fn pig_latin_word(word: &str) -> String {
    let mut chars_iter = word.chars();

    let mut first_last = if let Some(first) = chars_iter.next() {
        let mut rest: String = chars_iter.collect();
        rest.push(first);
        rest
    } else {
        "".to_owned()
    };
    add_to_end(&first_last, "ay")
}

#[test]
fn test_pig_latin_word() {
    let values = vec![
        ("hello", "ellohay"),
        ("world", "orldway"),
        ("ab", "baay"),
        ("a", "aay"),
        ("", "ay"),
    ];

    for (input, expected_res) in values {
        let res = pig_latin_word(input);
        assert_eq!(res, expected_res);
    }
}

fn pig_latin_phrase<F: Fn(&str) -> String>(phrase: &str, pig_latin_word_fn: F) -> String {
    phrase
        .split_whitespace()
        .map(|word| pig_latin_word_fn(word))
        .collect::<Vec<String>>()
        .join(" ")
}

#[test]
fn test_pig_latin_phrase() {
    let values = vec![("hello world", "ellohay orldway"), ("ab a", "baay aay")];

    for (input, expected_res) in values {
        let res = pig_latin_phrase(input, pig_latin_word);
        assert_eq!(res, expected_res);
    }
}

fn pig_latin_phrase_capitalized(phrase: &str) -> String {
    pig_latin_phrase(phrase, pig_latin_word_capitalized)
}

fn pig_latin_word_capitalized(word: &str) -> String {
    let mut chars_iter = word.chars();

    let mut first_last = if let Some(first) = chars_iter.next() {
        let first_decap = if first.is_uppercase() {
            first.to_ascii_lowercase()
        } else {
            first
        };

        let mut rest: String = chars_iter.collect();

        rest.push(first_decap);
        capitalize_first_char(&rest)
    } else {
        "".to_owned()
    };
    first_last.push_str("ay");
    first_last
}

fn capitalize_first_char(word: &str) -> String {
    let owned_word = word.to_owned();
    let mut word_iter = owned_word.chars();

    if let Some(first) = word_iter.next() {
        let rest: String = word_iter.collect();
        let mut new_string = String::new();
        new_string.push(first.to_ascii_uppercase());
        new_string.push_str(&rest);
        new_string
    } else {
        owned_word.clone()
    }
}

#[test]
fn test_pig_latin_phrase_capitalized() {
    let values = vec![("hello world", "Ellohay Orldway"), ("ab a", "Baay Aay")];

    for (input, expected_res) in values {
        let res = pig_latin_phrase_capitalized(input);
        assert_eq!(res, expected_res);
    }
}

fn pig_latin_phrase_capitalized_first_word_only(phrase: &str) -> String {
    let acc: Vec<String> = phrase.split_whitespace().fold(Vec::new(), |mut acc, word| {
        if acc.len() == 0 {
            acc.push(pig_latin_word_capitalized(word));
        } else {
            acc.push(pig_latin_word(word));
        }
        acc
    });

    acc.join(", ")
}

#[test]
fn test_pig_latin_phrase_capitalized_first_word_only() {
    let values = vec![("hello world", "Ellohay, orldway"), ("ab a", "Baay, aay")];

    for (input, expected_res) in values {
        let res = pig_latin_phrase_capitalized_first_word_only(input);
        assert_eq!(res, expected_res);
    }
}

fn pig_latin_word_no_reorder(word: &str) -> String {
    add_to_end(word, "ay")
}

fn add_to_end(word: &str, postfix: &str) -> String {
    let mut owned_word = word.to_owned();
    owned_word.push_str(postfix);
    owned_word
}

fn pig_latin_word_n_chars(word: &str, n: usize) -> String {
    let owned_word = word.to_owned();
    let mut chars: Vec<char> = owned_word.chars().collect();
    let mut i = 0;

    if chars.len() > n {
        while i < n {
            let first = chars.remove(0);
            chars.push(first);
            i += 1
        }
        let n_rev_word: String = chars.into_iter().collect();
        add_to_end(&n_rev_word, "ay")
    } else {
        add_to_end(word, "ay")
    }
}

#[test]
fn test_pig_latin_phrase_n_word_reorder() {
    let values = vec![("dragons strike quickly", "agonsdray rikestay icklyquay")];

    for (input, expected_res) in values {
        let res = pig_latin_phrase(input, |x| pig_latin_word_n_chars(x, 2));
        assert_eq!(res, expected_res);
    }
}