perf: Improve cost of chunk_idx compute

[ritchie46]

When you have many chunks, we must traverse them to compute the access indices. The worst case is get(-1) as that needs to traverse O(n).

get(-1) is now O(1) and worst case is now O(n / 2).

Fixes #16141

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.95%. Comparing base (b38aa4b) to head (e017af2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16154   +/-   ##
=======================================
  Coverage   80.95%   80.95%           
=======================================
  Files        1386     1386           
  Lines      178423   178462   +39     
  Branches     2881     2881           
=======================================
+ Hits       144437   144478   +41     
+ Misses      33493    33491    -2     
  Partials      493      493           

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