Indexing Cat Groups

cat_groups.rs

/// Cats, groups of cats, and cat ages obfuscate business objects
/// for an actual problem trying to be solved

use std::collections::HashMap;

struct Cat {
    age: u8
}

struct CatGroup {
    cats: Vec<Cat>,
    cat_index: HashMap<u8, Vec<usize>>
}

impl CatGroup {
    fn new() -> CatGroup {
        CatGroup{ cats: Vec::new(), cat_index: HashMap::new()}
    }

    fn add_cat(&mut self, cat: Cat) {
        self.cat_index
            .entry(cat.age)
            .or_insert(Vec::new())
            .push(self.cats.len());
        self.cats.push(cat);
    }

    fn iter(&self) -> impl Iterator<Item = &Cat> {
        self.cats.iter()
    }

    fn iter_cats_with_age<'a>(&'a self, age: u8) -> Box<dyn Iterator<Item = &Cat> + 'a> {
        match self.cat_index.get(&age) {
            Some(v) => Box::new(v.iter().map(move |idx| self.cats.get(*idx).unwrap())),
            None => Box::new(std::iter::empty())
        }
    }
}

fn main() {
    let mut cats = CatGroup::new();
    cats.add_cat(Cat{age: 4});
    cats.add_cat(Cat{age: 4});
    cats.add_cat(Cat{age: 5});
    cats.add_cat(Cat{age: 6});
    cats.add_cat(Cat{age: 4});

    println!("Four year old cats:");
    for cat in cats.iter_cats_with_age(4) {
        println!("Cat with age {}", cat.age);
    }

    println!("All Cats:");
    for cat in cats.iter() {
        println!("Cat with age {}", cat.age);
    }
}