Count the number of squares on a rectangular grid visited by a cleaning robot. The robot only moves forward, turning right if it cannot make a move.

Task description

There is a cleaning robot which is cleaning a rectangular grid of size N x M$,$ represented by array R consisting of N strings.

Rows are numbered from $0$ to N$1($from top to bottom$)$ and columns are numbered from $0$ to M$1($from left to right$).$

The robot starts cleaning in the top$-$left corner, facing rightwards.

It moves in a straight line for as long as it can, in other words, while there is an unoccupied grid square ahead of it$.$

When it cannot move forward, it rotates $90$ degrees clockwise and tries to move forward again until it encounters another obstacle, and so on$.$

Dots in the array $(".")$ represent empty squares and $"$X$"$s represent occupied squares $($ones the robot cannot move through$).$

Each square that the robot occupied at least once is considered clean. The robot moves indefinitely.

Write a function:

function solution$($R$)$;

that, given an array R consisting of N strings, each of length M$,$ representing the grid, returns the number of clean squares.

Examples:

$1.$ Given A $=["...$X$..","....$XX$","..$X$..."],$ your function should return $6.$

The robot starts at $(0,0),$ facing rightwards, and moves to $(0,2),$ where it turns due to the obstacle at $(0,3).$

Then it goes down from $(0,2)$ to $(1,2),$ where it changes direction again due to another obstacle.

Next it goes left from $(1,2)$ to $(1,0),$ where it turns once because of the grid boundary, then it moves once and turns once more, which makes it stand again at position $(0,0)$ facing rightwards, just as at the beginning, which means it will now repeat the loop indefinitely.

The total number of cleaned squares is $6.$

$2.$ Given A $=["....$X$..","$X$......",".....$X$.","......."],$ your function should return $15.$

$3.$ Given A $=["...$X$.",".$X$..$X$","$X$...$X$","..$X$.."],$ your function should return $9.$

$4.$ Given A $=["."],$ your function should return $1,$ because there is only one square on the grid and it is cleaned in the first move.

Assume that:

$$ N and M are integers within the range $[1.\mathrm{.}20]$;

$$ top$-$left cell is empty;

$$ each string in R consists only of the following characters: $"."$ and$/$or $"$X$".$

In your solution, focus on correctness. The performance of your solution will not be the focus of the assessment.