improve Decimal conversion performance using _mantissa

[mredig]

There seems to be a public-ish field called _mantissa in Decimal which should be faster than using the string representation here. Maybe we should use this and keep the above arround maybe commented out just in case _mantissa goes away one day.

Originally posted by @tgymnich in #123 (comment)

According to my tests, it looks like using the _mantissa approach is roughly 1/3 faster.

Starting 'Decimal String Init' with 999999 iterations.
Test 'Decimal String Init' with 999999 iterations took 14.572233080863953 seconds.
Finished 'Decimal String Init' with 999999 iterations.


Starting 'Decimal Mantissa Init' with 999999 iterations.
Test 'Decimal Mantissa Init' with 999999 iterations took 10.257248044013977 seconds.
Finished 'Decimal Mantissa Init' with 999999 iterations.


╔═══════════════════════╤══════════════╤══════════════╤══════════════╤══════════════╤══════════════╗
║ Label                 │ Avg          │ Max          │ Min          │ Delta        │ Median       ║
╟───────────────────────┼──────────────┼──────────────┼──────────────┼──────────────┼──────────────╢
║ Decimal Mantissa Init │ 0.0000102246 │ 0.0002379417 │ 0.0000089407 │ 0.0002290010 │ 0.0000110865 ║
╟───────────────────────┼──────────────┼──────────────┼──────────────┼──────────────┼──────────────╢
║ Decimal String Init   │ 0.0000144051 │ 0.0035669804 │ 0.0000129938 │ 0.0035539865 │ 0.0000140667 ║
╚═══════════════════════╧══════════════╧══════════════╧══════════════╧══════════════╧══════════════╝

I'll have another push in a bit to address the second suggestion.

[mredig]

Oh well look at that - linux doesn't like optimizations! Apparently "public-ish" was more on point than I thought.

[mredig]

Works, but setting the remaining bytes (0..<i) to zero after the loop below could be more efficient.

Originally posted by @tgymnich in #123 (comment)

✅

[mredig]

Bonus - I realized I needed Decimal(_ value: Big(U)Int and added that. I might be able to figure out a mantissa init instead of string based, but brain now dead. 🧟

[tgymnich]

👍