Optimize NaiveDate::add_days for small values

[pitdicker]

I expect this method to be used much more often with small values than with values that end up an x number of years into the past or future. So I would like to add a fast path.

In the common case where the result of add_checked_signed and similar ends up within the same year we can avoid a couple of divisions and lookups and just change the ordinal.

Before:

bench_naivedate_add_signed
                        time:   [32.797 ns 34.745 ns 36.560 ns]
Found 13 outliers among 100 measurements (13.00%)
  13 (13.00%) high severe

After:

bench_naivedate_add_signed
                        time:   [12.886 ns 13.422 ns 13.855 ns]
                        change: [-65.054% -63.390% -61.655%] (p = 0.00 < 0.05)
                        Performance has improved.

[pitdicker]

Does this have good test coverage?

I'll have a look later today

That must have been one of the longer days in history...

With a small change in add_days to make the result invalid we have 37 failing tests:

failures:
    datetime::tests::test_add_sub_months
    datetime::tests::test_datetime_add_assign_local
    datetime::tests::test_datetime_add_days
    datetime::tests::test_datetime_add_months
    datetime::tests::test_datetime_date_and_time
    datetime::tests::test_datetime_fixed_offset
    datetime::tests::test_datetime_from_local
    datetime::tests::test_datetime_offset
    datetime::tests::test_datetime_rfc2822
    datetime::tests::test_datetime_rfc3339
    datetime::tests::test_datetime_sub_assign_local
    datetime::tests::test_datetime_sub_days
    datetime::tests::test_datetime_sub_months
    datetime::tests::test_from_system_time
    datetime::tests::test_parse_datetime_utc
    datetime::tests::test_rfc3339_opts
    datetime::tests::test_utc_datetime_from_str
    format::formatting::tests::test_datetime_format_alignment
    format::parse::tests::test_rfc2822
    format::parse::tests::test_rfc3339
    format::parsed::tests::issue_551
    format::parsed::tests::test_parsed_to_naive_date
    format::parsed::tests::test_parsed_to_naive_datetime_with_offset
    month::tests::test_month_enum_primitive_parse
    month::tests::test_month_enum_try_from
    naive::date::tests::test_date_add
    naive::date::tests::test_date_add_days
    naive::date::tests::test_date_parse_from_str
    naive::date::tests::test_date_sub_days
    naive::date::tests::test_naiveweek
    naive::date::tests::test_week_iterator_limit
    naive::date::tests::test_weeks_from
    naive::datetime::tests::test_and_local_timezone
    naive::datetime::tests::test_and_utc
    naive::datetime::tests::test_datetime_add
    offset::fixed::tests::test_date_extreme_offset
    offset::local::tests::verify_correct_offsets_distant_past

More seriously: we have reasonable but not exhaustive coverage for values within a year, for values that cross the year boundary and have to account for leap years, and for values than can cause an overflow in naive::date::test_date_add_days.

Adding more tests does not seem that useful to me.

[jtmoon79]

I recommend adding a #[cfg(test)]-only code path within the add_days to turn on and turn off the fast path.
...
I recommend test cases for days values
0 to 367 inclusive that compares add_days when ADD_DAYS_ENABLE_FAST_PATH is true and false

Adding more tests does not seem that useful to me.

I think the confidence this provides is worth the relatively small amount of work. (admittedly, I'm not doing the work! though if you want me to then I can submit a PR for my suggestion)

[codecov]

Codecov Report

Merging #1214 (28c85aa) into 0.4.x (2960f2c) will increase coverage by 0.00%.
Report is 5 commits behind head on 0.4.x.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            0.4.x    #1214   +/-   ##
=======================================
  Coverage   91.40%   91.41%           
=======================================
  Files          38       38           
  Lines       16932    16959   +27     
=======================================
+ Hits        15477    15503   +26     
- Misses       1455     1456    +1     

Files Changed	Coverage Δ
src/naive/date.rs	96.30% <100.00%> (-0.05%)	⬇️

... and 3 files with indirect coverage changes

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[pitdicker]

@djc Did you want to give this a second look? Or was #1214 (comment) only a question and is this approved?