A neat trick to make the accumulator both collision-resistant and self-diagnosing.

  For every normalized link id x:
      y = (x << k) | h(x)   # append a k-bit hash to the id
      acc ^= y

If acc is non-zero, split it back into (x', h'):

* Re-compute h(x').

* If it equals h', exactly one link is unpaired and x' tells you which one (or an astronomically unlikely collision). Otherwise there are >= 2 problems.

This has collision-resistance like the parent comment and adds the ability to pinpoint a single offending link without a second pass or a hash table.