Formula bar: hide _xludf. like Excel, keep it when the edit is confirmed unchanged (#726)

crates/ui-egui/src/formula_locale.rs

@@ -9,6 +9,14 @@ pub(crate) fn canonical(
     sheet: usize,
     cell: gridcraft_engine::core::CellRef,
 ) -> Result<String, ParseError> {
+    // The editor hid the `_xludf.` prefixes of this cell's formula; confirming it unchanged keeps
+    // the user-defined calls instead of binding them to the built-ins of the same name.
+    if let Some(stored) = wb.sheet(sheet).and_then(|sh| sh.cell(cell)).filter(|c| c.formula.is_some()).map(|c| c.input_text())
+        && has_udf_prefix(&stored)
+        && text == display(&stored, locale, wb, sheet)
+    {
+        return Ok(stored);
+    }
     if locale == FormulaLocale::En || wb.sheet(sheet).is_some_and(|sh| wb.styles.get(sh.style_id(cell)).num_fmt.as_str() == "@") {
         return Ok(text.to_string());
     }
@@ -40,16 +48,42 @@ pub(crate) fn canonical(
 }
 
 pub(crate) fn display(text: &str, locale: FormulaLocale, wb: &Workbook, sheet: usize) -> String {
-    // English (and every language without a formula table) shows the stored text untouched,
-    // and skips a parse per frame.
+    // English (and every language without a formula table) shows the stored text, `_xludf.`
+    // prefixes aside, and skips a parse per frame.
     if locale == FormulaLocale::En {
-        return text.to_string();
+        return hide_udf_prefix(text, locale);
     }
     let Some(body) = text.strip_prefix('=') else { return text.to_string() };
-    formula::parse(body)
+    let shown = formula::parse(body)
         .and_then(|expr| formula::print_input(&expr, locale, |name| wb.name(name, sheet).is_some() || wb.table(name).is_some()))
         .map(|body| format!("={body}"))
-        .unwrap_or_else(|_| text.to_string())
+        .unwrap_or_else(|_| text.to_string());
+    hide_udf_prefix(&shown, locale)
+}
+
+const UDF_PREFIX: &str = "_xludf.";
+
+fn has_udf_prefix(text: &str) -> bool {
+    text.to_ascii_lowercase().contains(UDF_PREFIX)
+}
+
+/// Excel shows a user-defined call such as `_xludf.EOMONTH()` as `=EOMONTH()`. Only function
+/// names lose the prefix: text literals and names keep it.
+fn hide_udf_prefix(text: &str, locale: FormulaLocale) -> String {
+    let Some(body) = text.strip_prefix('=').filter(|b| has_udf_prefix(b)) else { return text.to_string() };
+    let Ok(tokens) = formula::lexer::tokenize_locale(body, locale) else { return text.to_string() };
+    let mut out = String::from("=");
+    let mut from = 0;
+    for t in tokens {
+        if matches!(t.tok, formula::lexer::Tok::Func(_))
+            && body.get(t.start..t.start + UDF_PREFIX.len()).is_some_and(|p| p.eq_ignore_ascii_case(UDF_PREFIX))
+        {
+            out.push_str(body.get(from..t.start).unwrap_or(""));
+            from = t.start + UDF_PREFIX.len();
+        }
+    }
+    out.push_str(body.get(from..).unwrap_or(""));
+    out
 }
 
 // Descriptions/signatures for the supported Spanish function table; untranslated functions

crates/ui-egui/tests/formula_locale.rs

@@ -1,7 +1,7 @@
 //! Language selection must affect UI entry, never the canonical engine/file boundary.
 use gridcraft_engine::{
     Session,
-    core::{CellRef, Value},
+    core::{CellError, CellRef, Value},
 };
 use gridcraft_ui_egui::{SheetApp, UiState, i18n::Language};
 use serde_json::json;
@@ -201,3 +201,21 @@ fn czech_interface_retains_its_canonical_formula_fallback() {
     let restored: UiState = serde_json::from_str(&serde_json::to_string(&app.ui).unwrap()).unwrap();
     assert_eq!(restored.language, Language::Cs);
 }
+
+#[test]
+fn user_defined_prefix_is_hidden_while_editing_and_kept_on_commit() {
+    // Excel shows `_xludf.EOMONTH()` as `=EOMONTH()`; the cell still holds the user-defined call
+    // (#726), and committing the edit unchanged must not turn it into the built-in.
+    let mut app = app();
+    app.run("cell.set", json!({"cell":"A1","input":"=_xludf.EOMONTH()&_xludf.CONCAT(\"_xludf.x\")"})).unwrap();
+    app.begin_edit(None, true);
+    assert_eq!(app.editor.as_ref().unwrap().text, "=EOMONTH()&CONCAT(\"_xludf.x\")");
+    assert!(app.commit_edit(0, 0, false, false));
+    assert_eq!(formula(&app), "_xludf.EOMONTH()&_xludf.CONCAT(\"_xludf.x\")");
+    assert_eq!(app.session.active().unwrap().wb.active().unwrap().value(CellRef::new(0, 0)), Value::Error(CellError::Name));
+    app.run("app.language.set", json!({"language":"es"})).unwrap();
+    app.begin_edit(None, true);
+    assert_eq!(app.editor.as_ref().unwrap().text, "=EOMONTH()&CONCAT(\"_xludf.x\")");
+    assert!(app.commit_edit(0, 0, false, false));
+    assert_eq!(formula(&app), "_xludf.EOMONTH()&_xludf.CONCAT(\"_xludf.x\")");
+}