1 files changed, 101 insertions, 0 deletions
diff --git a/src/simplify.rs b/src/simplify.rs
new file mode 100644
index 0000000..ee51fb9
--- /dev/null
+++ b/src/simplify.rs
@@ -0,0 +1,101 @@
+use crate::sexpr::{Atom, Expr, Number};
+
+fn simplify_once(expr: Expr) -> Expr {
+    match expr {
+        Expr::Atom(_) => expr,
+        Expr::Application(items) => {
+            let simplified_items: Vec<Expr> = items.into_iter().map(simplify_once).collect();
+
+            if simplified_items.is_empty() {
+                return Expr::Application(simplified_items);
+            }
+
+            match &simplified_items[0] {
+                Expr::Atom(Atom::Builtin(op)) if op == "+" => {
+                    let mut sum = 0.0;
+                    for arg in &simplified_items[1..] {
+                        if let Expr::Atom(Atom::Number(i)) = arg {
+                            sum += i.0;
+                        } else {
+                            return Expr::Application(simplified_items);
+                        }
+                    }
+                    Expr::Atom(Atom::Number(Number(sum)))
+                }
+                Expr::Atom(Atom::Builtin(op)) if op == "*" => {
+                    let mut product = 1.0;
+                    for arg in &simplified_items[1..] {
+                        if let Expr::Atom(Atom::Number(i)) = arg {
+                            product *= i.0;
+                        } else {
+                            return Expr::Application(simplified_items);
+                        }
+                    }
+                    Expr::Atom(Atom::Number(Number(product)))
+                }
+                _ => Expr::Application(simplified_items),
+            }
+        }
+        Expr::OrderedList(items) => {
+            Expr::OrderedList(items.into_iter().map(simplify_once).collect())
+        }
+    }
+}
+
+pub fn simplify(expr: Expr) -> Expr {
+    let mut current = expr;
+
+    loop {
+        let next = simplify_once(current.clone());
+        if next == current {
+            return next;
+        }
+        current = next;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn num(i: f64) -> Expr {
+        Expr::Atom(Atom::Number(Number(i)))
+    }
+
+    fn builtin(name: &str) -> Expr {
+        Expr::Atom(Atom::Builtin(name.to_string()))
+    }
+
+    fn app(items: Vec<Expr>) -> Expr {
+        Expr::Application(items)
+    }
+
+    #[test]
+    fn simplifies_addition_of_integers() {
+        let expr = app(vec![builtin("+"), num(2.0), num(3.0), num(4.0)]);
+        assert_eq!(simplify(expr), num(9.0));
+    }
+
+    #[test]
+    fn simplifies_multiplication_of_integers() {
+        let expr = app(vec![builtin("*"), num(2.0), num(3.0), num(4.0)]);
+        assert_eq!(simplify(expr), num(24.0));
+    }
+
+    #[test]
+    fn simplifies_nested_numeric_forms() {
+        let expr = app(vec![
+            builtin("+"),
+            app(vec![builtin("*"), num(2.0), num(3.0)]),
+            app(vec![builtin("+"), num(4.0), num(5.0)]),
+        ]);
+
+        assert_eq!(simplify(expr), num(15.0));
+    }
+
+    #[test]
+    fn does_not_simplify_when_non_integer_argument_present() {
+        let expr = app(vec![builtin("+"), num(2.0), builtin("x")]);
+        assert_eq!(simplify(expr.clone()), expr);
+    }
+}