Move loop-invariant offset calculation outside loop

[adlerjohn]

Fixes #34.

[codecov-commenter]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #35   +/-   ##
=========================================
  Coverage          ?   89.09%           
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  Files             ?        6           
  Lines             ?      376           
  Branches          ?        0           
=========================================
  Hits              ?      335           
  Misses            ?       20           
  Partials          ?       21           

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[liamsi]

👍🏼