average 0.6.0

Calculate statistics iteratively
Documentation 
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/// Estimate the reduction of a sequence of numbers ("population").
///
/// The reduction is a given function `Fn(f64, f64) -> f64`.
///
/// Everything is calculated iteratively using constant memory, so the sequence
/// of numbers can be an iterator.
#[derive(Debug, Clone)]
pub struct Reduce<F> {
    x: f64,
    reduce: F,
}

impl<F> Reduce<F>
    where F: Fn(f64, f64) -> f64
{
    /// Create a new reduction estimator given an initial value and a reduction.
    #[inline]
    pub fn from_value_and_fn(x: f64, f: F) -> Reduce<F> {
        Reduce { x: x, reduce: f }
    }

    /// Add an element sampled from the population.
    #[inline]
    pub fn add(&mut self, x: f64) {
        self.x = (self.reduce)(self.x, x);
    }

    /// Estimate the reduction of the population.
    #[inline]
    pub fn reduction(&self) -> f64 {
        self.x
    }

    /// Merge another sample into this one.
    #[inline]
    pub fn merge(&mut self, other: &Reduce<F>) {
        self.add(other.x);
    }
}