[Struct] Refactor negative sampling

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.32%. Comparing base (fe662e0) to head (3e98702). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #504 +/- ## ========================================== - Coverage 99.32% 99.32% -0.01% ========================================== Files 129 129 Lines 8452 8447 -5 ========================================== - Hits 8395 8390 -5 Misses 57 57 ``` | [Flag](https://app.codecov.io/gh/massquantity/LibRecommender/pull/504/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity) | Coverage Δ | | |---|---|---| | [CI](https://app.codecov.io/gh/massquantity/LibRecommender/pull/504/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity) | `99.32% <100.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity#carryforward-flags-in-the-pull-request-comment) to find out more.

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