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#BeABetterMe #女子美力盛典

📜 [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

「國中小特教研習」

今年治療師阿木最大的一場講座，將近170個人的線上講座，來跟國中小的老師探討「情緒障礙、自閉症、學障的專注力與班級經營輔導」

每次跟國中小的老師談特殊生議題，我不敢說要教會老師們什麼，而是想跟老師們分享許多關於「過動兒、自閉症、學習障礙」孩子們的生命故事。更從專注力角度切入，激發老師們嘗試有更多教學模式，提高孩子的動機與專心。

我期待老師們能夠從「認識」他們的問題，到願意「同理」他們，進而用適合的方式「協助」他們。

這些孩子們，雖然成績沒有很好，能力沒有很棒，但是仍然期待老師們「用心」看到他們「能做到的那些事」。
願意「降低一點標準」，讓他們能夠「完成簡單任務」，試著為他們「加分」，給他們更多的「鼓勵」能往好的方向繼續前進。

期待師長們能給孩子的是「生命的教育」，而不是知識的教育而已！

感謝老師們熱情的參與….

😍😍親子天下嚴選:同理心教育從尊重特教生開始~
https://www.parenting.com.tw/article/5087994-%E3%80%90%E6%95%99%E8%82%B2%E7%8F%BE%E5%A0%B4%E3%80%91%E5%90%8C%E7%90%86%E5%BF%83%E6%95%99%E8%82%B2%EF%BC%8C%E5%BE%9E%E5%B0%8A%E9%87%8D%E7%89%B9%E6%95%99%E7%94%9F%E9%96%8B%E5%A7%8B~/

訂閱《Today is my day》👉https://bit.ly/2Roz2qd
#宇宙人外信 全收錄：https://bit.ly/3wO4KwA
來IG找我們玩吧：https://bit.ly/2PNGJ8J

魷魚遊戲不只是遊戲，還反映了社會體系和階級的殘酷！韓國的椪糖跟台灣椪糖差在哪裡？美國的123木頭人原來是這個！

02:00 childhood game 童年遊戲
06:30 unfair 不公平
06:42 It's a stink and con game 這遊戲是一場又臭又爛的騙局
10:37 무궁화 꽃이 피었습니다 mugunghwa kkoch-i pieossseubnida
11:08 red light, green light 紅燈綠燈（美版123木頭人）
12:42 tag 鬼抓人
13:38 you're it 你當鬼
13:57 hide and seek 躲貓貓
14:06 I'll be it first 我先當鬼
14:22 mother may I 媽媽我可以...?

中英日韓四聲道
🌍英文主播：Ethan &國際編譯：Ryan／隔壁老王
📺從時事新聞教你英日韓單字
🗣 閒話家常學微知識
💬讓你輕鬆掌握生活用語

想聲歷其境，來收聽高音質Podcast
搜尋訂閱★宇宙人外信
《Apple Podcast》： http://apple.co/3eSztCS
《Spotify》：http://spoti.fi/3cEwlrF
《SoundOn》：http://bit.ly/3bUeVYU
《KKBOX》：http://bit.ly/2OzeQ42
《Google Podcast》：https://bit.ly/3vUvBr1


#Podcast​ #魷魚遊戲 #童年 #Netflix #韓劇

Introducing Aden!
OPEN FOR MORE INFO 打開我有更多資訊
__

✧ Extra Notes 補充內容
第一次聽到ADEN的歌『想了你6次』實在是太愛了每聽就會粉紅泡泡那種！於是問他我可不可以cover~沒想到他直接讓我寫全新女生版的主歌！！！希望你們喜歡啦～

想了你6次 Dena 版 Version
music & lyrics by Aden & Dena

Baby come here to be my only
我受夠了只是想妳 這道選擇題他很簡單
Money 再多我也選擇妳 我|在這里等妳
喔! 請妳不要想太多 簡單點請妳跟我說

Is it real or tell me if it’s all just in my head
Oh I can’t sleep, cus every time I start to think about us
The way you grabbed my hand and told me
You wanna go out and let’s eat
I can’t breathe cus you’re exactly what I need aye

They say I’m falling too fast
But I don’t mind
Cus it you that I’m falling into you
Something about you makes me feel
Like we can build sth brand new
Maybe
I’m exactly what you need aye

Baby come here to be my only
我受夠了只是想妳 這道選擇題他很簡單
Money 再多我也選擇妳 我|在這里等妳
喔! 請妳不要想太多 簡單點請妳跟我說

喔 那麼多的feelings get involved
喔 我纏了線讓妳操控我
喔 今天想了妳6次
哎 我可不想裝沒事
So 請妳不要想太多 我在這, 等妳跟我說

Music Prod: ADEN
Video Prod: Dena
__

✧ What's On Me 我身上的

Suit Set 套裝:
https://www.pazzo.com.tw/zh-tw/market/n/18694?c=41853

__

✧ Listen to my original“July” 來聽Dena的新歌
Youtube MV: https://youtu.be/sEJypi8BIls​​
Spotify: https://tinyurl.com/u83d2ckk​​
KKBOX: https://tinyurl.com/3sr2bd76​​
iTunes: https://tinyurl.com/zjuvsc7d​​
Apple Music: https://tinyurl.com/zjuvsc7d​​

__

✧ Social Medias 社群
INSTAGRAM
https://www.instagram.com/dena_chang/​​ @dena_chang
FACEBOOK 粉專
https://www.facebook.com/denachmusic​​ ​
__

✧ Business Inquiries 工作洽談
[email protected]
__

✧ About Dena 有關Dena
哈囉哈囉～叫我Dena就好～我平常喜歡唱歌寫歌（歡迎上網找我的作品們！也有寫給別的歌手唱喔）拍拍影片上傳到Youtube！喜歡可以讓自己變美或是讓心情變美的一切事物！也喜歡一個人在房間跳舞。雖然沒有很厲害但是感覺認真練起來會不得了！歡迎來到我的世界！耶！很開心認識大家！

Hey peeps! It's Dena here! I'm a singer/songwriter/producer based in Taiwan! Ok and apparently I also make videos about what I love and my life! Welcome to my world!!

#想了你6次 #cover

小額贊助Gina，請Gina喝咖啡，用綠界平台付款，謝謝支持~
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