basis1/excel

Verified Excel semantics

Every page below is a real bug we hit porting Excel's own logic into a Rust engine, verified against the OOXML spec, against LibreOffice as an independent implementation, and — where it matters most — against the cached values Excel itself wrote into real files. Each ends with a reproduction you can run yourself.

• Why a formula that worked yesterday shows #NAME? today

  LET, XLOOKUP, UNIQUE and other modern functions are stored on disk with an _xlfn. prefix Excel strips in the formula bar. Drop it on write and every one of those cells opens broken.

• Why Excel says it found unreadable content in your workbook

  A stale xl/calcChain.xml — a derived, optional performance cache — no longer matching the file's formula set is the single most common trigger for Excel's repair prompt.

• Why your dates are off by exactly 4 years, or exactly 1 day

  Excel dates are numbers plus an epoch flag. Get the flag wrong and every date shifts by exactly 1,462 days — reproduced with two files differing in one byte.

• Why SUM breaks when a cell errors, but COUNT keeps working

  SUM, AVERAGE and MAX carry an error out of a range; COUNT and COUNTA quietly walk past it. Text in the same range is ignored, not coerced, by all of them.

• Why COUNTIF doesn't find the row you know is there

  A bare number criteria matches numbers only; a text criteria that looks like a number matches both — by value and by literal characters. We shipped this bug ourselves; here's the fix and the corpus impact.