laser light to measure distances to objects, in our case track cones.


.. _Humble: https://docs.ros.org/en/humble/index.html