xlsx: share each custom number format across cell formats, cap font names at 31 characters

Every cell format record copied its font (name included) and built its
own copy of its number-format code, so many short records naming one
long font or code multiplied it in memory. Custom codes are now built
once and shared, and font names are cut to Excel's 31 characters.

crates/xlsx/src/styles.rs

@@ -59,11 +59,14 @@ fn bool_prop(e: &El, name: &str) -> bool {
 
 const DEFAULT_FONT_FAMILY: u8 = 2;
 const MAX_FONT_FAMILY: u8 = 14;
+/// Longest font name kept, in characters (Excel's own limit). Every cell format copies its
+/// font, so a long name repeated by many formats would otherwise multiply in memory.
+const MAX_FONT_NAME: usize = 31;
 
 pub fn read_font(e: &El, palette: &[u32]) -> Font {
     let mut f = Font::default();
     if let Some(n) = e.child("name").or_else(|| e.child("rFont")).and_then(|n| n.attr("val")) {
-        f.name = n.to_string();
+        f.name = n.chars().take(MAX_FONT_NAME).collect();
     }
     if let Some(sz) = e.child("sz").and_then(|s| s.attr_f64("val")) {
         f.size = sz.clamp(1.0, 409.0) as f32;
@@ -277,14 +280,14 @@ fn read_protection(p: &El) -> Protection {
     Protection { locked: p.flag("locked", true), hidden: p.flag("hidden", false) }
 }
 
-fn num_fmt_code(id: u32, custom: &HashMap<u32, String>) -> NumFmt {
+fn num_fmt_code(id: u32, custom: &HashMap<u32, NumFmt>) -> NumFmt {
     if let Some(c) = custom.get(&id) {
-        return NumFmt::new(c);
+        return c.clone();
     }
     NumFmt::new(builtin_format(id).unwrap_or("General"))
 }
 
-fn read_xf(xf: &El, fonts: &[Font], fills: &[Fill], borders: &[Borders], fmts: &HashMap<u32, String>, checkboxes: &[bool]) -> Style {
+fn read_xf(xf: &El, fonts: &[Font], fills: &[Fill], borders: &[Borders], fmts: &HashMap<u32, NumFmt>, checkboxes: &[bool]) -> Style {
     let mut s = Style::default();
     if let Some(f) = xf.attr_u32("fontId").and_then(|i| fonts.get(i as usize)) {
         s.font = f.clone();
@@ -336,6 +339,8 @@ pub fn read_styles(root: &El, checkboxes: &[bool], minor_font: &str) -> StylesIn
             }
         }
     }
+    // One shared copy of each custom code, which every format using it points to.
+    let shared_fmts: HashMap<u32, NumFmt> = fmts.iter().map(|(id, code)| (*id, NumFmt::new(code))).collect();
     // The scheme link only means something on a font in the body face; elsewhere it stays as a new font has it.
     let body_font = |e: &El| {
         let mut f = read_font(e, &palette);
@@ -345,10 +350,14 @@ pub fn read_styles(root: &El, checkboxes: &[bool], minor_font: &str) -> StylesIn
     let fonts: Vec<Font> = root.child("fonts").map(|f| f.kids("font").map(body_font).collect()).unwrap_or_default();
     let fills: Vec<Fill> = root.child("fills").map(|f| f.kids("fill").map(|e| read_fill(e, &palette, false)).collect()).unwrap_or_default();
     let borders: Vec<Borders> = root.child("borders").map(|f| f.kids("border").map(|e| read_border(e, &palette)).collect()).unwrap_or_default();
-    let xfs =
-        root.child("cellXfs").map(|x| x.kids("xf").map(|xf| read_xf(xf, &fonts, &fills, &borders, &fmts, checkboxes)).collect()).unwrap_or_default();
-    let style_xfs: Vec<Style> =
-        root.child("cellStyleXfs").map(|x| x.kids("xf").map(|xf| read_xf(xf, &fonts, &fills, &borders, &fmts, &[])).collect()).unwrap_or_default();
+    let xfs = root
+        .child("cellXfs")
+        .map(|x| x.kids("xf").map(|xf| read_xf(xf, &fonts, &fills, &borders, &shared_fmts, checkboxes)).collect())
+        .unwrap_or_default();
+    let style_xfs: Vec<Style> = root
+        .child("cellStyleXfs")
+        .map(|x| x.kids("xf").map(|xf| read_xf(xf, &fonts, &fills, &borders, &shared_fmts, &[])).collect())
+        .unwrap_or_default();
     let mut named = Vec::new();
     if let Some(cs) = root.child("cellStyles") {
         for c in cs.kids("cellStyle") {
@@ -359,12 +368,12 @@ pub fn read_styles(root: &El, checkboxes: &[bool], minor_font: &str) -> StylesIn
             }
         }
     }
-    let dxfs = root.child("dxfs").map(|d| d.kids("dxf").map(|x| read_dxf(x, &palette, &fmts)).collect()).unwrap_or_default();
+    let dxfs = root.child("dxfs").map(|d| d.kids("dxf").map(|x| read_dxf(x, &palette, &shared_fmts)).collect()).unwrap_or_default();
     StylesIn { xfs, dxfs, palette, named, num_fmts: fmts }
 }
 
 /// One differential format (`dxf`): the style it sets over a cell's own.
-fn read_dxf(x: &El, palette: &[u32], fmts: &HashMap<u32, String>) -> Style {
+fn read_dxf(x: &El, palette: &[u32], fmts: &HashMap<u32, NumFmt>) -> Style {
     let mut s = Style::default();
     if let Some(e) = x.child("font") {
         let f = read_font(e, palette);

crates/xlsx/src/tests/malformed.rs

@@ -385,3 +385,22 @@ fn outline_summary_directions_round_trip() {
     let xml = String::from_utf8(crate::package::Package::open(&bytes).unwrap().read("xl/worksheets/sheet1.xml").unwrap().unwrap()).unwrap();
     assert!(!xml.contains("outlinePr"), "{xml}");
 }
+
+/// Many cell formats naming one long font and one long custom number format: the code is shared
+/// by every format and the font name is cut to Excel's 31 characters, so neither is copied per
+/// format.
+#[test]
+fn cell_formats_share_their_number_format_and_keep_short_font_names() {
+    let xml = format!(
+        r#"<styleSheet><numFmts><numFmt numFmtId="164" formatCode="{}"/></numFmts><fonts><font><name val="{}"/></font></fonts><cellXfs>{}</cellXfs></styleSheet>"#,
+        "0".repeat(100_000),
+        "F".repeat(100_000),
+        r#"<xf numFmtId="164" fontId="0"/>"#.repeat(20_000)
+    );
+    let root = crate::xml::parse(xml.as_bytes()).unwrap();
+    let st = crate::styles::read_styles(&root, &[], "Calibri");
+    assert_eq!(st.xfs.len(), 20_000);
+    assert!(st.xfs.iter().all(|s| std::sync::Arc::ptr_eq(&s.num_fmt.0, &st.xfs[0].num_fmt.0)));
+    assert_eq!(st.xfs[0].num_fmt.as_str().len(), 100_000);
+    assert!(st.xfs.iter().all(|s| s.font.name.chars().count() == 31));
+}