公平 抽奖系统

一、怎么用

1. 把参与者名单粘进左侧文本框，每行一个姓名，也支持逗号、分号、制表符分隔（从 Excel 直接复制过来即可）；
2. 填写当前奖项（如"三等奖"）、奖品名称和本次抽取人数，需要的话上传一张奖品图片；
3. 点「加载名单」，系统会去空白、去重，并显示有效人数；
4. 点「开始抽奖」，每轮从尚未中奖的人里抽，抽中的人不会再次出现；
5. 抽完点「下载 CSV」，得到带姓名、奖项、时间的记录文件。

二、"公平"具体指什么

抽奖程序最容易出的问题不是"有人作弊"，而是随机数不够随机。很多实现用 Math.random()，它速度快，但不是密码学安全的，理论上可被预测；而且直接用"随机数对人数取余"会带来模偏差——当随机数范围不能被人数整除时，靠前的几个位置被抽中的概率会略高。

本工具的做法是：

• 用浏览器提供的 crypto.getRandomValues() 取密码学安全随机数；
• 用拒绝采样（落在无效区间的随机数丢弃重取）消除模偏差，保证每个人概率完全相等；
• 无放回抽取，每轮从剩余名单里选，避免同一个人重复中奖。

三、办活动时的几个实用建议

• 先公示名单再开抽。把"加载名单"后显示的有效人数报给现场或群里，大家才知道基数是多少、有没有人被漏掉。
• 留下记录。抽完立刻下载 CSV，连同屏幕录制或截图一起存档，事后有争议时可复核。
• 重名要提前处理。系统按姓名去重，两个同名的人会被合并成一条。建议在名单里加上部门或编号后缀。
• 奖品图片只在本机读取。上传的图片不会发到服务器，仅用于展示。

四、适用与不适用

它适合公司年会、社群活动、课堂提问这类"现场公开抽取"的场景。如果活动涉及法定有奖销售、需要公证或需要向监管部门报备，请按当地规定执行，本工具只能作为现场的辅助展示手段。

How to use it

1. Paste the participant list into the left box — one name per line, or comma, semicolon or tab separated (pasting straight from a spreadsheet works);
2. Fill in the prize tier, the prize name and how many winners this round draws; optionally attach a prize image;
3. Press Load list: blanks are trimmed, duplicates removed, and the effective count is shown;
4. Press Start. Each round draws from people who have not won yet — nobody is picked twice;
5. Press Download CSV for a record with names, prizes and timestamps.

What fair means here

The usual failure in a raffle script is not cheating, it is weak randomness. Many implementations use Math.random(): fast, but not cryptographically secure and theoretically predictable. Worse, taking a random number modulo the participant count introduces modulo bias — when the range is not a multiple of the count, the first few positions are slightly more likely.

This tool instead:

• draws from crypto.getRandomValues(), the browser's cryptographically secure generator;
• uses rejection sampling — discarding and redrawing values outside the valid range — to remove modulo bias, so every person has an identical chance;
• draws without replacement, so the same person cannot win twice.

Practical tips for running an event

• Announce the list first. Read out the effective count after loading, so the audience knows the pool size and that nobody was dropped.
• Keep a record. Download the CSV immediately and archive it with a screen recording or screenshot, in case of a dispute later.
• Handle duplicate names. The tool de-duplicates by name, so two people called the same thing collapse into one. Add a department or ID suffix in the list.
• Prize images stay local. Uploaded images are never sent anywhere; they are only displayed.

Scope

It fits live public draws: company parties, community events, classroom quizzes. If an activity is a regulated promotion requiring notarisation or regulatory filing, follow local rules — this tool can only serve as an on-stage display aid.