macro_rules! assert_approx_eq {
    ($a:expr, $b:expr, $eps:expr) => {{
        let (a, b) = (&$a, &$b);
        assert!(
            (*a - *b).abs() < $eps,
            "assertion failed: `(left !== right)` \
             (left: `{:?}`, right: `{:?}`, expect diff: `{:?}`, real diff: `{:?}`)",
            *a,
            *b,
            $eps,
            (*a - *b).abs()
        );
    }};
}