bytewords: replace match with precomputed perfect has function

[dspicher]

The two giant match statements with 256 arms incur linear lookup cost. To avoid this, we have defined a simple family of hash functions linear in the first and last character of the encoded string (this lines up nicely with the fact that those two characters are identical for the default and minimal encodings). Exhaustive search has then yielded a perfect hash function (with no collisions) that only needs a domain of roughly double the theoretical minimum (which would be 256 elements).

Other families of hash functions might theoretically yield smaller domains, but we don't judge this to be a relevant optimization for now.

This design achieves the following goals:

• Constant lookup time at runtime.

• no_std compliance, i.e. no OnceLocks and no HashMaps.

• No unsafe code.

The recently introduced decoding test [1] suggests performance improvements of 66%.

[1] bbf09e4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.56%. Comparing base (bbf09e4) to head (a804fa3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
- Coverage   98.92%   98.56%   -0.37%     
==========================================
  Files           5        5              
  Lines        1766     1251     -515     
==========================================
- Hits         1747     1233     -514     
+ Misses         19       18       -1     

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