黑马点评：先画业务与数据导图

import java.util.*;
import java.util.function.ToLongFunction;

public class Main {
  record User(long id) {}
  record Shop(long id) {}
  record Voucher(long id, long shopId, long currentPrice) {}
  record Order(long id, long userId, long voucherId, long shopId, long paidCents) {}
  record Pair(long userId, long voucherId) {}
  record Stats(long count, long paidCents) {}
  static <T> Map<Long,T> index(List<T> rows, ToLongFunction<T> id) {
    Map<Long,T> map = new HashMap<>();
    for (T row : rows) {
      long key = id.applyAsLong(row);
      if (key <= 0 || map.putIfAbsent(key,row) != null) throw new IllegalArgumentException("id");
    }
    return map;
  }
  static Map<Long,Stats> rebuild(List<User> users, List<Shop> shops,
      List<Voucher> vouchers, List<Order> orders) {
    Map<Long,User> us = index(users,User::id);
    Map<Long,Shop> ss = index(shops,Shop::id);
    Map<Long,Voucher> vs = index(vouchers,Voucher::id);
    index(orders,Order::id); // 先校验订单主键，不能由聚合掩盖重复事实
    for (Voucher v : vouchers)
      if (!ss.containsKey(v.shopId()) || v.currentPrice()<0) throw new IllegalArgumentException("voucher");
    Set<Pair> unique = new HashSet<>();
    Map<Long,Stats> result = new HashMap<>();
    ss.keySet().forEach(id -> result.put(id,new Stats(0,0))); // 零订单商户也有查询投影
    for (Order o : orders) {
      Voucher v = vs.get(o.voucherId());
      if (!us.containsKey(o.userId()) || v==null || v.shopId()!=o.shopId() || o.paidCents()<0)
        throw new IllegalArgumentException("relation");
      if (!unique.add(new Pair(o.userId(),o.voucherId())))
        throw new IllegalArgumentException("one order per user and voucher");
      Stats old = result.get(o.shopId());
      result.put(o.shopId(),new Stats(Math.addExact(old.count(),1),
          Math.addExact(old.paidCents(),o.paidCents()))); // 成交快照不随当前券价变化
    }
    return Map.copyOf(result); // 只有全部输入通过后才发布结果
  }
  static void check(boolean ok) { if(!ok) throw new AssertionError(); }
  static void bad(Runnable action) {
    try { action.run(); } catch (IllegalArgumentException expected) { return; }
    throw new AssertionError("expected rejection");
  }
  public static void main(String[] args) {
    List<User> u=List.of(new User(1),new User(2));
    List<Shop> s=List.of(new Shop(10),new Shop(20));
    List<Voucher> v=List.of(new Voucher(100,10,999));
    List<Order> o=List.of(new Order(1,1,100,10,90),new Order(2,2,100,10,80));
    Map<Long,Stats> r=rebuild(u,s,v,o);
    check(r.get(10L).equals(new Stats(2,170)));
    check(r.get(20L).equals(new Stats(0,0)));
    bad(() -> rebuild(u,s,v,List.of(new Order(1,3,100,10,90))));
    bad(() -> rebuild(u,s,v,List.of(new Order(1,1,100,20,90))));
    bad(() -> rebuild(u,s,v,List.of(o.get(0),new Order(2,1,100,10,80))));
    bad(() -> rebuild(u,s,v,List.of(o.get(0),o.get(0))));
    bad(() -> rebuild(u,s,List.of(new Voucher(100,30,10)),List.of()));
    check(rebuild(u,s,v,List.of()).get(10L).count()==0);
    System.out.println("8 business projection checks passed");
  }
}