extdot-impl 0.2.0

Please see `extdot` crate for actual usage
Documentation 
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// See https://users.rust-lang.org/t/iterator-need-to-identify-the-last-element/8836

//! This module contains general helper traits.
use std::{ iter, mem };

/// This trait defines the iterator adapter `identify_first_last()`.
/// The new iterator gives a tuple with an `(element, is_first, is_last)`.
/// `is_first` is true when `element` is the first we are iterating over.
/// `is_last` is true when `element` is the last and no others follow.
pub trait IdentifyFirstLast: Iterator + Sized {
    fn identify_first_last(self) -> Iter<Self>;
}

/// Implement the iterator adapter `identify_first_last()`
impl<I> IdentifyFirstLast for I where I: Iterator {
    fn identify_first_last(self) -> Iter<Self> {
        Iter(true, self.peekable())
    }
}

/// A struct to hold the iterator's state
/// Our state is a bool telling if this is the first element.
pub struct Iter<I>(bool, iter::Peekable<I>) where I: Iterator;


impl<I> Iterator for Iter<I> where I: Iterator {
    type Item = (bool, bool, I::Item);

    /// At `next()` we copy false to the state variable.
    /// And `peek()` adhead to see if this is the last one.
    fn next(&mut self) -> Option<Self::Item> {
        let first = mem::replace(&mut self.0, false);
        self.1.next().map(|e| (first, self.1.peek().is_none(), e))
    }
}


#[test]
fn test_iterator_adaptor_identify_first_last(){

    let mut iter = "abcde".chars().identify_first_last();
    assert_eq!(iter.next(), Some((true,  false, 'a')));
    assert_eq!(iter.next(), Some((false, false, 'b')));
    assert_eq!(iter.next(), Some((false, false, 'c')));
    assert_eq!(iter.next(), Some((false, false, 'd')));
    assert_eq!(iter.next(), Some((false, true , 'e')));

    let mut iter = "a".chars().identify_first_last();
    assert_eq!(iter.next(), Some((true,  true,  'a')));
}