WIP: add second derivative test (#4)

samples/tests/mod.rs

@@ -2,7 +2,7 @@
 
 mod forward;
 mod reverse;
-
+mod second;
 mod higher;
 mod neohookean;
 mod traits;

samples/tests/second/mod.rs

@@ -0,0 +1,30 @@
+samples::test! {
+    forward_of_reverse;
+    // ANCHOR: forward_of_reverse
+    #[autodiff(df, Reverse, Duplicated, Active)]
+    fn f(x: &[f32; 2]) -> f32 {
+        x[0] * x[0] + x[1] * x[0]
+    }
+
+    #[autodiff(dg, Forward, Dual, Dual)]
+    fn g(x: &[f32; 2], gx: &mut [f32; 2]) {
+        let mut bx = [0.0, 0.0];
+        df(x, &mut bx, 1.0);
+        *gx = bx;
+    }
+
+    fn main() {
+        let x = [2.0, 3.0];
+        let fx = f(&x);
+        assert_eq!(10.0, fx);
+
+        let mut gx = [0.0, 0.0];
+        g(&x, &mut gx);
+        assert_eq!([7.0, 2.0], gx);
+
+        let dx = [1.0, 0.0];
+        let mut dgx = [0.0, 0.0];
+        dg(&x, &dx, &mut gx, &mut dgx);
+    }
+    // ANCHOR_END: forward_of_reverse
+}