Long operator chains: evaluate them without the depth limit

A formula such as =A1+A2+…+A500 nests one level per operator, and the
evaluator recursed once per level, so past its depth limit (about 400
operators) the result was #CALC!. Excel allows such formulas up to its
8,192-character limit. Chains of arithmetic, comparison and & operators
are now folded in a loop.

crates/calc/src/eval.rs

@@ -465,9 +465,22 @@ impl<'h> Evaluator<'h> {
                 }
             }
             _ => {
-                let x = self.value(a);
-                let y = self.value(b);
-                Ev::V(binary_values(op, &x, &y))
+                // A long chain such as A1+A2+…+A500 nests to the left: fold it in a loop instead
+                // of one level of recursion per operator (past the depth limit it was #CALC!).
+                let mut rest = vec![(op, b)];
+                let mut left = a;
+                while let Expr::Binary(o, l, r) = left
+                    && !matches!(o, BinOp::Range | BinOp::Union | BinOp::Intersect)
+                {
+                    rest.push((*o, r.as_ref()));
+                    left = l.as_ref();
+                }
+                let mut acc = self.value(left);
+                while let Some((o, r)) = rest.pop() {
+                    let y = self.value(r);
+                    acc = binary_values(o, &acc, &y);
+                }
+                Ev::V(acc)
             }
         }
     }

crates/calc/src/tests.rs

@@ -549,6 +549,17 @@ fn text_function() {
     assert_eq!(t.get("A1"), Value::from("1,234.50"));
 }
 
+#[test]
+fn long_operator_chains() {
+    let mut t = T::new();
+    t.set("A1", "1");
+    // 1,000 additions: a formula of 3,000 characters, well within Excel's 8,192.
+    t.set("B1", &format!("={}", vec!["A1"; 1000].join("+")));
+    assert_eq!(t.num("B1"), 1000.0);
+    t.set("B2", &format!("=5{}", "+1-1".repeat(600)));
+    assert_eq!(t.num("B2"), 5.0);
+}
+
 #[test]
 fn subtotal_skips_hidden() {
     let mut t = T::new();