有人能解释 EI 的这段代码在做什么吗

Poly Poly::quo(const Poly &rhs) const {
  if (rhs.deg() == 0)
    return a[0] * (ll) ::inv(rhs[0]) % P;
  Poly g = ::inv(rhs[0]);
  int t = 0, n;
  for (n = 1; (n << 1) <= rhs.deg(); ++t, n <<= 1) {
    Poly nttg = g;
    nttg.redeg((n << 1) - 1);
    ntt.fft(nttg.base(), t + 1, 1);
    Poly prod = rhs.slice((n << 1) - 1);
    ntt.fft(prod.base(), t + 1, 1);
    for (int i = 0; i < (n << 1); ++i)
      prod[i] = prod[i] * (ll) nttg[i] % P;
    ntt.fft(prod.base(), t + 1, -1);
    for (int i = 0; i < n; ++i)
      prod[i] = 0;
    ntt.fft(prod.base(), t + 1, 1);
    for (int i = 0; i < (n << 1); ++i)
      prod[i] = prod[i] * (ll) nttg[i] % P;
    ntt.fft(prod.base(), t + 1, -1);
    for (int i = 0; i < n; ++i)
      prod[i] = 0;
    g = g - prod;
  }
  Poly nttg = g;
  nttg.redeg((n << 1) - 1);
  ntt.fftLead(nttg.base(), t + 1);
  Poly eps1 = rhs.slice((n << 1) - 1);
  ntt.fft(eps1.base(), t + 1, 1);
  for (int i = 0; i < (n << 1); ++i)
    eps1[i] = eps1[i] * (ll) nttg[i] % P;
  ntt.fft(eps1.base(), t + 1, -1);
  memcpy(eps1.base(), eps1.base() + n, sizeof(int) << t);
  memset(eps1.base() + n, 0, sizeof(int) << t);
  ntt.fftLead(eps1.base(), t + 1);
  Poly h0 = slice(n - 1);
  h0.redeg((n << 1) - 1);
  ntt.fftLead(h0.base(), t + 1);
  Poly h0g0 = zeroes((n << 1) - 1);
  for (int i = 0; i < (n << 1); ++i)
    h0g0[i] = h0[i] * (ll)nttg[i] % P;
  ntt.fft(h0g0.base(), t + 1, -1);
  Poly h0eps1 = zeroes((n << 1) - 1);
  for (int i = 0; i < (n << 1); ++i)
    h0eps1[i] = h0[i] * (ll)eps1[i] % P;
  ntt.fft(h0eps1.base(), t + 1, -1);
  for (int i = 0; i < n; ++i) {
    h0eps1[i] = operator[](i + n) - h0eps1[i];
    if (h0eps1[i] < 0)
      h0eps1[i] += P;
  }
  memset(h0eps1.base() + n, 0, sizeof(int) << t);
  ntt.fftLead(h0eps1.base(), t + 1);
  for (int i = 0; i < (n << 1); ++i)
    h0eps1[i] = h0eps1[i] * (ll)nttg[i] % P;
  ntt.fft(h0eps1.base(), t + 1, -1);
  memcpy(h0eps1.base() + n, h0eps1.base(), sizeof(int) << t);
  memset(h0eps1.base(), 0, sizeof(int) << t);
  return (h0g0 + h0eps1).slice(rhs.deg());
}

目前有一个猜测是他在求 f\times rhs^{-1}，不确定