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/// The `Measurement` trait and the `implement_measurement!` macro
/// provides a common way for various measurements to be implemented.
/// 
/// # Example
/// ```
/// // Importing the `implement_measurement` macro from the external crate is important
/// #[macro_use]
/// extern crate measurements;
/// 
/// use measurements::Measurement;
/// 
/// struct Cubits {
///     forearms: f64
/// }
/// 
/// impl Measurement for Cubits {
///     fn get_base_units(&self) -> f64 {
///         self.forearms
///     }
///     
///     fn from_base_units(units: f64) -> Self {
///         Cubits { forearms: units }
///     }
/// }
///
/// // Invoke the macro to automatically implement Add, Sub, etc...
/// implement_measurement! { Cubits }
///
/// // The main function here is only included to make doc tests compile.
/// // You should't need it in your own code.
/// fn main() { }
/// ```
pub trait Measurement {
    fn get_base_units(&self) -> f64;
    fn from_base_units(units: f64) -> Self;
}

/// This is a special macro that creates the code to implement
/// operator and comparison overrides.
#[macro_export]
macro_rules! implement_measurement {
    ($($t:ty)*) => ($(
        impl ::std::ops::Add for $t {
            type Output = Self;
            
            fn add(self, rhs: Self) -> Self {
                Self::from_base_units(self.get_base_units() + rhs.get_base_units())
            }
        }

        impl ::std::ops::Sub for $t {
            type Output = Self;
            
            fn sub(self, rhs: Self) -> Self {
                Self::from_base_units(self.get_base_units() - rhs.get_base_units())
            }
        }

        /// 
        /// Dividing a `$t` by another `$` returns a ratio.
        /// 
        impl ::std::ops::Div<$t> for $t {
            type Output = f64;
            
            fn div(self, rhs: Self) -> f64 {
                self.get_base_units() / rhs.get_base_units()
            }
        }

        /// 
        /// Dividing a `$` by a factor returns a new portion of the measurement.
        /// 
        impl ::std::ops::Div<f64> for $t {
            type Output = Self;
            
            fn div(self, rhs: f64) -> Self {
                Self::from_base_units(self.get_base_units() / rhs)
            }
        }

        /// 
        /// Multiplying a `$t` by another `$t` returns the product of those measurements.
        /// 
        impl ::std::ops::Mul<$t> for $t {
            type Output = Self;
            
            fn mul(self, rhs: Self) -> Self {
                Self::from_base_units(self.get_base_units() * rhs.get_base_units())
            }
        }

        /// 
        /// Multiplying a `$t` by a factor increases (or decreases) that measurement a number of times.
        /// 
        impl ::std::ops::Mul<f64> for $t {
            type Output = Self;
            
            fn mul(self, rhs: f64) -> Self {
                Self::from_base_units(self.get_base_units() * rhs)
            }
        }

        impl ::std::cmp::Eq for $t { }
        impl ::std::cmp::PartialEq for $t {
            fn eq(&self, other: &Self) -> bool {
                self.get_base_units() == other.get_base_units()
            }
        }

        impl ::std::cmp::PartialOrd for $t {
            fn partial_cmp(&self, other: &Self) -> Option<::std::cmp::Ordering> {
                self.get_base_units().partial_cmp(&other.get_base_units())
            }
        }
    )*)
}