Add split offset function v0.6

[shayzluf]

[Part of] #2307

---

Add helper function SplitOffset needed for shuffling

includes:
TestSplitOffset_OK
SplitOffset

Write why you are making the changes in this pull request
to comply with 0.6 spec

Write a summary of the changes you are making
changes in:

• beacon-chain/utils/shuffle_test.go
• beacon-chain/utils/shuffle.go
  Link anything that would be helpful or relevant to the reviewers
  https://github.com/ethereum/eth2.0-specs/blob/dev/specs/core/0_beacon-chain.md#get_split_offset

[terencechain]

There's already a split function in the same file 😅 https://github.com/prysmaticlabs/prysm/blob/master/beacon-chain/utils/shuffle.go#L63

[shayzluf]

there is no splitoffset function @terencechain

    """
    Returns a value such that for a list L, chunk count k and index i,
    split(L, k)[i] == L[get_split_offset(len(L), k, i): get_split_offset(len(L), k, i+1)]
    """
    return (list_size * index) // chunks```

[terencechain]

there is no splitoffset function @terencechain

It's a split function. SplitIndices essentially does the same thing and we only need one, so I'd say delete that and use your SplitOffset or update the name of SplitIndices to SplitOffset. We are not just implementing the new functions we have to replace/fix current ones too : )

[shayzluf]

you are right, i thought it was an added function. didn't notice that it changed the old function. its an improvement to the other one. fixing
Thanks

[codecov]

Codecov Report

Merging #2365 into spec-v0.6 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           spec-v0.6    #2365   +/-   ##
==========================================
  Coverage      69.16%   69.16%           
==========================================
  Files            119      119           
  Lines           9594     9594           
==========================================
  Hits            6636     6636           
  Misses          2264     2264           
  Partials         694      694

[prestonvanloon]

Can you add a test that tests splitoffset only? This test seems to depend on SplitIndicies

[nisdas]

addressed here d0e4a47