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//! Provides a macro make checked math pretty easy.
//! 
//! ```
//! # #[macro_use] extern crate cheque;
//! # fn main() {
//! let a = 10u8;
//! let b = 20u8;
//! 
//! let_checked![a, b];
//! 
//! assert_eq!(a + b, 30);
//! assert_eq!(b * b, None);
//! assert_eq!(b / 0, None);
//! assert_eq!(a - 20, None);
//! assert_eq!((a - b) + 1, None);
//! # }
//! ```
//! 
//! `let_checked!` redeclares each identifier as a checked numeric value.
//! You can then use `+`, etc. on the checked variables, and then deref the result to get an
//! `Option<_>`.
//! 
//! You can also use numeric literals/unchecked values, so long as they are on the right side of
//! the operation.
//! 
//! ```
//! # #[macro_use] extern crate cheque;
//! # fn main() {
//! let c = 20usize;
//! let_checked![c];
//! 
//! if let Some(scary) = *(c - 100) {
//!     panic!("Ahh! {:?}", scary);
//! }
//! # }
//! ```
//! 
//! 
//! 
//! If you are doing generic programming, you should add the [checked num_traits] to your
//! `where` bounds.
//! [checked num_traits]: http://rust-num.github.io/num/num_traits/ops/checked/index.html

extern crate num_traits;

use std::ops::*;
use std::cmp::PartialEq;

use num_traits::ops::checked::*;

#[macro_export]
macro_rules! let_checked {
    ($($ident:ident),*) => {$(
        let $ident = $crate::Checker(Some($ident));
    )*};
    ($($ident:ident,)*) => {$(
        let $ident = $crate::Checker(Some($ident));
    )*};
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
/// This implements `Deref<Option<T>>`.
pub struct Checker<T>( #[doc(hidden)] pub /*(macro)*/ Option<T>);

impl<T> Deref for Checker<T> {
    type Target = Option<T>;
    fn deref(&self) -> &Self::Target { &self.0 }
}

impl<T> DerefMut for Checker<T> {
    fn deref_mut(&mut self) -> &mut Self::Target { &mut self.0 }
}

impl<T> PartialEq<T> for Checker<T>
where T: PartialEq<T> + Copy
{
    fn eq(&self, other: &T) -> bool {
        self.0 == Some(*other)
    }
}

impl<T> PartialEq<Option<T>> for Checker<T>
where T: PartialEq<T>
{
    fn eq(&self, other: &Option<T>) -> bool {
        &self.0 == other
    }
}

macro_rules! impl_checked {
    ($Vanilla:ident, $vanilla_fn:ident, $Checked:ident, $checked_fn:ident) => {
        impl<T> $Vanilla<Self> for Checker<T>
        where T: $Checked
        {
            type Output = Self;
            #[inline]
            fn $vanilla_fn(self, rhs: Checker<T>) -> Self {
                if let (Some(l), Some(r)) = (self.0, rhs.0) {
                    Checker(l.$checked_fn(&r))
                } else {
                    Checker(None)
                }
            }
        }

        impl<T> $Vanilla<T> for Checker<T>
        where T: $Checked
        {
            type Output = Self;
            #[inline]
            fn $vanilla_fn(self, rhs: T) -> Self {
                Checker(self.0.and_then(|l| l.$checked_fn(&rhs)))
            }
        }
    }
}

impl_checked![Add, add, CheckedAdd, checked_add];
impl_checked![Sub, sub, CheckedSub, checked_sub];
impl_checked![Mul, mul, CheckedMul, checked_mul];
impl_checked![Div, div, CheckedDiv, checked_div];


#[cfg(test)]
mod test {
    #[test]
    fn compiles() {
        let a = 10;
        let_checked![a];
        a + a;
        a - a;
        a * a;
        a / a;
        a + 1;
        a - 1;
        a * 1;
        a / 1;
    }

    #[test]
    fn divide_by_zero() {
        let b = 1u8;
        let z = 0u8;
        let_checked![b, z, /* assert compilation w/ comma */ ];
        assert_eq!(b / z, None);
    }

    #[test]
    fn empty_invoke() {
        let_checked![]; //... how does it disambiguate? o_O
    }
}