质量、成本与尾延迟：关注等待较久的请求

import java.math.*;
import java.util.*;
public class Main {
    record Usage(Long input,Long cached,Long output){}
    record Rates(BigDecimal input,BigDecimal cached,BigDecimal output){}
    record Bill(BigDecimal known,int complete,int unknown){}
    static void nonnegative(Long x){if(x!=null&&x<0)throw new IllegalArgumentException("negative");}
    static Bill bill(List<Usage> usages,Rates rates){
        if(rates.input()==null||rates.cached()==null||rates.output()==null||rates.input().signum()<0||rates.cached().signum()<0||rates.output().signum()<0)throw new IllegalArgumentException("rates");
        BigDecimal sum=BigDecimal.ZERO;int complete=0,unknown=0;
        for(Usage u:usages){
            nonnegative(u.input());nonnegative(u.cached());nonnegative(u.output());
            if(u.input()!=null&&u.cached()!=null&&u.cached()>u.input())throw new IllegalArgumentException("cached");
            if(u.input()==null||u.cached()==null||u.output()==null){unknown++;continue;} // 缺失不是零消费。
            BigDecimal cost=BigDecimal.valueOf(u.input()-u.cached()).multiply(rates.input())
                .add(BigDecimal.valueOf(u.cached()).multiply(rates.cached()))
                .add(BigDecimal.valueOf(u.output()).multiply(rates.output())).movePointLeft(3); // 缓存输入只计一次。
            sum=sum.add(cost);complete++; // 累加不提前舍入。
        }
        return new Bill(sum,complete,unknown);
    }
    static void eq(String amount,int c,int u,Bill b){if(b.known().compareTo(new BigDecimal(amount))!=0||b.complete()!=c||b.unknown()!=u)throw new AssertionError(b);}
    public static void main(String[] args){
        Rates r=new Rates(new BigDecimal("1"),new BigDecimal("0.5"),new BigDecimal("2"));
        eq("1.1",1,0,bill(List.of(new Usage(1000L,200L,100L)),r));
        eq("0.5",1,0,bill(List.of(new Usage(1000L,1000L,0L)),r));
        eq("0",0,1,bill(List.of(new Usage(null,0L,3L)),r));
        eq("0",0,0,bill(List.of(),r));
        Bill tiny=bill(Collections.nCopies(10,new Usage(1L,0L,0L)),r);eq("0.010",10,0,tiny);
        if(!tiny.known().setScale(2,RoundingMode.HALF_UP).equals(new BigDecimal("0.01")))throw new AssertionError();
        try{bill(List.of(new Usage(1L,2L,0L)),r);throw new AssertionError();}catch(IllegalArgumentException expected){}
        try{bill(List.of(new Usage(null,-1L,0L)),r);throw new AssertionError();}catch(IllegalArgumentException expected){}
        System.out.println("PASS 7 tests");
    }
}