lib.rs

pub fn sing(from: i32, to: i32) -> String {
    (to..(from+1))
        .rev()
        .map(|i| verse(i) ).collect::<Vec<_>>()
        .join("\n")
}

pub fn verse(n: i32) -> String {
    let amount = bottles_of_beer(n);

    let action = match n {
        0 => "Go to the store and buy some more",
        1 => "Take it down and pass it around",
        _ => "Take one down and pass it around"
    };

    let next = match n {
        0 => bottles_of_beer(99),
        _ => bottles_of_beer(n-1)
    }.to_lowercase();

    format!("{} on the wall, {}.\n{}, {} on the wall.\n", amount, amount.to_lowercase(), action, next)
}

fn bottles_of_beer(n: i32) -> String {
    match n {
        0 => "No more bottles of beer".to_owned(),
        1 => "1 bottle of beer".to_owned(),
        _ => format!("{} bottles of beer", n)
    }
}