ImoHash.SumFile returns "Seek: invalid offset" error if called on a file with a size that is greater than sampleThreshold but less than sampleSize

[Hakkin]

See schollz/croc#786 (comment)

The offending code is here:

imohash/imohash.go

Lines 107 to 131 in 3f885db

	if f.Size() < int64(imo.sampleThreshold) || imo.sampleSize < 1 {
	if _, err := io.Copy(imo.hasher, f); err != nil {
	return emptyArray, err
	}
	} else {
	buffer := make([]byte, imo.sampleSize)
	if _, err := f.Read(buffer); err != nil {
	return emptyArray, err
	}
	imo.hasher.Write(buffer) // these Writes never fail
	if _, err := f.Seek(f.Size()/2, 0); err != nil {
	return emptyArray, err
	}
	if _, err := f.Read(buffer); err != nil {
	return emptyArray, err
	}
	imo.hasher.Write(buffer)
	if _, err := f.Seek(int64(-imo.sampleSize), 2); err != nil {
	return emptyArray, err
	}
	if _, err := f.Read(buffer); err != nil {
	return emptyArray, err
	}
	imo.hasher.Write(buffer)
	}

imohash tries to seek -imo.sampleSize from the end of the reader, but does not verify imo.sampleSize <= f.Size() before doing so, leading to Seek returning an error.

Here's a playground link demonstrating this with Sum (panics rather than returning an error): https://go.dev/play/p/wKCwBAo6nsZ

Note that this situation (seemingly?) isn't possible with the default SampleSize and SampleThreshold, since the default SampleThreshold>SampleSize. It's only an issue if you set custom values and SampleSize>SampleThreshold. I don't know the intricacies of the algorithm, so I'm not sure if it's even correct or makes sense to set the values like this.

[kalafut]

This is a good catch and not a particularly obscure edge case. Let me think about the correct way to handle it. Note: I'm traveling, and a PR might take a few days.