const CustomError = require('../extensions/custom-error');

module.exports = function transform(arr) {

let result = [];

if (!Array.isArray(arr)) {

throw new Error();

}

let cmd = false;

for (let i = 0; i < arr.length; i++) {

switch (arr[i]) {

case '--discard-next':

cmd = true;

i++;

break;

case '--discard-prev':

!cmd && result.pop();

cmd = true;

break;

case '--double-next':

cmd = true;

(i + 1) < arr.length && result.push(arr[i + 1]);

break;

case '--double-prev':

i !== 0 && !cmd && result.push(arr[i - 1]);

cmd = true;

break;

default:

cmd = false;

result.push(arr[i]);

}

}

return result;

};