Need Help to solve an string challenge C#, Java or JavaScript - arrays

Question

I have been trying to solve this challenge, and I cannot find a way to do it. I have solved similar problems before, but I found this particularly difficult, more even trying to solve while keeping a linear time complexity. This was part of an interview assessment that I obviously failed.
I will appreciate any help.
Thanks
Here is the challenge
You are to parse the string into pieces that are no more than "pieceLength" characters long
INCLUDING the commas.
If pieceLength = 3 the result for the above test string would be
result[0] = "1,2"
result[1] = ",3,"
result[2] = "5,8"
result[3] = ","
result[4] = "131"
result[5] = ",21"
result[6] = ",34"
You can look at the test above. This is a sample string and result, however
your code will be run with multiple input strings and piece length parameters.
Note how result[3] is just ",". Including any more characters would break apart
the next number, 131, and that's not allowed.

Answer 1

One way to do this is to iterate through the string, looking at the character beyond the next piecelength. If this is past end of string, or is a comma, there is no problem and you can push all the characters up to that point. Otherwise iterate backwards from that position until you find a comma and push the string up to the comma. If a comma is not found, the portion of the string was all digits and can be pushed to the array:
let str = "1,2,3,5,8,131,21,34";
let pieces = (str, piecelength) => {
let res = [];
for (let i = 0; i < str.length;) {
if (i + piecelength >= str.length || str[i + piecelength] == ',') {
// no problem, just grab the characters
res.push(str.substring(i, Math.min(i + piecelength, str.length)));
i += piecelength;
} else {
// search back for the last comma
for (j = piecelength - 1; j >= 0; j--) {
if (str[i + j] == ',') {
res.push(str.substr(i, j + 1));
i += j + 1;
break;
}
}
if (j == -1) {
// didn't find a comma, everything must be a digit
res.push(str.substr(i, piecelength));
i += piecelength;
}
}
}
return res;
}
console.log(pieces(str, 3));
console.log(pieces(str, 4));