fix: plus evaluated before minus

src/nodes/evaluate.rs

@@ -112,24 +112,6 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) {
         end -= 1;
     }
 
-    while let Some(operator) = nodes[start..end]
-        .iter()
-        .position(|n| *n == Node::Operator(Operator::Plus))
-    {
-        let lhs = match nodes.remove(start + operator - 1) {
-            Node::Number(n) => n,
-            n => panic!("operator has an invalid operand: {n:?}"),
-        };
-        end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.add_from(lhs),
-            n => panic!("operator has an invalid operand: {n:?}"),
-        };
-
-        nodes.remove(start + operator - 1);
-        end -= 1;
-    }
-
     while let Some(operator) = nodes[start..end]
         .iter()
         .position(|n| *n == Node::Operator(Operator::Minus))
@@ -148,6 +130,24 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) {
         end -= 1;
     }
 
+    while let Some(operator) = nodes[start..end]
+        .iter()
+        .position(|n| *n == Node::Operator(Operator::Plus))
+    {
+        let lhs = match nodes.remove(start + operator - 1) {
+            Node::Number(n) => n,
+            n => panic!("operator has an invalid operand: {n:?}"),
+        };
+        end -= 1;
+        match &mut nodes[start..end][operator] {
+            Node::Number(n) => n.add_from(lhs),
+            n => panic!("operator has an invalid operand: {n:?}"),
+        };
+
+        nodes.remove(start + operator - 1);
+        end -= 1;
+    }
+
     if nodes.len() > 1 {
         nodes.remove(end);
         nodes.remove(start);

src/nodes/tests.rs

@@ -5,6 +5,24 @@ use rug::Float;
 
 use crate::nodes::{Function, Node, Nodes, Operator, PREC};
 
+#[test]
+fn plus_minus_order() {
+    assert_eq!(
+        Nodes::from_str("1 - 4 - 5").unwrap().evaluate().to_f64(),
+        -8.
+    );
+
+    assert_eq!(
+        Nodes::from_str("1 - 4 + 5").unwrap().evaluate().to_f64(),
+        2.
+    );
+
+    assert_eq!(
+        Nodes::from_str("1 + 4 - 5").unwrap().evaluate().to_f64(),
+        0.
+    );
+}
+
 #[test]
 fn e_edge_case() {
     assert_eq!(Nodes::from_str("1e-1").unwrap().evaluate().to_f64(), 0.1)