https://www.reddit.com/r/dailyprogrammer/comments/3h9pde/20150817_challenge_228_easy_letters_in/

challenge_228.rs

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

use std::fmt;

#[cfg(not(test))]
fn main() {
    let data = r#"
        billowy
        biopsy
        chinos
        defaced
        chintz
        sponged
        bijoux
        abhors
        fiddle
        begins
        chimps
        wronged
    "#;

    for word in data.lines().map(|line| line.trim()).filter(|word| !word.is_empty()) {
        println!("{}\t{}", word, order_status(word));
    }
}

#[derive(Debug, PartialEq)]
pub enum OrderStatus {
    InOrder,
    ReverseOrder,
    NotInOrder,
}

impl fmt::Display for OrderStatus {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let status = match *self {
            OrderStatus::InOrder => "IN ORDER",
            OrderStatus::ReverseOrder => "REVERSE ORDER",
            OrderStatus::NotInOrder => "NOT IN ORDER",
        };
        fmt.write_str(status)
    }
}

pub fn order_status(word: &str) -> OrderStatus {
    if word.chars().zip(word.chars().skip(1)).all(|(a, b)| a <= b) {
        OrderStatus::InOrder
    } else if word.chars().zip(word.chars().skip(1)).all(|(a, b)| a >= b) {
        OrderStatus::ReverseOrder
    } else {
        OrderStatus::NotInOrder
    }
}

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

    #[test]
    fn in_order() {
        expect!(order_status("abc")).to(be_equal_to(OrderStatus::InOrder));
        expect!(order_status("b")).to(be_equal_to(OrderStatus::InOrder));
        expect!(order_status("bb")).to(be_equal_to(OrderStatus::InOrder));
    }

    #[test]
    fn reverse_order() {
        expect!(order_status("cba")).to(be_equal_to(OrderStatus::ReverseOrder));
        expect!(order_status("cbb")).to(be_equal_to(OrderStatus::ReverseOrder));
        expect!(order_status("cca")).to(be_equal_to(OrderStatus::ReverseOrder));
    }

    #[test]
    fn not_in_order() {
        expect!(order_status("cda")).to(be_equal_to(OrderStatus::NotInOrder));
        expect!(order_status("abcba")).to(be_equal_to(OrderStatus::NotInOrder));
        expect!(order_status("abcbc")).to(be_equal_to(OrderStatus::NotInOrder));
    }
}