Redis 数据结构入门：从需要的操作反推存储方式

import java.util.*;
public class Main {
    record Entry(String member,long score) {}
    static class Board {
        final int capacity;
        final Map<String,Long> scores=new HashMap<>();
        final TreeSet<Entry> order=new TreeSet<>(
            Comparator.comparingLong(Entry::score).reversed()
                .thenComparing(Entry::member,Comparator.reverseOrder()));
        Board(int capacity) {
            if (capacity<1) throw new IllegalArgumentException("capacity");
            this.capacity=capacity;
        }
        void put(String member,long score) {
            if (member==null||!member.matches("[A-Za-z0-9_-]+")||
                    score < -1_000_000_000L||score > 1_000_000_000L)
                throw new IllegalArgumentException("value");
            Long old=scores.get(member);
            if (old==null&&scores.size()==capacity) throw new IllegalStateException("full");
            // 更新前移除旧排序项，防止同一成员出现两个分数。
            if (old!=null) order.remove(new Entry(member,old));
            scores.put(member,score);
            order.add(new Entry(member,score));
        }
        void remove(String member) {
            Long old=scores.remove(member);
            // 删除同时维护两个索引；不存在成员不产生新条目。
            if (old!=null) order.remove(new Entry(member,old));
        }
        List<String> top(int k) {
            if (k<0) throw new IllegalArgumentException("k");
            List<String> result=new ArrayList<>();
            for (Entry e:order) {
                if (result.size()==k) break;
                result.add(e.member()); // 迭代顺序已经包含同分规则。
            }
            return List.copyOf(result);
        }
    }
    static void eq(Object expected,Object actual) {
        if (!Objects.equals(expected,actual)) throw new AssertionError(actual);
    }
    public static void main(String[] args) {
        Board b=new Board(3);
        b.put("alice",10); b.put("bob",10); b.put("carol",20);
        eq(List.of("carol","bob","alice"),b.top(5));
        b.put("alice",30);
        eq(List.of("alice","carol","bob"),b.top(3)); eq(3,b.scores.size());
        b.put("bob",10); eq(3,b.order.size());
        try { b.put("dave",40); throw new AssertionError("capacity"); }
        catch (IllegalStateException expected) {}
        b.remove("carol"); eq(List.of("alice","bob"),b.top(3));
        eq(List.of(),b.top(0));
        try { b.put("中文",1); throw new AssertionError("encoding boundary"); }
        catch (IllegalArgumentException expected) {}
        System.out.println("PASS 8 leaderboard tests");
    }
}