📜 [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

在 Mac 上使用 JetBrains IDE，如果想不碰滑鼠的話，很多人會在想用 Opt+shortkey 的 mnemonic 輔助功能時，碰到 mac os 智慧輔助鍵盤卡住的問題。舉例來說，extract field 選擇 initialize 位置，得按 Opt + i 就會卡住。

我找到根本解的設定了，大家可以試試看，不要再因為這個小問題搞得自己節奏被打亂。

設定請參考：https://tdd.best/blog/jetbrains-ide-on-mac-using-mnemonic/

【初始化】野台開唱之後，一切開始飛快地展開，我們受 Freddy 之邀在 Vibe 和閃靈共演，那是 Echo 第一次在台北 live house 演出。沒多久「聖界」開張，我們也成了頭一批去演出的樂團，和過去在酒吧裡演出最大的不同在於，不論台下的人多人少，觀眾們總是很專注地看著表演、很用力地鼓掌和尖叫，那感覺很特別，像是一群平常不被理解的邊緣份子，聚在一起時要更認真地為彼此加油打氣，我們很快地融入了這個場景，成了其中一份子——一個「地下樂團」。

差不多同一段時間，滾石可樂開站，經過前面幾個月的累積，除了感官駕馭和瞬間之外，我們又錄好了限度和晚宴兩首新 demo，於是一口氣放了四首歌到上面。不到一年之前，我們唯一能被聽見的方式還只有發行合輯 CD、找唱片行鋪貨，現在只要把音樂上傳到某個網站，就會看到歌曲的點播次數不斷增加，這是一種全新的體驗，但當時我們並不知道，自己正目睹著一個新時代的誕生，整個世界正開始劇烈地改變。

幾天之後，就發現四首歌都上了排行榜，排名最前面的是感官駕馭，緊接著又收到了站方寄來的 email，希望採訪我們，聯繫的人是站長阿凱，他也有個樂團，就是去年在春天吶喊驚鴻一瞥的 1976。他來電做了我們人生中第一次的專訪，寫成一篇報導名為「風城來的迴音」，我們登上了網站首頁，感官駕馭也在排行榜冠軍待了好一陣子。

和阿凱第一次見面是在聖界，他邀我們和 76 共演，當天印象最深刻的就是他緊握著我的手說：「我真的很喜歡你們的音樂」，那個誠摯的眼神和雙手的溫度令人難忘，對我們這群窩在新竹、渴望逃離畢業後就進入隔壁科學園區宿命、卻往往被身邊的人視為異類的大學生而言，能被同樣頻率的人接納一直是種奢侈的盼望，而從那段時間開始，我們不但不再邊緣，也給新竹的學弟妹對玩團這條路有了更多的信心。

也因此，迴聲社第二張合輯進度大大超前，有好多個新團加入，大家都知道如果努力創作和練團，就會有機會被看見聽見。其中 Keiichi 的「草莓野狼」算是創作力最旺盛的，在社團錄了四五首歌，當然第一張參與人馬所組的 Echo、Black Illusion 等也不惶多讓，錄音混音的技術也越來越好，大家都很期待第二張合輯的發行，那將是整個社團跨出的一大步。

當時錄音還不是用大家現在熟悉的錄音介面加上電腦 DAW，我們的Roland VS-880 硬碟錄音座最多只有八軌，所有操作都在機器上的按鈕和小小的單色液晶上完成，不過裡面也內建了不少效果器，以當時來說功能已經相當完整。而且這台機器有個貼心的設計，如果要執行刪除或覆蓋等動作，它會連續跳出兩次警告要你確認，但在錄得起勁時挺惱人的就是，我們大多都反射性地連續按兩次 Yes。

學期結束前，合輯錄音工作終於完成了，一切只待最後的母帶處理，我和春佑也把新的封面設計和攝影都準備就緒。某天到社團，冠文一如往常在錄音座前摸來摸去，他轉頭看到我露出招牌笑容，但神情有點怪異和尷尬，接著他說：「哈哈，我把硬碟 initialize 了⋯⋯」
「什麼意思？」
「就，我要刪東西不小心按到 intialize，它問我：『Are you sure?』，我按 Yes，然後它又跳：『Are you really sure?』，我按 Yes，然後它又問：『Are you really really sure?』，我就再按 Yes，然後才想說怎麼這次問三次，但⋯來不及了⋯⋯」

就這樣，所有人錄好的心血，就在冠文的神之 Yes 三連發之下化為烏有，從此他也多了一個新的稱號——Initializer。

—
12/04 處女秀演唱會
回聲樂團處女空氣十週年+吳柏蒼《42》專輯首演
購票：https://blackmarket.kktix.cc/events/seg13r
吳柏蒼《42》專輯
11/13 數位發行
黑膠預購+門票：pochang.com

いつもお世話になっております。運営の井堀と申します。
このたび、アカウントが復活しましたのでお知らせします。

ことの経緯なのですが、
8月11日に極楽レンジャーのGoogleアカウントがハイジャックされ
、そのハイジャック犯は、
ログインパスワードやメールアドレス、バックアップコード、
再設定用のメールアドレスなどを上書き変更し、

私は その後、ログイン出来ない状態となりました。

すぐに、Youtubeのサポートセンターにメールで
、アカウントが不正利用されたので、復元をお願いします！
と、連絡したところ、約２日でアカウントの復元ができました。

視聴者様をはじめ、関係者のみなさまにご迷惑と、ご心配をお掛けしてしまい、大変申し訳ありませんでした。
また、暖かいお声をかけて下さった方に本当に感謝しております。

乗っ取りの原因として考えられるのが
海外の案件を装ったフィッシングーメールで
。詳細のファイルを誤ってダウンロードしてしまったことだと思います。

対策としてパソコンを初期化し、
復元したアカウントのセキュリティー強化をしました。
また、今後もこのようなことが起こらないよう
万全のセキュリティー対策を行っていきます。

今後は、通常通り動画投稿のほうを再開しますので、引き続き楽しんで頂ければ幸いです。
今後とも極楽レンジャーチャンネルをどうぞよろしくお願いいたします。

Thank you very much for your support. My name is Ihori.
I would like to inform you that my account has been restored.

Here's how it happened.
The Google account of Gokuraku Ranger was hijacked on August 11.
The hijackers took the following information
The hijacker overwrote the login password, email address, backup code, and
The hijacker overwrote the login password, email address, backup code, and email address for resetting the account.

After that, I could not login.

I immediately sent an email to Youtube's support center.
My account has been abused, please restore it!
I was able to restore my account in about 2 days.

I'm very sorry for the inconvenience and worry I caused to the viewers and everyone else involved.
I am also really grateful to those who gave me a warm welcome.

The possible causes of the hijacking are
Phishing emails posing as overseas projects.
If you have any questions, please do not hesitate to contact us. I think I accidentally downloaded a file with details.

As a countermeasure, initialize your computer and
As a countermeasure, we initialized the computer and strengthened the security of the restored account.
We are also taking all possible security measures to prevent this from happening again in the future.
We will take all possible security measures to prevent this from happening again in the future.

We will resume posting videos as usual, and we hope you will continue to enjoy them.
Thank you for your continued support of the Gokuraku Ranger Channel.

Lenovo Think Pad X220にてOBS Studioを使おうとしていたのですが、インストールは問題無いものの起動時に「failed to initialize video. your gpu may not be supported」というエラーが表示されました。
GPUエラーということでグラフィックドライバーのバージョンを最新にしたり下げたりしましたが全然効果が無く、試しに同じ世代のCPUが搭載したパソコンでOBS Studioを入れてみた所すんなりと動作しました^^;
結果的には昔のOBSデータ（ユーザ設定データ）が悪さをしていたようで、アンインストール時にユーザデータも削除するようにしてもう一度入れ直した所何事もなかったかのように起動しました。

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主題曲完整版: http://www.youtube.com/watch?v=dW4HUPIQ5II

希望你地睇完可以同媽咪輕鬆笑下

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拍攝: 近南哥團隊
(Cam: 威仔，Boom: Kenrick，Lighting: Initialize)
聯絡: 近南哥instgram@nearthesouth

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主題曲MV :
http://www.youtube.com/watch?v=dW4HUPIQ5II

MV direcotr/editor: 周聰
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Vocal: 歐陽雨
Lyrics: 銘輝 fb:https://facebook.com/hkmusicisgreat
Song by: Jamie Fong
Arranged&Produced by: SKHP@APAS學生作品
Website: www.apasmusic.com
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