library Checkpoints:Transform openzeppelin

使用时间戳替代区块号的 Checkpoints 库实现（OpenZeppelin 风格）

在基于区块号的投票、锁仓等场景中，用区块高度作为时间点会遇到分叉回滚、出块不稳定等问题。一种更稳健的做法是使用「时间戳」作为时间点，并配合 uint256 类型的数值来记录数量（quantity）。下面给出一个可复用的 Checkpoints 库实现。

一、Checkpoints 库核心代码

1. // SPDX-License-Identifier: MIT
   
2. 
   
3. pragma solidity ^0.8.0;
   
4. 
   
5. library Checkpoints {
   
6.         struct data{
   
7.         uint32 fromTime;
   
8.         uint256 quantity;
   
9.         }
   
10. 
    
11.     struct Checkpoint {
    
12.                 mapping (address => mapping (uint32 => data)) checkpoints;
    
13.                 mapping (address => uint32) numCheckpoints;
    
14.     }
    
15. 
    
16.     // --------------checkpoint-----------
    
17.         function _getCurrentQuantity(Checkpoint storage checkpoint,address account) internal view returns (uint256) {
    
18.         uint32 nCheckpoints = checkpoint.numCheckpoints[account];
    
19.         return nCheckpoints > 0 ? checkpoint.checkpoints[account][nCheckpoints - 1].quantity : 0;
    
20.     }
    
21. 
    
22.         function getCurrentQuantity(Checkpoint storage checkpoint,address account) external view returns (uint256) {
    
23.         return _getCurrentQuantity(checkpoint,account);
    
24.     }
    
25. 
    
26.     function getPriorQuantity(Checkpoint storage checkpoint,address account, uint blockTimestamp) external view returns (uint256) {
    
27.         return _getPriorQuantity(checkpoint,account,blockTimestamp);
    
28.     }
    
29. 
    
30.     function _getPriorQuantity(Checkpoint storage checkpoint,address account, uint blockTimestamp) internal view returns (uint256) {
    
31.         require(blockTimestamp < block.timestamp, "ShipSell::getPriorQuantity: not yet determined");
    
32. 
    
33.         uint32 nCheckpoints = checkpoint.numCheckpoints[account];
    
34.         if (nCheckpoints == 0) {
    
35.             return 0;
    
36.         }
    
37. 
    
38.         // First check most recent balance
    
39.         if (checkpoint.checkpoints[account][nCheckpoints - 1].fromTime <= blockTimestamp) {
    
40.             return checkpoint.checkpoints[account][nCheckpoints - 1].quantity;
    
41.         }
    
42. 
    
43.         // Next check implicit zero balance
    
44.         if (checkpoint.checkpoints[account][0].fromTime > blockTimestamp) {
    
45.             return 0;
    
46.         }
    
47. 
    
48.         uint32 lower = 0;
    
49.         uint32 upper = nCheckpoints - 1;
    
50.         while (upper > lower) {
    
51.             uint32 center = upper - (upper - lower) / 2; // ceil, avoiding overflow
    
52.             data memory cp = checkpoint.checkpoints[account][center];
    
53.             if (cp.fromTime == blockTimestamp) {
    
54.                 return cp.quantity;
    
55.             } else if (cp.fromTime < blockTimestamp) {
    
56.                 lower = center;
    
57.             } else {
    
58.                 upper = center - 1;
    
59.             }
    
60.         }
    
61.         return checkpoint.checkpoints[account][lower].quantity;
    
62.     }
    
63. 
    
64.     function _writeCheckpoint(Checkpoint storage checkpoint,address account, uint256 newQuantity) internal {
    
65.       uint32 blockTimestamp = uint32(block.timestamp);
    
66.       uint32 nCheckpoints = checkpoint.numCheckpoints[msg.sender];
    
67. 
    
68.       if (nCheckpoints > 0 && checkpoint.checkpoints[account][nCheckpoints - 1].fromTime == blockTimestamp) {
    
69.           checkpoint.checkpoints[account][nCheckpoints - 1].quantity = newQuantity;
    
70.       } else {
    
71.           checkpoint.checkpoints[account][nCheckpoints] = data(blockTimestamp, newQuantity);
    
72.           checkpoint.numCheckpoints[account] = nCheckpoints + 1;
    
73.       }
    
74.     }
    
75. 
    
76.     function WriteCheckpoint(Checkpoint storage checkpoint,address account, uint256 newQuantity) external {
    
77.         _writeCheckpoint(checkpoint,account,newQuantity);
    
78.     }
    
79. }

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二、配套 Mock 合约（用于测试）

1. // SPDX-License-Identifier: MIT
   
2. 
   
3. pragma solidity 0.8.7;
   
4. 
   
5. import "./Checkpoints.sol";
   
6. 
   
7. contract CheckpointsMock{
   
8.     using Checkpoints for Checkpoints.Checkpoint;
   
9.     Checkpoints.Checkpoint example;
   
10. 
    
11.     function write(uint256 amount)external{
    
12.         uint256 current = Checkpoints.getCurrentQuantity(example,msg.sender);
    
13.         Checkpoints.WriteCheckpoint(example,msg.sender,current+amount);
    
14.     }
    
15. 
    
16.     function readCurrent()public view returns (uint256){
    
17.         return Checkpoints.getCurrentQuantity(example,msg.sender);
    
18.     }
    
19. 
    
20.     function readPrior(uint256 fromTime)public view returns (uint256){
    
21.         return Checkpoints.getPriorQuantity(example,msg.sender,fromTime);
    
22.     }
    
23. }

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三、设计要点

• 用 uint32 时间戳代替区块号，避免出块不稳定带来的时间点漂移；用 uint256 记录数量，容量更大。
• _getPriorQuantity 采用二分查找在历史时间点取值，require 约束传入时间必须小于当前时间，防止取到尚未确定的值。
• _writeCheckpoint 在「同一时间戳已存在记录」时直接覆盖数量，否则追加一条新记录，保证每个时间点只保留最新快照。
• Mock 合约通过 using for 语法为 Checkpoint 类型绑定库函数，方便单元测试读写当前值与历史值。
  

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