📜 [專欄新文章] Gas Efficient Card Drawing in Solidity

✍️ Ping Chen

📥 歡迎投稿： https://medium.com/taipei-ethereum-meetup #徵技術分享文 #使用心得 #教學文 #medium

Assign random numbers as the index of newly minted NFTs

Scenario

The fun of generative art NFT projects depends on randomness. The industry standard is “blind box”, where both the images’ serial number and the NFTs’ index are predetermined but will be shifted randomly when the selling period ends. (They call it “reveal”) This approach effectively solves the randomness issue. However, it also requires buyers to wait until the campaign terminates. What if buyers want to know the exact card right away? We’ll need a reliable onchain card drawing solution.

The creator of Astrogator🐊 isn’t a fan of blind boxes; instead, it thinks unpacking cards right after purchase is more interesting.

Spec

When initializing this NFT contract, the creator will determine the total supply of it. And there will be an iterable function that is randomly picking a number from the remaining pool. The number must be in range and must not collide with any existing ones.

Our top priority is accessibility/gas efficiency. Given that gas cost on Ethereum is damn high nowadays, we need an elegant algorithm to control gas expanse at an acceptable range.

Achieving robust randomness isn’t the primary goal here. We assume there’s no strong financial incentive to cheat, so the RNG isn’t specified. Implementers can bring their own source of randomness that they think is good enough.

Implementation

Overview

The implementation is pretty short and straightforward. Imagine there’s an array that contains all remaining(unsold) cards. When drawIndex() is called, it generates a (uniform) random seed to draw a card from the array, shortens the array, and returns the selected card.

Algorithm

Drawing X cards from a deck with the same X amount of cards is equal to shuffling the deck and dealing them sequentially. It’s not a surprise that our algorithm is similar to random shuffling, and the only difference is turning that classic algo into an interactive version.

A typical random shuffle looks like this: for an array with N elements, you randomly pick a number i in (0,N), swap array[0] and array[i], then choose another number i in (1,N), swap array[1] and array[i], and so on. Eventually, you’ll get a mathematically random array in O(N) time.

So, the concept of our random card dealing is the same. When a user mints a new card, the smart contract picks a number in the array as NFT index, then grabs a number from the tail to fill the vacancy, in order to keep the array continuous.

Tweak

Furthermore, as long as the space of the NFT index is known, we don’t need to declare/initialize an array(which is super gas-intensive). Instead, assume there’s such an array that the n-th element is n, we don’t actually initialize it (so it is an array only contains “0”) until the rule is broken.

For the convenience of explanation, let’s call that mapping cache. If cache[i] is empty, it should be interpreted as i instead of 0. On the other hand, when a number is chosen and used, we’ll need to fill it up with another unused number. An intuitive method is to pick a number from the end of the array, since the length of the array is going to decrease by 1.

By doing so, the gas cost in the worst-case scenario is bound to be constant.

Performance and limitation

Comparing with the normal ascending index NFT minting, our random NFT implementation requires two extra SSTORE and one extra SLOAD, which cost 12600 ~ 27600 (5000+20000+2600) excess gas per token minted.

Theoretically, any instantly generated onchain random number is vulnerable. We can restrict contract interaction to mitigate risk. The mitigation is far from perfect, but it is the tradeoff that we have to accept.

ping.eth

Gas Efficient Card Drawing in Solidity was originally published in Taipei Ethereum Meetup on Medium, where people are continuing the conversation by highlighting and responding to this story.

👏 歡迎轉載分享鼓掌

【教務處開學防疫相關措施】
Relevant prevention measures for the upcoming semester of the Office of Academic Affairs

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各位好：

因應嚴重特殊傳染性肺炎疫情發展，除校方發布各項防疫措施外，有關教學及其相關申辦業務，請各單位依據以下措施配合辦理：

一、110-1學期開學後前三週全面採遠距教學，授課教師應將上課網址及課程相關訊息公告於課程大綱 (有關NTU COOL課程網頁如何設為公開，詳見 https://www.dlc.ntu.edu.tw/2021/09/09/visibility/ )。實驗、實作或人數較少的研究所課程若需採實體授課，須依以下規範辦理：

(一) 落實課堂點名並禁止在教室內飲食。

(二) 上課教室應保持通風良好及定時清消。

(三) 全程配戴口罩，並維持安全社交距離 (1.5米或2.25平方米)。

(四)於110年9月24日前提出申請送教務處核定：申請表詳見課務組公告 https://www.space.ntu.edu.tw/navigate/s/CB679CFFA6134C83B4964E40A30A460AQQY

二、開學後第四週起上課方式，修課人數80人(含)以上課程採遠距教學，80人以下以實體課程為原則，特殊情形請於110年10月12日前提出申請送教務處核定：申請表詳見課務組公告 https://www.space.ntu.edu.tw/navigate/s/CB679CFFA6134C83B4964E40A30A460AQQY

(一) 採實體授課者，仍須依前述防疫規範辦理。

(二)本校鼓勵教師以數位和實體並行的混成方式進行教學準備，詳細資訊請見數習中心網站： https://www.dlc.ntu.edu.tw/2021/09/08/blended-teaching/

三、本學期加退選、停修彈性做法如下：

(一) 解除擋修、超修及減修申請書: 老師及系主任可親簽或用郵件代替簽名，請同學將申請書及相關同意證明以email寄至所屬教務單位承辦人。

(二) 開學後第一、二週加退選:

1.第1、3類方式加選：照舊(上網登記分發)。

2.第2類方式加選(授權碼): 建議老師可用信件傳遞授權碼予學生加選，並記錄學號留存，以利日後查核。

(三) 開學後第三週人工加選及停修：老師及系主任可親簽或用郵件代替簽名，請同學將申請書及相關同意證明，合併成1個pdf檔上傳至教務處。上傳網址將再另行公告。

(四) 因本學期前3週皆採遠距授課，同學在加退選期間若有加簽或聽課需求，請主動聯繫授課老師。

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教務處 敬上

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Dear NTU community members,

Given the current pandemic situation, the University is asking all academic and administrative units to comply with the following measures regarding teaching and learning for the upcoming semester despite the epidemic prevention measures released by the school:

A. All courses shall adopt distance learning in the first three weeks since 110-1 semester. Teachers shall announce course URL and relevant information on course description.(If you want to set the visibility of your course page on NTU COOL to be public, please see this document: https://www.dlc.ntu.edu.tw/2021/09/09/visibility/ ) If experiments, hands-on course,graduate courses with fewer people,wish to give physical course, please follow the measures listed below:

1. Teachers shall do the roll call, while eating and drinking in the classroom is forbidden.

2. Classrooms shall be cleansed and disinfected regularly, while kept ventilated.

3. Face masks must be worn throughout, while social distancing shall be maintained (e.g., 1.5 meters or 2.25 square meters).

4. Please submit an application form ( https://www.space.ntu.edu.tw/navigate/s/CB679CFFA6134C83B4964E40A30A460AQQY ) to the Office of Academic Affairs before September 24th.

B. In principle, 80 or above people selecting a course shall adopt distance learning from the fourth week of 110-1 semester. Courses under 80 people will be in-person classes. In special cases, please fill out this application form ( https://www.space.ntu.edu.tw/navigate/s/CB679CFFA6134C83B4964E40A30A460AQQY ) before October 12th.

1. In-person class shall comply with the regulations listed above.

2. Teachers are encouraged to adopt a blended teaching approach －a combination of online learning and in-person class - to prepare the courses. For more information, please see the website of Digital Learning Center: https://www.dlc.ntu.edu.tw/2021/09/08/blended-teaching/

C. The flexible measures of this semester for adding, dropping, and withdrawing classes are as follows.

1. The “Unblock Course Request Form”, “Course Overload Request Form”, and “Course Underload Request Form”:
The lecturer and the director may sign these request forms in person or reply by email to these request forms to indicate their approval of the request. Please email the request form and the proof for approval to the contact person in the Office of Academic Affairs.

2. Adding and dropping classes in the first and second week of this semester:

(1) . Adding classes in the first and the third category:
The process is as usual. Students may register for classes online.

(2). The second category (adding classes with a permission number):
It is recommended that the lecturer email a permission number to students and record the student number for future verification.

3. Adding or withdrawing classes in the third week of this semester:
The lecturer and the director may sign the application form in person or reply by email to the application form to indicate their approval of the request. Please combine the application form and the proof for approval into a single PDF and upload it to the Office of Academic Affairs. The website address for uploading the file will be announced separately.

4. Because online teaching is conducted during the first three weeks of this semester, if students want to add or attend the classes during the period, they need to contact the lecturer.

.
Office of Academic Affairs

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