Help me dissect this code for suffix array

So I was learning suffix array and how to implement it. I was not able to implement it myself so I took unused code for constructing suffix array I was able to understand the initial steps but was not able to understand what is happening after the 'Start' in code below. Please help me. Thanks.

Code

vector<int> suffix_array(const string& in) {
    int n = (int)in.size(), c = 0;
    vector<int> temp(n), pos2bckt(n), bckt(n), bpos(n), out(n);
    for (int i = 0; i < n; i++) out[i] = i; 
    sort(out.begin(), out.end(), [&](int a, int b) { return in[a] < in[b]; });
    for (int i = 0; i < n; i++) {
        bckt[i] = c;
        if (i + 1 == n || in[out[i]] != in[out[i + 1]]) c++;
    }
    /*Start*/
    for (int h = 1; h < n && c < n; h <<= 1) {// executes log n times
        for (int i = 0; i < n; i++) pos2bckt[out[i]] = bckt[i];
	for (int i = n - 1; i >= 0; i--) bpos[bckt[i]] = i;
	for (int i = 0; i < n; i++)
            if (out[i] >= n - h) temp[bpos[bckt[i]]++] = out[i];
        for (int i = 0; i < n; i++)
            if (out[i] >= h) temp[bpos[pos2bckt[out[i] - h]]++] = out[i] - h;
        c = 0;
        for (int i = 0; i + 1 < n; i++) {
            int a = (bckt[i] != bckt[i + 1]) || (temp[i] >= n - h)
                || (pos2bckt[temp[i + 1] + h] != pos2bckt[temp[i] + h]);
            bckt[i] = c;
            c += a;
        }
        bckt[n - 1] = c++;
        temp.swap(out);
    }
    return out;
}