epoch: derive Hash

[gwbres]

Hash is required if we want structures like "HashMap" or "BTreeMap"
indexed by an Epoch

Signed-off-by: Guillaume W. Bres guillaume.bressaix@gmail.com

[ChristopherRabotin]

Thanks! That is definitely useful.

I'm not sure why Clippy doesn't like it. Here is the explanation: https://rust-lang.github.io/rust-clippy/master/index.html#derive_hash_xor_eq . Any idea how to fix this? Should Hash be implemented manually instead of derived? I want to keep the PartialEq and Eq "derivations" for backward compatibility (and it's a feature I use a lot).

[gwbres]

Hello @ChristopherRabotin

Any idea how to fix this? Should Hash be implemented manually instead of derived? I want to keep the PartialEq and Eq "derivations" for backward compatibility (and it's a feature I use a lot).

https://www.philipdaniels.com/blog/2019/rust-equality-and-ordering/

My mere understanding is that Hash and Eq are closely related.
When implementing Eq yourself you can do whatever you want, like disregarding inner objects contained in a structure.
An example would be a Duration equality, based only off "centuries".
But auto derivation / auto implementation makes use of all inner objects (I presume).
So one must be very careful when mixing Auto and self implemented traits like this, especially these two that intend to work on the same variables/conditions

[codecov-commenter]

Codecov Report

Base: 78.79% // Head: 78.57% // Decreases project coverage by -0.21% ⚠️

Coverage data is based on head (0a1b05d) compared to base (0b25940).
Patch coverage: 12.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   78.79%   78.57%   -0.22%     
==========================================
  Files           9        9              
  Lines        2579     2586       +7     
==========================================
  Hits         2032     2032              
- Misses        547      554       +7     

Impacted Files	Coverage Δ
src/duration.rs	83.90% <0.00%> (-0.62%)	⬇️
src/epoch.rs	86.34% <0.00%> (-0.21%)	⬇️
src/timescale.rs	90.54% <100.00%> (ø)

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[ChristopherRabotin]

Perfect, I think that implementing the hasher manually works well (and makes clippy happy). Also, the Epoch hash is based only on the duration since J1900 TAI, which is what the equality checks too, so that's the correct behavior. Thanks!