Reconstructing data with corrupted shard(s)

[Ilia Shchepin]

Hello,
I have a question regarding the reconstruction of original data when some of the data shards are corrupted (for example some of the bytes in shard []byte are set to 0).
In this case I can detect that my dataset is invalid by using Verify function.
But then, if I pass the dataset to ReconstructData, it will do nothing, because of this code:

	// Quick check: are all of the shards present?  If so, there's
	// nothing to do.
	numberPresent := 0
	dataPresent := 0
	for i := 0; i < r.Shards; i++ {
		if len(shards[i]) != 0 {
			numberPresent++
			if i < r.DataShards {
				dataPresent++
			}
		}
	}
	if numberPresent == r.Shards || dataOnly && dataPresent == r.DataShards {
		// Cool.  All of the shards data data.  We don't
		// need to do anything.
		return nil
	}

Is it possible to reconstruct the data in this case?

[Klaus Post]

You should not use Verify for integrity check. It is very weak and it cannot tell you which shard is broken. See the "usage" section of the README:

The encoder does not know which parts are invalid, so if data corruption is a likely scenario, you need to implement a hash check for each shard. If a byte has changed in your set, and you don't know which it is, there is no way to reconstruct the data set.

[Ilia Shchepin]

Ok, thank you!