fix(formats): skip full rows before walking them for HTML rowspans

crates/formats/src/model.rs

@@ -58,6 +58,23 @@ mod tests {
         assert!(t.rows.iter().all(|r| r.len() <= 64), "a row grew past the column cap");
     }
 
+    #[test]
+    fn insert_covered_skips_full_rows_without_walking_them() {
+        // 50,000 spanning cells over a 50,000-cell row: each walked the row below to its grid
+        // column before finding it full (billions of steps). A full row is now skipped at once.
+        let n = 50_000;
+        let mut t = FTable {
+            rows: vec![vec![Cell { rowspan: 2, ..Default::default() }; n], vec![Cell::default(); n]],
+            widths: Vec::new(),
+            borderless: false,
+        };
+        let start = std::time::Instant::now();
+        t.insert_covered();
+        assert!(start.elapsed() < std::time::Duration::from_secs(10), "{:?}", start.elapsed());
+        assert_eq!(t.rows[1].len(), n, "a full row takes no covered cell");
+        assert!(t.rows[1].iter().all(|c| !c.covered));
+    }
+
     #[test]
     fn base64_round_trip() {
         for data in [&b""[..], b"f", b"fo", b"foo", b"foob", b"fooba", b"foobar", &[0, 255, 128, 7]] {

crates/formats/src/model/types.rs

@@ -418,6 +418,11 @@ impl FTable {
                             return;
                         }
                         let Some(row) = self.rows.get_mut(r + k) else { break };
+                        // A full row takes no covered cell: checked before it is walked, so a
+                        // wide row isn't scanned for every span above it.
+                        if row.len() >= 64 {
+                            continue;
+                        }
                         // Find the index in that row at grid column `g`.
                         let mut x = 0usize;
                         let mut at = row.len();
@@ -428,9 +433,7 @@ impl FTable {
                             }
                             x += cc.colspan.clamp(1, 63) as usize;
                         }
-                        if row.len() < 64 {
-                            row.insert(at, Cell { colspan: span as u32, covered: true, ..Default::default() });
-                        }
+                        row.insert(at, Cell { colspan: span as u32, covered: true, ..Default::default() });
                     }
                 }
                 g += span;

crates/formats/src/sheet/xlsx.rs

@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use super::date::{DateKind, builtin_date, custom_date, iso_datetime, serial_date};
 use super::{
     BTreeMap, Ev, Grid, HashMap, MAX_CELLS, MAX_COLS, MAX_ROWS, MAX_SHARED, MAX_SHEETS, MAX_XFS, Package, TextBudget, attr, push_capped, walk,
@@ -263,7 +261,7 @@ pub(super) fn xlsx_rows(pkg: &mut Package<'_>, path: &str) -> Result<Vec<Vec<Str
                     row.resize(c.col + 1, String::new());
                 }
                 if let Some(slot) = row.get_mut(c.col) {
-                    *slot = text.into_owned();
+                    *slot = text;
                 }
             }
             _ => {}
@@ -276,15 +274,12 @@ pub(super) fn xlsx_rows(pkg: &mut Package<'_>, path: &str) -> Result<Vec<Vec<Str
     Ok(grid.rows)
 }
 
-/// A cell's text: a shared string borrowed, so a cell the sheet then refuses (past the text
-/// budget, the columns or the rows) costs no copy of it.
-fn xlsx_value<'a>(c: &XCell, shared: &'a [String], formats: &[Option<DateKind>], date1904: bool) -> Cow<'a, str> {
+fn xlsx_value(c: &XCell, shared: &[String], formats: &[Option<DateKind>], date1904: bool) -> String {
     let v = c.value.trim();
     let kind = c.kind.trim();
     if kind.eq_ignore_ascii_case("s") {
-        return Cow::Borrowed(v.parse::<usize>().ok().and_then(|i| shared.get(i)).map_or("", String::as_str));
-    }
-    Cow::Owned(if kind.eq_ignore_ascii_case("inlineStr") {
+        v.parse::<usize>().ok().and_then(|i| shared.get(i)).cloned().unwrap_or_default()
+    } else if kind.eq_ignore_ascii_case("inlineStr") {
         if c.inline.is_empty() { c.value.clone() } else { c.inline.clone() }
     } else if kind.eq_ignore_ascii_case("str") || kind.eq_ignore_ascii_case("e") {
         c.value.clone()
@@ -295,12 +290,12 @@ fn xlsx_value<'a>(c: &XCell, shared: &'a [String], formats: &[Option<DateKind>],
     } else if v.is_empty() {
         c.inline.clone()
     } else {
-        let Ok(x) = v.parse::<f64>() else { return Cow::Owned(v.to_string()) };
+        let Ok(x) = v.parse::<f64>() else { return v.to_string() };
         match formats.get(c.style).copied().flatten() {
             Some(kind) => serial_date(x, date1904, kind).unwrap_or_else(|| number(x)),
             None => number(x),
         }
-    })
+    }
 }
 
 /// A number as a spreadsheet shows it in General format: up to 15 significant digits, no