Replaced the loop-based year/month/day extraction in update_from_seconds
with Ben Joffe's "Very Fast 64-bit" date algorithm using multiply-shift
division (4 multiplications, 0 hardware divisions). Also replaced the
mktime inverse with Ben Joffe's inverse date algorithm.
Updated is_leap_year to use the Drepper-Neri-Schneider algorithm (% 25
instead of % 100). Replaced the O(n) yday month-length loop with a
constant-time cumulative-days table lookup.
Uses UInt128 for portable 128-bit support across 64-bit (native) and
32-bit (software fallback) targets.
Raman's benchmarks on x86-64 show a ~13% wall-time improvement:
Before: real 2.215s, user 2.214s
After: real 1.919s, user 1.913s
Algorithm references:
https://www.benjoffe.com/fast-date-64https://www.benjoffe.com/fast-date#inversehttps://www.benjoffe.com/fast-leap-year#drepper-neri-schneider
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Co-authored-by: Raman Tenneti <rtenneti@google.com>
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