const updateGameState = () => {
  // If the user chose a valid direction update the snake direction.
  const chosenDirectionIsValid = state.chosenDirection !== undefined
    && !wouldEatItself(
      arenaSize,
      state.snake.body,
      state.chosenDirection
    );
  if (chosenDirectionIsValid) {
    state.snake.direction = state.chosenDirection;
  }
  state.chosenDirection = undefined;

  // If the snake stands still, the game is frozen.
  if (state.snake.direction === Direction.NONE) {
    return;
  }

  // If by moving the head we get a collision with the body,
  // the snake dies.
  const body = initial(state.snake.body);
  const head = last(state.snake.body);
  const newHead = moveTo(head, state.snake.direction, arenaSize);
  if (body.some(part => coincident(part, newHead))) {
    state.snake.status = Status.DEAD;
    return;
  }

  // If the snake is dead, the game is frozen.
  if (state.snake.status === Status.DEAD) {
    return;
  }

  // If the snake eats the food, consume it
  // and increment score and stocks.
  if (
    state.food !== undefined
    && coincident(newHead, state.food.coords)
  ) {
    state.score += state.food.strength;
    state.snake.stocks = state.food.strength;
    state.food = undefined;
  }

  // Make sure there is always some food to eat.
  if (state.food === undefined) {
    state.food = {
      coords: {
        x: random(0, arenaSize - 1),
        y: random(0, arenaSize - 1),
      },
      strength: random(1, 5),
    };
  }

  // Update the snake position and consume one stock, if available.
  const newBody = state.snake.stocks > 0
    ? state.snake.body
    : tail(state.snake.body);
  state.snake.body = [...newBody, newHead];
  state.snake.stocks = Math.max(0, state.snake.stocks - 1);
};