fix(xls/xlsb): correct column letters in reconstructed formulas (columns >= AA)

src/utils.rs

@@ -65,20 +65,27 @@ pub fn read_f64(s: &[u8]) -> f64 {
     f64::from_le_bytes(s[..8].try_into().unwrap())
 }
 
-/// Push literal column into a String buffer
-pub fn push_column(mut col: u32, buf: &mut String) {
-    if col < 26 {
-        buf.push((b'A' + col as u8) as char);
-    } else {
-        let mut rev = String::new();
-        while col >= 26 {
-            let c = col % 26;
-            rev.push((b'A' + c as u8) as char);
-            col -= c;
-            col /= 26;
-        }
-        buf.extend(rev.chars().rev());
+/// Convert a 0-based column index to an Excel column name (0 -> "A", 26 -> "AA").
+pub fn push_column(col: u32, buf: &mut String) {
+    let mut digits = [0u8; 6];
+    let len = column_name_digits(col, &mut digits);
+    for &d in &digits[..len] {
+        buf.push(d as char);
+    }
+}
+
+/// Write the Excel letters for a 0-based column index into `out` (most
+/// significant first) and return how many were written.
+pub(crate) fn column_name_digits(col: u32, out: &mut [u8]) -> usize {
+    let mut num = col + 1;
+    let mut n = 0;
+    while num > 0 {
+        out[n] = b'A' + ((num - 1) % 26) as u8;
+        n += 1;
+        num = (num - 1) / 26;
     }
+    out[..n].reverse();
+    n
 }
 
 // Utility function to unescape standard XML entities or character references
@@ -1165,6 +1172,24 @@ pub const FTAB_ARGC: [u8; FTAB_LEN] = [
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_push_column() {
+        let check = |col: u32, expected: &str| {
+            let mut got = String::new();
+            push_column(col, &mut got);
+            assert_eq!(got, expected, "push_column({col})");
+        };
+        check(0, "A");
+        check(25, "Z");
+        check(26, "AA");
+        check(27, "AB");
+        check(51, "AZ");
+        check(53, "BB");
+        check(701, "ZZ");
+        check(702, "AAA");
+        check(16383, "XFD");
+    }
+
     #[test]
     fn sound_to_u32() {
         let data = b"ABCDEFGH";

src/xls.rs

@@ -1773,11 +1773,27 @@ fn parse_formula(
                     write!(&mut formula, "${row_last}").unwrap();
                     rgce = &rgce[6..];
                 } else {
-                    formula.push('$');
-                    push_column(read_u16(&rgce[4..6]) as u32, &mut formula);
-                    write!(&mut formula, "${}:$", read_u16(&rgce[0..2]) as u32 + 1).unwrap();
-                    push_column(read_u16(&rgce[6..8]) as u32, &mut formula);
-                    write!(&mut formula, "${}", read_u16(&rgce[2..4]) as u32 + 1).unwrap();
+                    // columnFirst/columnLast are ColRelU: 14-bit column + fColRel/fRwRel flags.
+                    let col_first = read_u16(&[rgce[4], rgce[5] & 0x3F]);
+                    let col_last = read_u16(&[rgce[6], rgce[7] & 0x3F]);
+                    let row_first = read_u16(&rgce[0..2]) as u32 + 1;
+                    let row_last = read_u16(&rgce[2..4]) as u32 + 1;
+                    if rgce[5] & 0x80 != 0x80 {
+                        formula.push('$');
+                    }
+                    push_column(col_first as u32, &mut formula);
+                    if rgce[5] & 0x40 != 0x40 {
+                        formula.push('$');
+                    }
+                    write!(&mut formula, "{row_first}:").unwrap();
+                    if rgce[7] & 0x80 != 0x80 {
+                        formula.push('$');
+                    }
+                    push_column(col_last as u32, &mut formula);
+                    if rgce[7] & 0x40 != 0x40 {
+                        formula.push('$');
+                    }
+                    write!(&mut formula, "{row_last}").unwrap();
                     rgce = &rgce[8..];
                 }
             }

src/xlsx/mod.rs

@@ -3340,14 +3340,9 @@ pub(crate) fn column_number_to_name(num: u32, buf: &mut Vec<u8>) -> Result<(), X
     if num >= MAX_COLUMNS {
         return Err(XlsxError::ColumnNumberOverflow);
     }
-    let start = buf.len();
-    let mut num = num + 1;
-    while num > 0 {
-        let integer = ((num - 1) % 26 + 65) as u8;
-        buf.push(integer);
-        num = (num - 1) / 26;
-    }
-    buf[start..].reverse();
+    let mut digits = [0u8; 6];
+    let len = crate::utils::column_name_digits(num, &mut digits);
+    buf.extend_from_slice(&digits[..len]);
     Ok(())
 }
 

tests/test.rs

@@ -3617,6 +3617,18 @@ fn xls_empty_string() {
     assert_eq!(range.get_value((0, 0)), Some(&String("".to_string())));
 }
 
+#[test]
+fn xls_formula_columns_beyond_z() {
+    // Formula column references at/after column AA (index 26) exercise the
+    // column-name rendering and the BIFF8 PtgArea column masking.
+    let mut wb: Xls<_> = wb("xls_formula_columns_beyond_z.xls");
+    let formula = wb.worksheet_formula("Sheet1").unwrap();
+    let mut rows = formula.rows();
+    assert_eq!(rows.next(), Some(&["SUM(AA1:AA3)".to_owned()][..]));
+    assert_eq!(rows.next(), Some(&["AA1+AB1".to_owned()][..]));
+    assert_eq!(rows.next(), None);
+}
+
 #[test]
 fn xls_embedded_cross_sheet_chart_does_not_leak_cells() {
     // Regression test: the chart on "Report" is a nested substream whose cached