Map index for normal arrays and pixel arrays (RGB) which runs cols by col in inverted order. top to bottom then bottom to top and so on

Map for LED strips with RGB

function ledStripMap(cols, rows) {
  var total = cols * rows;
  // var map
  var map = new Uint8Array(total);
  var index = 0;
  var count = 0

  for(var i = 0; i < cols; i++) {
    for (var j = 0; j < rows; j++) {
      if (i % 2 === 0) {
        index = j * cols + i;
      } else {
        index = (rows - j - 1) * cols + i;
      }
      map[count++] = index;
    }
  }
  return map;
}

function applyMap(arr, map) {
  var mappedValues = new Uint8Array(arr.length);
  for (var i = 0; i < mappedValues.length; i++) {
    mappedValues[i] = arr[map[i]];
  }
  return mappedValues;
}

function applyMapRGB(arr, map) {
  var mappedValues = new Uint8Array(arr.length);
  var mapIndex;
  for (var i = 0; i < mappedValues.length; i += 3) {
    mapIndex = map[Math.floor(i / 3)] * 3;
    mappedValues[i] = arr[mapIndex];
    mappedValues[i + 1] = arr[mapIndex + 1];
    mappedValues[i + 2] = arr[mapIndex + 2];
  }
  return mappedValues;
}

function printArray(a, num) {
  for (var k = 0; k < a.length; k+= num) {
    console.log(a.slice(k, k + num).toString());
  }
}


var map = ledStripMap(5, 20);

// test with 100 values
var arr = new Uint8Array(100);
arr.forEach(function(item, index) { arr[index] = Math.random() * 10 >> 0; });

console.log('original array');
printArray(arr, 5);

console.log('map');
printArray(map, 20);

var mapped = applyMap(arr, map);
console.log('mapped values');
printArray(mapped, 20)


console.log('\n\n\nRGB\n\n');
var mapRGB = ledStripMap(5, 5);
printArray(mapRGB, 5)
// test with 100 pixel so 3 values per pixel
var arrRGB = new Uint8Array(25 * 3);
//arrRGB.forEach(function(item, index) { arrRGB[index] = index; });
arrRGB.forEach(function(item, index) { arrRGB[index] = Math.random() * 255 >> 0; });
var mappedRGB = applyMapRGB(arrRGB, mapRGB);

printArray(arrRGB, 15);
console.log('\n---\n');
printArray(mappedRGB, 15);