add gcd() function

src/ast/functions.rs

@@ -59,5 +59,28 @@ pub fn default_functions() -> FnMap {
     def_fn!(round);
     def_fn!(abs);
 
+    def_fn!(
+        gcd = |expr, args| {
+            let mut args = args.iter().map(|a| expr.eval(a));
+            let mut r = args.next().unwrap()?;
+            for n in args {
+                r = gcd(r, n?);
+            }
+            Ok(r)
+        }
+    );
+
     map
 }
+
+fn gcd(mut a: f64, mut b: f64) -> f64 {
+    if a == 0.0 {
+        return b;
+    }
+    while b > 0.0 {
+        let r = a % b;
+        a = b;
+        b = r;
+    }
+    return a;
+}

src/test.rs

@@ -202,7 +202,13 @@ fn functions() {
 
     test_fn!(
         log_7(expr!(num!(7.0), op!(Exponent), num!(6.0))),
-        "log_7(7 ** 6)", // TODO: fractions in input
+        "log_7(7 ** 6)",
         6.0
     );
+
+    test_fn!(
+        gcd(expr!(num!(3.0)), expr!(num!(6.0)), expr!(num!(54.0))),
+        "gcd(3, 6, 54)",
+        3.0
+    );
 }