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📜 [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

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“Alliances with businessmen were temporary as part of the Party’s goal of total societal control,” Mr. Shum writes in his book. “Once we were no longer needed—to build the economy, invest overseas, or help restrict freedoms in Hong Kong—we, too, would become the enemy.”

In 2013, Ms. Zhang told Ms. Duan and Mr. Shum that she and her children had donated their assets to the state in return for immunity from prosecution, according to Mr. Shum’s book. Mr. Wen and Ms. Zhang couldn’t be reached for comment.

疫情初期，新加坡採取了積極的 「清零」 策略，但在 6 月，意識到 COVID 有可能永遠不會消失，政府宣佈將轉向與病毒共存，用疫苗控制疫情爆發。隨著限制放寬，每日本土病例激增突破一千大關，新加坡能否找到與病毒共存的方式？

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本集 timestamps
0:00 Intro
0:56 第一遍英文朗讀
3:24 新聞 & 相關單字解說
15:36 額外單字片語
22:20 第二遍英文朗讀

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臉書社團 (朗讀文字)：https://www.facebook.com/groups/ssyingwen/posts/307655551120691/

朗讀內容參考了
Al Jazeera: https://www.aljazeera.com/news/2021/9/20/can-we-live-with-covid-19-singapore-tries-to-show-how
CNN: https://edition.cnn.com/2021/09/07/asia/singapore-covid-19-restrictions-intl-hnk/index.html
CNBC: https://www.cnbc.com/2021/09/20/singapores-daily-covid-cases-breach-1000-levels-over-the-weekend.html

———

本集提到的單字片語：
Singapore 新加坡
Aggressive 積極手段的
Social distancing 社交距離
Eradicate 根除
Strategy 策略
Policy 政策
Covid zero
Living with Covid
Restrict 限制
Vaccines 疫苗
Outbreaks  疫情爆發
Monitoring 監測
Hospitalizations 住院 (狀況、人數)
Denmark 丹麥
South Africa 南非
Chile 智利
Thailand 泰國
Pandemic 全球大流行病
Epidemic 疫情、流行病
Endemic 地方性流行病
Ease off 放鬆
Impose 推行、 施加
Re-impose 重新推行
Lockdown-weary 厭倦了封鎖的
Virtual 虛擬的、線上的
A rite of passage
Asymptomatic  無症狀的
Mild symptoms 症狀輕微
Free up 釋放空間
General practitioner (GP)
Vaccine passport 疫苗護照
Skeptics / sceptics 持​​懷疑態度的人
Booster jabs
Deja vu 似曾相識
Handout 講義
Flyer 傳單
Catalog
Brochure / pamphlet
Autumn / fall equinox 秋分



♥️ 喜歡時事英文 podcast 嗎？♥️
你可以支持我繼續錄製 podcast 👉 https://bit.ly/zeczec_ssyingwen

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#podcast #國際新聞 #英文聽力 #學英文 #英文筆記 #英文學習 #英文 #每日英文 #托福 #雅思 #雅思英語 #雅思托福 #多益 #多益單字 #播客 #英文新聞 #taiwanpodcast

歡迎光臨~我是樂筆！

前陣子因為疫情關係買了Netflix。

但為了避免自己過太廢，所以規定自己只能看十集以內的劇，免得因為看劇變成一灘爛泥耽誤生活(當然已經破戒這只是安慰自己……)。這次跟大家分享的《火神的眼淚》與《我是遺物整理師》分別是台灣、韓國2021年5月同期推出的職人劇，都是十分滿分的十集劇喔！

兩部劇都是從「大眾不熟悉甚至有很多誤解的職業」切入，進而帶出複雜、值得深思的社會議題。

很多時候，我若只站在自己的角度去思考、只相信眼睛看到的，很容易被現象界蒙蔽，試著把眼睛閉起來，學會「用心」去溝通、傾聽，那麼人與人的心就會更靠近。

世界好複雜，但我們永遠可以選擇多一點真誠、尊重與擁抱！歡迎帶包衛生紙，一起來了解這兩部賺人熱淚的好劇吧！

Welcome to Sunlight~
Recently, I subscribed to Netflix due to Level 3 COVID-19 alert.
To avoid becoming a couch potato, I restrict myself to watch dramas which are within 10 episodes, or I'll waste too much time in them(Of course I break the rule so it's only for comforting myself......).
Topic today, "Tears on Fire" and "Move to Heaven" are Taiwanese and Korean drama of craftsmen, respectively, released in May, 2021.
They're ten out of ten, with 10 episodes!
The two dramas draw on occupation that the public is unfamilliar with or even misunderstands, and then bring out some social issues.
Most of the time, if I don't think in a different light and only believe visible things, I'll be blinded by material universe.
Try to close your eyes, and use your heart to communicate and listen to others, so that our hearts can be closer.
The world is not simple, but we can always be sincere, respectful and embrace everyone's differences more!
Prepare a box of tissues, and watch the two tear-jerker dramas!

📁節目收聽方式：
Apple Podcast、KKBOX、Spotify 🔍歡迎光臨
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主持人兼編輯 Host/Editor/：樂筆
剪接師 Sound Editor：Papa.H
翻譯 Translator：Youli

Lockdown!

Duel Replay:
https://duellinks.konami.net/att/00485864684adfa67b4ccbad87fa63983dac686fba

Decklist:
https://duellinks.konami.net/att/00911fef5ba0e1d428dc236db1f0d7f0a602dcb84d
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Skill: Ojama Overflow
Fortune Lady Light ×1
Thunder Dragonduo ×1
Invader of Darkness ×1
Amano-Iwato ×1
Gilasaurus ×1
Fortune Lady Past ×1
Fortune Fairy Chee ×1
Fortune Lady Water ×1
Star Blast ×1
Fortune Lady Calling ×1
Fortune's Future ×3
Offerings to the Doomed ×1
Closed Forest ×1
Advance Draw ×1
Double Summon ×1
Wonder Wand ×3
Owner's Seal ×1
Mask of Restrict ×1
Seismic Shockwave ×1

Ojama King ×1
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中文版：https://youtu.be/MRzvk2oTpAk
日本語版：https://youtu.be/5mOhS5Ce_AU