// start times of each frame, in ms
double *framestart;

// end times of each frame, in ms
// these are equal to the start time of the next frame, except the last frame, which we BS
double *frameend;

// length of one CFR frame, in ms
double cfrlength;

// number of frames total
int nframes;

// for each output frame, what input frame it corresponds to
int *remaps;

// this is strictly concept code
...

for (int i = 0, int outpos = 0; i < nframes; i++)
{
  // start of the space we have to insert frame into
  double cfrstart = cfrlength * outpos;
  // total length of the window we have to insert a frame into (can be negative, this is ok)
  double diff = frameend[i] - cfrstart;

  // try to insert one copy of frame at low threshold (so likely to succeed)
  if (diff >= th1)
  {
    diff -= cfrlength; // consumed an entire frame's worth of space
    remaps[outpos] = i; // show this frame
    outpos++;
  }
  else
  { // record that we dropped a frame
    ndrop++;
  }

  // try to insert as many as needed copies of this frame at high threshold (so less likely to succeed)
  while (diff >= th2)
  {
    diff -= cfrlength;
    remaps[outpos] = i;
    outpos++;
    // record that we duped a frame
    ndup++;
  }
}
// the most important significant thing that is missing is a special case for frames before frame 0 of the vfr stream,
// which can be filled with blank if there's enough space