How can I guarantee that my Rubik's cube scrambling algorithm doesn't select the same direction twice in a row?

Question

I am working on a Rubik's Cube Timer Website, (JavaScript) and it requires a scrambling algorithm. It would be easy if it was just picking random letters from an array, but there are requirements that it needs to meet. Each Letter Represents a move in the Rubik's Cube Algorithmic notation, so for example "L" would mean to move the Left side clockwise. Or "U2" would mean to move the Upper side two times, and " B' " would mean to move the Backside anticlockwise. and so on.

The thing is that there cannot be two of the same letters next to each other, and not even if they are in different directions. For example, U cannot be next to U', or U2 and so on. It must be a different letter. Sometimes, my code generates 2 of the same letter next to each other.

Here's my code:

function generateScramble() {

    //Possible Letters
    var array = new Array(" U", " D", " R", " L", " F", " B", " U\'", " D\'", " R\'", " L\'", " F\'", " B\'", " U2", " D2", " R2", " L2", " F2", " B2"); 

    var array2 = new Array(); // The Scramble.

    var rdArr = new Array(); // The Array of random numbers.

    for (var i = 0; i < 20; i++) {
        var random = Math.floor(Math.random() * array.length);
        rdArr.unshift(random);

        if (rdArr[1] - rdArr[0] == 0 ||
            rdArr[0] - rdArr[1] == 0 ||
            rdArr[1] - rdArr[0] == 6 ||
            rdArr[0] - rdArr[1] == 6 ||
            rdArr[1] - rdArr[0] == 12 ||
            rdArr[0] - rdArr[1] == 12) { // Check whether a D is next to D' or D2, or if F is next to F' or F2, R next to R' or R2, and so on
            if (random < 17) {
                random++;
            } else {
                random--;
            }
        }

        array2.push(array[random]); // Get letters in random order in the array.
    }

    var scramble = "Scramble: " + array2[0] + array2[1] + array2[2] + array2[3] + array2[4]
                                + array2[5] + array2[6] + array2[7] + array2[8] + array2[9]
                                + array2[10] + array2[11] + array2[12] + array2[13] + array2[14]
                                + array2[15] + array2[16] + array2[17] + array2[18] + array2[19];

    document.getElementById("Scramble").innerHTML = scramble; // Display the scramble

}

Answer 1

I think, the code could be as it:

generateScramble();

function generateScramble() {

  // Possible Letters
  var array = new Array(" U", " D", " R", " L", " F", " B")

  // Possible switches
  var switches = ["", "\'", "2"]; 

  var array2 = new Array(); // The Scramble.

  var last = ''; // Last used letter

  var random = 0;

  for (var i = 0; i < 20; i++) {
      // the following loop runs until the last one 
      // letter is another of the new one
      do {
         random = Math.floor(Math.random() * array.length);
      } while (last == array[random]) 

      // assigns the new one as the last one
      last = array[random];

      // the scramble item is the letter
      // with (or without) a switch
      var scrambleItem = array[random] + switches[parseInt(Math.random()*switches.length)];

      array2.push(scrambleItem); // Get letters in random order in the array.
  }

  var scramble = "Scramble: ";
  
  // Appends all scramble items to scramble variable
  for(i=0; i<20; i++) {
     scramble += array2[i];
  }
  
  document.getElementById("Scramble").innerHTML = scramble; // Display the scramble
}

<div id="Scramble"></div>

Hope it helps.