Excel shows 15 significant digits. When a number is an exact tie at the 16th
(its exact decimal expansion is 15 digits, a 5, then only zeros), Excel
rounds it toward zero: TEXT(1234567890123455,"0") is 1234567890123450.
GridCraft took Rust's `{:.14e}`, which rounds that tie to even, so any tie
with an odd 15th digit came out one unit too high (…460).
decimal_digits now formats 16 digits (correct unless the 16th is a 5),
and only on a 5 checks the exact expansion (a 30-digit screen, then the
full one) and truncates a true tie. Non-ties, including values just above
or below a tie, round as before. One format serves almost every number.
Tests: ties of every 15th-digit parity, negatives, fractional ties, the
neighbours just above and below a tie, the carry after fifteen 9s
(999999999999999.5 shows 999999999999999), and unchanged ordinary cases.
Evidence: Windows Excel 16.0.20430 TEXT() over a 174-row probe and a 24-row
carry probe agree on every row except those under a percent format and
General over negatives (the two bugs named below). The suite's number-format
golden goes from 183,843 to 184,900 of 187,810 passing with no row that
passed before failing now.
modified: crates/numfmt/src/decimal.rs
modified: crates/numfmt/src/tests.rs
Sebastian Whincop