https://www.reddit.com/r/dailyprogrammer/comments/3aqvjn/20150622_challenge_220_easy_mangling_sentences/

challenge_220.rs

#[cfg(test)]
#[macro_use(expect)]
extern crate expectest;

#[cfg(not(test))]
fn main() {
    println!("{}", mangle("This challenge doesn't seem so hard."));
}

#[derive(Copy, Clone)]
enum MaskedChar {
    Lowercase,
    Uppercase,
    Misc(char),
}

fn mask_char(ch: char) -> MaskedChar {
    if ch.is_lowercase() {
        MaskedChar::Lowercase
    } else if ch.is_uppercase() {
        MaskedChar::Uppercase
    } else {
        MaskedChar::Misc(ch)
    }
}

fn to_mask(word: &[char]) -> Vec<MaskedChar> {
    word.iter().cloned().map(mask_char).collect()
}

fn char_to_char((c, m): (char, MaskedChar)) -> char {
    match m {
        MaskedChar::Misc(ch) => ch,
        MaskedChar::Uppercase => c.to_uppercase().next().unwrap_or(c),
        MaskedChar::Lowercase => c.to_lowercase().next().unwrap_or(c),
    }
}

fn fill_mask(word: &[char], mask: &[MaskedChar]) -> String {
    word.iter().cloned().zip(mask.iter().cloned()).map(char_to_char).collect()
}

fn sorted_chars(word: &str) -> Vec<char> {
    use std::cmp::Ordering;
    let mut chars: Vec<char> = word.to_lowercase().chars().collect();
    chars.sort_by(|a, b| {
        if a.is_alphabetic() && b.is_alphabetic() {
            a.cmp(b)
        } else {
            Ordering::Equal
        }
    });
    chars
}

fn chars(word: &str) -> Vec<char> {
    word.chars().collect()
}

pub fn mangle(s: &str) -> String {
    s.split_whitespace().map(|word| {
        fill_mask(&sorted_chars(word), &to_mask(&chars(word)))
    }).collect::<Vec<_>>().connect(" ")
}

#[cfg(test)]
mod tests {
    use super::*;
    use expectest::prelude::*;

    #[test]
    fn phrase_1() {
        let string = "This challenge doesn't seem so hard.";
        expect!(mangle(string)).to(be_equal_to("Hist aceeghlln denos't eems os adhr."));
    }

    #[test]
    fn phrase_2() {
        let string = "There are more things between heaven and earth, Horatio, than are dreamt of in your philosophy.";
        expect!(mangle(string)).to(be_equal_to("Eehrt aer emor ghinst beeentw aeehnv adn aehrt, Ahioort, ahnt aer ademrt fo in oruy hhilooppsy."));
    }
}