function tinf_build_tree(t, lengths, off, num) {
  var i, sum;

  /* clear code length count table */
  for (i = 0; i < 16; ++i) t.table[i] = 0;

  /* scan symbol lengths, and sum code length counts */
  for (i = 0; i < num; ++i) t.table[lengths[off + i]]++;

  t.table[0] = 0;

  /* compute offset table for distribution sort */
  for (sum = 0, i = 0; i < 16; ++i) {
    offs[i] = sum;
    sum += t.table[i];
  }

  /* create code->symbol translation table (symbols sorted by code) */
  for (i = 0; i < num; ++i) {
    if (lengths[off + i]) t.trans[offs[lengths[off + i]]++] = i;
  }
}
This decorator caches the results of a getter or method such that
the results are lazily computed once, and then cached.
@private