rustlings/exercises/conversions/from_str.rs

from_str.rs

// from_str.rs
// This is similar to from_into.rs, but this time we'll implement `FromStr`
// and return errors instead of falling back to a default value.
// Additionally, upon implementing FromStr, you can use the `parse` method
// on strings to generate an object of the implementor type.
// You can read more about it at https://doc.rust-lang.org/std/str/trait.FromStr.html
use std::num::ParseIntError;
use std::ops::Not;
use std::str::FromStr;

#[derive(Debug, PartialEq)]
struct Person {
    name: String,
    age: usize,
}

// We will use this error type for the `FromStr` implementation.
#[derive(Debug, PartialEq)]
enum ParsePersonError {
    // Empty input string
    Empty,
    // Incorrect number of fields
    BadLen,
    // Empty name field
    NoName,
    // Wrapped error from parse::<usize>()
    ParseInt(ParseIntError),
}


// Steps:
// 1. If the length of the provided string is 0, an error should be returned
// 2. Split the given string on the commas present in it
// 3. Only 2 elements should be returned from the split, otherwise return an error
// 4. Extract the first element from the split operation and use it as the name
// 5. Extract the other element from the split operation and parse it into a `usize` as the age
//    with something like `"4".parse::<usize>()`
// 6. If while extracting the name and the age something goes wrong, an error should be returned
// If everything goes well, then return a Result of a Person object

impl FromStr for Person {
    type Err = ParsePersonError;
    fn from_str(s: &str) -> Result<Person, Self::Err> {
        if s.is_empty() {
            return Err(ParsePersonError::Empty);
        }

        let mut split = s.split(',');
        // Wrong, nth discard previous values
        // if split.nth(2).is_some() {
        //     return Err(ParsePersonError::BadLen);
        // }
        // let (name, age) = (
        //     split.next().ok_or(ParsePersonError::BadLen)?,
        //     split.next().ok_or(ParsePersonError::BadLen)?,
        // );

        // let (name, age, _) = (
        //     split.next().ok_or(ParsePersonError::BadLen)?,
        //     split.next().ok_or(ParsePersonError::BadLen)?,
        //     split
        //         .next()
        //         .map_or(Ok(()), |_| Err(ParsePersonError::BadLen))?,
        // );

        let (name, age) = match (split.next(), split.next(), split.next()) {
            (Some(name), Some(age), None) => (
                name.is_empty()
                    .not()
                    .then(|| name.into())
                    .ok_or(ParsePersonError::NoName)?,
                age.parse::<usize>().map_err(ParsePersonError::ParseInt)?,
            ),
            _ => return Err(ParsePersonError::BadLen),
        };

        Ok(Person { name, age })
    }
}

fn main() {
    let p = "Mark,20".parse::<Person>().unwrap();
    println!("{:?}", p);
}

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

    #[test]
    fn empty_input() {
        assert_eq!("".parse::<Person>(), Err(ParsePersonError::Empty));
    }
    #[test]
    fn good_input() {
        let p = "John,32".parse::<Person>();
        assert!(p.is_ok());
        let p = p.unwrap();
        assert_eq!(p.name, "John");
        assert_eq!(p.age, 32);
    }
    #[test]
    fn missing_age() {
        assert!(matches!(
            "John,".parse::<Person>(),
            Err(ParsePersonError::ParseInt(_))
        ));
    }

    #[test]
    fn invalid_age() {
        assert!(matches!(
            "John,twenty".parse::<Person>(),
            Err(ParsePersonError::ParseInt(_))
        ));
    }

    #[test]
    fn missing_comma_and_age() {
        assert_eq!("John".parse::<Person>(), Err(ParsePersonError::BadLen));
    }

    #[test]
    fn missing_name() {
        assert_eq!(",1".parse::<Person>(), Err(ParsePersonError::NoName));
    }

    #[test]
    fn missing_name_and_age() {
        assert!(matches!(
            ",".parse::<Person>(),
            Err(ParsePersonError::NoName | ParsePersonError::ParseInt(_))
        ));
    }

    #[test]
    fn missing_name_and_invalid_age() {
        assert!(matches!(
            ",one".parse::<Person>(),
            Err(ParsePersonError::NoName | ParsePersonError::ParseInt(_))
        ));
    }

    #[test]
    fn trailing_comma() {
        assert_eq!("John,32,".parse::<Person>(), Err(ParsePersonError::BadLen));
    }

    #[test]
    fn trailing_comma_and_some_string() {
        assert_eq!(
            "John,32,man".parse::<Person>(),
            Err(ParsePersonError::BadLen)
        );
    }
}