Collections Enums

Enums สำหรับ collection operations

Entry API

use std::collections::HashMap;
use std::collections::hash_map::Entry;

fn main() {
    let mut map: HashMap<&str, i32> = HashMap::new();

    // Entry enum: Occupied or Vacant
    match map.entry("key") {
        Entry::Vacant(entry) => {
            println!("Vacant: inserting");
            entry.insert(42);
        }
        Entry::Occupied(entry) => {
            println!("Occupied: {}", entry.get());
        }
    }

    // or_insert - insert if vacant
    map.entry("a").or_insert(1);
    map.entry("a").or_insert(100);  // Not inserted
    println!("a: {}", map.get("a").unwrap());

    // or_insert_with - lazy insert
    map.entry("b").or_insert_with(|| {
        println!("Computing default...");
        expensive_computation()
    });

    // or_default
    let counts: &mut i32 = map.entry("count").or_default();
    *counts += 1;

    // and_modify
    map.entry("a")
        .and_modify(|v| *v += 10)
        .or_insert(0);
    println!("a after modify: {}", map.get("a").unwrap());

    // Key access
    map.entry("key").and_modify(|v| *v *= 2);
    println!("key: {:?}", map.get("key"));

    println!("Map: {:?}", map);
}

fn expensive_computation() -> i32 {
    42
}