refactor simulation decoder

[fedekunze]

Closes: #5664

Description

cc: @alexanderbez

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[alexanderbez]

LGTM 🎉

[codecov]

Codecov Report

Merging #6037 into master will decrease coverage by 0.02%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #6037      +/-   ##
==========================================
- Coverage   54.65%   54.63%   -0.03%     
==========================================
  Files         424      424              
  Lines       25820    25834      +14     
==========================================
+ Hits        14113    14115       +2     
- Misses      10736    10742       +6     
- Partials      971      977       +6     

[alexanderbez]

ACK

[alexanderbez]

There looks to be issues with the x/params decoder.

[fedekunze]

@alexanderbez If you're ok, I'd suggest merging this since the bug is from #6002 and this PR's changes are unrelated