playground.rs

use std::collections::HashSet;

#[derive(Debug)]
enum Check {
    Known(String),
    Duplicate(String),
    Missing(String),
    Unknown(String)
}

fn main() {
    let known_values: HashSet<_> = [
        "a".to_string(),
        "b".to_string(),
        "c".to_string()]
            .iter().cloned().collect();
        
    let provided_values = vec![
        "a".to_string(),
        "b".to_string(),
        "z".to_string(),
        "b".to_string()
    ];
    
    let mut found = HashSet::new();
        
    let mut check_values: Vec<_> = provided_values.iter().cloned()
        .map(|v| {
            if known_values.contains(&v) {
                if found.contains(&v) {
                    Check::Duplicate(v)
                } else {
                    found.insert(v.clone());
                    Check::Known(v)
                }
            } else {
                Check::Unknown(v)
            }
        }).collect();
        
    let missing = known_values.difference(&found);
    
    check_values = missing
        .cloned()
        .fold(check_values, |mut cv, m| {
            cv.push(Check::Missing(m));
            cv
        });
        
    println!("check_values: {:#?}", check_values);
}