code: function compressRun() {
      if (runLen > 1 && offset) {
        let ri = bi - runLen;
        diff[ri] = [offset, runLen];
        for (ri++; ri < bi; ri++) {
          delete diff[ri];
        }
      }
    }
docs: /*
  transform things like:
  
  {
  0: 1,
  1: 2,
  2: 3,
  3: 4,
  n: 4
  }
  
  into things like:
  
  {
  0: [ 1, 4 ],   // targetIndex: [ offset, runLength ]
  n: 4
  }