numfmt: TEXT returns #VALUE! when its result passes a cell's 32,767 characters

A text section repeats its input once per @ placeholder, and TEXT handed
back whatever that built, so nested TEXT calls could multiply a long
text until memory ran out. Results past the cell text limit are now
#VALUE!.

crates/numfmt/src/lib.rs

@@ -425,6 +425,9 @@ impl NumberFormat {
 /// The `TEXT()` worksheet function: formats a value with a format code. Numeric text is
 /// converted to a number first; other text and booleans (as `TRUE`/`FALSE`) go through the text
 /// section, or are returned as is.
+/// Longest text a cell holds, in characters (Excel's limit), and so the longest `TEXT` result.
+const MAX_CELL_TEXT_CHARS: usize = 32_767;
+
 pub fn text_function(v: &Value, code: &str, sys: DateSystem) -> Result<String, CellError> {
     if code.is_empty() {
         return Ok(String::new());
@@ -440,7 +443,15 @@ pub fn text_function(v: &Value, code: &str, sys: DateSystem) -> Result<String, C
         Value::Bool(b) => return Ok(fmt.format_text(if b { "TRUE" } else { "FALSE" }).text),
         Value::Text(t) => match gridcraft_core::parse::parse_number_text(&t) {
             Some(n) => n,
-            None => return Ok(fmt.format_text(&t).text),
+            // Every `@` repeats the text: a result past a cell's limit is `#VALUE!`, so nested
+            // TEXT calls can't multiply it further.
+            None => {
+                let out = fmt.format_text(&t).text;
+                if out.len() > MAX_CELL_TEXT_CHARS && out.chars().count() > MAX_CELL_TEXT_CHARS {
+                    return Err(CellError::Value);
+                }
+                return Ok(out);
+            }
         },
         Value::Array(_) => return Err(CellError::Value),
     };

crates/numfmt/src/tests.rs

@@ -396,6 +396,10 @@ fn text_fn() {
     assert_eq!(text_function(&Value::Number(1234.5), "$#,##0.00", S), Ok("$1,234.50".into()));
     assert_eq!(text_function(&Value::from("5"), "0.00", S), Ok("5.00".into()));
     assert_eq!(text_function(&Value::from("abc"), "0.00", S), Ok("abc".into()));
+    // Each `@` repeats the text: up to a cell's 32,767 characters, then #VALUE!.
+    let at = "@".repeat(255);
+    assert_eq!(text_function(&Value::from("x".repeat(128).as_str()), &at, S).map(|s| s.len()), Ok(128 * 255));
+    assert_eq!(text_function(&Value::from("x".repeat(32_000).as_str()), &at, S), Err(CellError::Value));
     assert_eq!(text_function(&Value::Error(CellError::NA), "0", S), Err(CellError::NA));
     assert_eq!(text_function(&Value::Number(46302.0), "yyyy", S), Ok("2026".into()));
     assert_eq!(text_function(&Value::Number(0.5), "General", S), Ok("0.5".into()));