Function col_to_label 

pub fn col_to_label(col: usize) -> String

Converts a 1-based column index to an Excel column label (e.g. 1 → “A”, 27 → “AA”).

Excel’s full column range is 1..=16384, where column 16384 is XFD.

Panics

Panics in debug builds if col == 0. A zero column is an internal programming error; calamine returns 0-based coordinates and every call site must add 1 before calling this function.