Rust: Rust 集合：HashMap 与 HashSet 详解

let mut candidates = Vec::new();
let mut votes = Vec::new();

candidates.push("Alice");
votes.push(0);

candidates.push("Bob");
votes.push(0);

// 给 Alice 投票
let pos = candidates.iter().position(|&c| c == "Alice").unwrap();
votes[pos] += 1;

// 查询 Bob 的票数
let pos = candidates.iter().position(|&c| c == "Bob").unwrap();
println!("Bob 的票数: {}", votes[pos]);

use std::collections::HashMap;

fn main() {
    let mut votes = HashMap::new();

    // 给候选人投票
    *votes.entry("Alice").or_insert(0) += 1;
    *votes.entry("Bob").or_insert(0) += 1;
    *votes.entry("Alice").or_insert(0) += 1;  // 再投 Alice
    *votes.entry("Charlie").or_insert(0) += 1;

    // 查询票数
    for (candidate, count) in &votes {
        println!("{}: {} 票", candidate, count);
    }

    // 查询特定候选人
    println!("Alice 的票数: {}", votes.get("Alice").unwrap());
}

Alice: 2 票
Bob: 1 票
Charlie: 1 票
Alice 的票数: 2

// ============================================
// 投票统计系统：展示 HashMap 基本 API
// ============================================

use std::collections::HashMap;

fn main() {
    // Create a new empty HashMap
    let mut vote_counts: HashMap<String, u32> = HashMap::new();

    // --- insert ---
    // Insert key-value pairs (overwrites existing value)
    vote_counts.insert(String::from("Alice"), 0);
    vote_counts.insert(String::from("Bob"), 0);
    vote_counts.insert(String::from("Charlie"), 0);

    println!("After initial insert:");
    print_votes(&vote_counts);

    // --- get ---
    // Get a value by key (returns Option<&V>)
    let alice_votes = vote_counts.get("Alice");
    match alice_votes {
        Some(count) => println!("Alice's votes (via get): {}", count),
        None => println!("Alice not found"),
    }

    // --- entry API ---
    // The idiomatic way: insert or update
    // entry() returns an Entry enum, or_insert() inserts default if missing
    println!("\n--- Voting round ---");
    let candidates = ["Alice", "Bob", "Alice", "Charlie", "Alice", "Bob", "David"];
    for name in &candidates {
        let count = vote_counts.entry(String::from(*name)).or_insert(0);
        *count += 1;
        println!("Voted for {} (total: {})", name, count);
    }

    // --- contains_key ---
    println!("\n--- Checking candidates ---");
    for name in &["Alice", "David", "Eve"] {
        if vote_counts.contains_key(*name) {
            println!("{} is a candidate with {} votes", name, vote_counts.get(*name).unwrap());
        } else {
            println!("{} is NOT a candidate", name);
        }
    }

    // --- len and is_empty ---
    println!("\nTotal candidates: {}", vote_counts.len());
    println!("Is empty: {}", vote_counts.is_empty());

    // --- Final results ---
    println!("\n--- Final Results ---");
    print_votes(&vote_counts);
}

fn print_votes(votes: &HashMap<String, u32>) {
    // Note: HashMap iteration order is NOT guaranteed
    for (name, count) in votes {
        println!("  {}: {} votes", name, count);
    }
}

After initial insert:
  Alice: 0 votes
  Charlie: 0 votes
  Bob: 0 votes

Alice's votes (via get): 0

--- Voting round ---
Voted for Alice (total: 1)
Voted for Bob (total: 1)
Voted for Alice (total: 2)
Voted for Charlie (total: 1)
Voted for Alice (total: 3)
Voted for Bob (total: 2)
Voted for David (total: 1)

--- Checking candidates ---
Alice is a candidate with 3 votes
David is a candidate with 1 votes
Eve is NOT a candidate

Total candidates: 4
Is empty: false

--- Final Results ---
  Alice: 3 votes
  Charlie: 1 votes
  David: 1 votes
  Bob: 2 votes

// ============================================
// HashMap 所有权规则：什么类型能做 key/value
// ============================================

use std::collections::HashMap;

#[derive(Debug, Hash, Eq, PartialEq)]
struct ProductId(u32);

#[derive(Debug, Clone)]
struct Product {
    name: String,
    price: f64,
    stock: u32,
}

fn main() {
    // --- Rule 1: Owned types as keys ---
    // String (owned) can be a key; &str (borrowed) needs lifetime management
    let mut inventory: HashMap<String, Product> = HashMap::new();

    let product = Product {
        name: String::from("Rust Book"),
        price: 29.99,
        stock: 100,
    };

    // insert takes ownership of key and value
    inventory.insert(String::from("RB-001"), product);
    // println!("{:?}", product);  // ❌ product was moved into the HashMap

    // --- Rule 2: Inserting a reference ---
    // Borrowed keys need lifetime annotations on the HashMap
    // This works because the string literals have 'static lifetime
    let mut lookup: HashMap<&str, u32> = HashMap::new();
    lookup.insert("apple", 5);
    lookup.insert("banana", 3);
    println!("Lookup table: {:?}", lookup);

    // --- Rule 3: Getting values returns references ---
    // get() returns Option<&V>, not V
    let stock_ref = inventory.get("RB-001");
    match stock_ref {
        Some(p) => println!("Product: {}, price: {}", p.name, p.price),
        None => println!("Not found"),
    }
    // inventory is still valid (we only borrowed)

    // --- Rule 4: Custom types as keys ---
    // Keys must implement Eq + Hash
    let mut product_map: HashMap<ProductId, String> = HashMap::new();
    product_map.insert(ProductId(1), String::from("Laptop"));
    product_map.insert(ProductId(2), String::from("Mouse"));

    // --- Rule 5: Updating values with get_mut ---
    // get_mut() returns Option<&mut V> for mutable access
    if let Some(product) = inventory.get_mut("RB-001") {
        product.stock -= 1;  // Sell one unit
        println!("Updated stock: {}", product.stock);
    }

    // --- Rule 6: The entry API for sophisticated updates ---
    let mut word_count: HashMap<String, u32> = HashMap::new();
    let text = "hello world hello rust hello again";

    for word in text.split_whitespace() {
        // or_insert returns &mut V, which we dereference and increment
        let counter = word_count.entry(String::from(word)).or_insert(0);
        *counter += 1;
    }
    println!("\nWord count: {:?}", word_count);

    // Advanced: modify entry with and_modify + or_insert
    let mut scores: HashMap<String, u32> = HashMap::new();
    for team in &["red", "blue", "red", "green", "blue", "red"] {
        scores.entry(String::from(*team))
            .and_modify(|count| *count += 1)  // if exists, increment
            .or_insert(1);                     // if not, insert 1
    }
    println!("Scores: {:?}", scores);
}

Lookup table: {"banana": 3, "apple": 5}
Product: Rust Book, price: 29.99
Updated stock: 99

Word count: {"again": 1, "hello": 2, "rust": 1, "world": 1}
Scores: {"green": 1, "blue": 2, "red": 3}

// ============================================
// HashSet：去重、交集、并集、差集运算
// ============================================

use std::collections::HashSet;

fn main() {
    // --- Basic HashSet: deduplication ---
    println!("--- HashSet Deduplication ---");
    let mut unique_numbers: HashSet<i32> = HashSet::new();

    let numbers = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5];
    for &n in &numbers {
        unique_numbers.insert(n);
    }
    println!("Original: {:?}", &numbers[..]);
    println!("Unique: {:?}", unique_numbers);
    println!("Count: {} (original: {})", unique_numbers.len(), numbers.len());

    // --- contains ---
    println!("\n--- Contains Check ---");
    for &n in &[1, 7, 9] {
        if unique_numbers.contains(&n) {
            println!("{} is in the set", n);
        } else {
            println!("{} is NOT in the set", n);
        }
    }

    // --- Set operations ---
    println!("\n--- Set Operations ---");

    let set_a: HashSet<i32> = [1, 2, 3, 4, 5].iter().cloned().collect();
    let set_b: HashSet<i32> = [4, 5, 6, 7, 8].iter().cloned().collect();

    println!("Set A: {:?}", set_a);
    println!("Set B: {:?}", set_b);

    // Union: elements in A OR B
    let union: HashSet<&i32> = set_a.union(&set_b).collect();
    println!("Union (A ∪ B): {:?}", union);

    // Intersection: elements in A AND B
    let intersection: HashSet<&i32> = set_a.intersection(&set_b).collect();
    println!("Intersection (A ∩ B): {:?}", intersection);

    // Difference: elements in A but NOT in B
    let diff_ab: HashSet<&i32> = set_a.difference(&set_b).collect();
    println!("Difference (A - B): {:?}", diff_ab);

    let diff_ba: HashSet<&i32> = set_b.difference(&set_a).collect();
    println!("Difference (B - A): {:?}", diff_ba);

    // Symmetric difference: elements in A or B but NOT both
    let sym_diff: HashSet<&i32> = set_a.symmetric_difference(&set_b).collect();
    println!("Symmetric Difference: {:?}", sym_diff);

    // --- Practical example: finding common friends ---
    println!("\n--- Practical: Common Friends ---");

    let alice_friends: HashSet<&str> =
        ["Bob", "Charlie", "David", "Eve"].iter().cloned().collect();
    let bob_friends: HashSet<&str> =
        ["Alice", "Charlie", "Eve", "Frank"].iter().cloned().collect();

    println!("Alice's friends: {:?}", alice_friends);
    println!("Bob's friends: {:?}", bob_friends);

    // Mutual friends (intersection)
    let mutual: HashSet<&&str> = alice_friends.intersection(&bob_friends).collect();
    println!("Mutual friends: {:?}", mutual);

    // Friends only Alice knows (difference)
    let alice_only: HashSet<&&str> = alice_friends.difference(&bob_friends).collect();
    println!("Only Alice knows: {:?}", alice_only);

    // All unique friends (union)
    let all_friends: HashSet<&&str> = alice_friends.union(&bob_friends).collect();
    println!("All unique friends: {:?}", all_friends);
}

--- HashSet Deduplication ---
Original: [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]
Unique: {3, 2, 1, 6, 4, 9, 5}
Count: 7 (original: 11)

--- Contains Check ---
1 is in the set
7 is NOT in the set
9 is in the set

--- Set Operations ---
Set A: {2, 3, 4, 5, 1}
Set B: {4, 7, 6, 5, 8}
Union (A ∪ B): {7, 2, 3, 6, 4, 5, 1, 8}
Intersection (A ∩ B): {4, 5}
Difference (A - B): {2, 3, 1}
Difference (B - A): {6, 7, 8}
Symmetric Difference: {1, 2, 3, 6, 7, 8}

--- Practical: Common Friends ---
Alice's friends: {"Charlie", "David", "Eve", "Bob"}
Bob's friends: {"Charlie", "Frank", "Alice", "Eve"}
Mutual friends: {"Charlie", "Eve"}
Only Alice knows: {"David", "Bob"}
All unique friends: {"Charlie", "David", "Frank", "Alice", "Eve", "Bob"}

// ============================================
// 遍历 HashMap + 集合选择策略对比
// ============================================

use std::collections::HashMap;

fn main() {
    // --- Build a sample dataset ---
    let mut sales: HashMap<String, f64> = HashMap::new();
    sales.insert(String::from("Laptop"), 1200.0);
    sales.insert(String::from("Mouse"), 25.0);
    sales.insert(String::from("Keyboard"), 80.0);
    sales.insert(String::from("Monitor"), 350.0);
    sales.insert(String::from("Headphones"), 150.0);

    // --- Method 1: Iterate over key-value pairs ---
    println!("--- All Products (iter) ---");
    for (product, revenue) in &sales {
        println!("  {}: ${:.2}", product, revenue);
    }

    // --- Method 2: Iterate over keys only ---
    println!("\n--- Product Names (keys) ---");
    for product in sales.keys() {
        println!("  - {}", product);
    }

    // --- Method 3: Iterate over values only ---
    println!("\n--- Revenue Values (values) ---");
    let total: f64 = sales.values().sum();
    println!("  Total revenue: ${:.2}", total);
    println!("  Average: ${:.2}", total / sales.len() as f64);

    // --- Method 4: Mutable iteration over values ---
    println!("\n--- Apply 10% Discount (values_mut) ---");
    for revenue in sales.values_mut() {
        *revenue *= 0.9;  // Apply 10% discount
    }
    for (product, revenue) in &sales {
        println!("  {}: ${:.2}", product, revenue);
    }

    // --- Method 5: drain to consume the HashMap ---
    let mut backup = sales.clone();
    println!("\n--- Drain (consumes HashMap) ---");
    while let Some((product, revenue)) = backup.drain().next() {
        println!("  Removed: {} (${:.2})", product, revenue);
    }
    println!("  backup is empty: {}", backup.is_empty());

    // --- When to use what: Collection selection guide ---
    println!("\n--- Collection Selection Guide ---");

    // Scenario 1: Vec (ordered, indexed access)
    let mut todo_list: Vec<&str> = Vec::new();
    todo_list.push("Buy milk");
    todo_list.push("Write report");
    todo_list.push("Call mom");
    println!("Vec (ordered todo list):");
    for (i, item) in todo_list.iter().enumerate() {
        println!("  {}. {}", i + 1, item);
    }

    // Scenario 2: HashMap (key-value lookup)
    let mut phone_book: HashMap<&str, &str> = HashMap::new();
    phone_book.insert("Alice", "123-4567");
    phone_book.insert("Bob", "987-6543");
    println!("HashMap (phone book):");
    println!("  Alice's number: {}", phone_book.get("Alice").unwrap());

    // Scenario 3: HashSet (membership check)
    let mut admin_users: HashSet<&str> = HashSet::new();
    admin_users.insert("admin");
    admin_users.insert("root");
    let user = "admin";
    println!("HashSet (admin check):");
    println!("  Is '{}' admin? {}", user, admin_users.contains(user));
}

// Import HashSet for the last scenario
use std::collections::HashSet;

--- All Products (iter) ---
  Laptop: $1200.00
  Mouse: $25.00
  Keyboard: $80.00
  Monitor: $350.00
  Headphones: $150.00

--- Product Names (keys) ---
  - Laptop
  - Mouse
  - Keyboard
  - Monitor
  - Headphones

--- Revenue Values (values) ---
  Total revenue: $1805.00
  Average: $361.00

--- Apply 10% Discount (values_mut) ---
  Laptop: $1080.00
  Mouse: $22.50
  Keyboard: $72.00
  Monitor: $315.00
  Headphones: $135.00

--- Drain (consumes HashMap) ---
  Removed: Laptop ($1080.00)
  Removed: Mouse ($22.50)
  Removed: Keyboard ($72.00)
  Removed: Monitor ($315.00)
  Removed: Headphones ($135.00)
  backup is empty: true

--- Collection Selection Guide ---
Vec (ordered todo list):
  1. Buy milk
  2. Write report
  3. Call mom
HashMap (phone book):
  Alice's number: 123-4567
HashSet (admin check):
  Is 'admin' admin? true

// ============================================
// 综合示例：HashMap + HashSet 文本分析
// ============================================

use std::collections::{HashMap, HashSet};

fn word_frequency(text: &str) -> HashMap<String, u32> {
    let mut freq: HashMap<String, u32> = HashMap::new();
    for word in text.split_whitespace() {
        let clean: String = word.chars()
            .filter(|c| c.is_alphabetic())
            .map(|c| c.to_lowercase().next().unwrap())
            .collect();
        if !clean.is_empty() {
            *freq.entry(clean).or_insert(0) += 1;
        }
    }
    freq
}

fn unique_words(text: &str) -> HashSet<String> {
    text.split_whitespace()
        .map(|w| w.to_lowercase())
        .collect()
}

fn top_n(freq: &HashMap<String, u32>, n: usize) -> Vec<(&str, u32)> {
    let mut entries: Vec<_> = freq.iter().map(|(k, &v)| (k.as_str(), v)).collect();
    entries.sort_by(|a, b| b.1.cmp(&a.1));
    entries.into_iter().take(n).collect()
}

fn main() {
    let text1 = "the cat sat on the mat and the cat slept on the mat";
    let text2 = "the dog ran on the grass and the dog slept on the rug";

    println!("=== 文本 1 词频 ===");
    let freq1 = word_frequency(text1);
    for (word, count) in top_n(&freq1, 5) {
        println!("  '{}': {} 次", word, count);
    }

    println!("\n=== 文本 2 词频 ===");
    let freq2 = word_frequency(text2);
    for (word, count) in top_n(&freq2, 5) {
        println!("  '{}': {} 次", word, count);
    }

    let words1 = unique_words(text1);
    let words2 = unique_words(text2);

    let common: HashSet<_> = words1.intersection(&words2).collect();
    println!("\n共同词汇: {:?}", common);

    let only1: HashSet<_> = words1.difference(&words2).collect();
    println!("文本1独有: {:?}", only1);

    let only2: HashSet<_> = words2.difference(&words1).collect();
    println!("文本2独有: {:?}", only2);

    let all: HashSet<_> = words1.union(&words2).collect();
    println!("所有词汇数: {}", all.len());
}

=== 文本 1 词频 ===
  'the': 3 次
  'cat': 2 次
  'on': 2 次
  'mat': 2 次
  'sat': 1 次

=== 文本 2 词频 ===
  'the': 3 次
  'dog': 2 次
  'on': 2 次
  'grass': 1 次
  'ran': 1 次

共同词汇: {"the", "and", "on", "slept"}
文本1独有: {"mat", "cat", "sat"}
文本2独有: {"ran", "rug", "grass", "dog"}

所有词汇数: 11