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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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いつ伺ってもワクワクする直系店の１杯！神奈川県横浜市『杉田家』に伺いました。ライブ終了後や休日などよく伺ってきた大好きなお店で「横浜家系ラーメン総本山吉村家」の直系一号店です。今の場所にお店を構えたのは昭和の時代。その当時からその確かな職人技で作られた家系ラーメンは多くのファンを唸らせてきました。早朝から深夜まで（自粛期間中は営業時間を短縮）濃厚な家系ラーメンが食べられるお店で、いつ伺ってもその味を求めるお客さんが途絶えることがありません。今回も楽しみに伺いましたので早速ご覧ください！

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＊感染対策を徹底して撮影を行っています。
＊撮影に際しては、お店の方や周りのお客様に充分配慮して撮影をおこなっています。

One cup of a direct line store that is exciting whenever you visit! We visited the "Sugitaya" in Yokohama City, Kanagawa Prefecture. This is my favorite shop that I often visit after the concert and on holidays, and it is the first shop directly affiliated with "Yokohama Yoshimuraya". It was in the Showa period that the store was set up at the current location. Since that time, Iekei Ramen made with its reliable craftsmanship has made many fans groan. From early morning to late night (shortening business hours during the self-restraint period), you can eat rich Iekei ramen, and customers who want the taste will never stop. I'm looking forward to seeing you again this time, so please have a look!
* It may differ from the actual topping.
* We take thorough measures against infection.
* When shooting, we give due consideration to the shop and customers around us.
いつもありがとうございます！( ´ ▽ ` )
高評価＆チャンネル登録もよろしくお願いいたします！

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—————《関連動画》———————————

なぜ吉村家へ修行に？あのテレビは事実？杉田家誕生時代の壮絶な思い出を語る【家系ラーメン物語】
https://youtu.be/4DcwgmDDpBA

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https://youtu.be/Y4UMSF0bvQY

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Jeder sollte tanzen und auf Tik Tok und Instagram posten.
Für die Welttour müssen wir mit allen zusammenarbeiten, um weiter zu expandieren.

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Total Produced by S.TIME STYLE RECORDS
VIDEO by CES HI-FIMEDIA
Beat by CHRIS RICH
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Yeah Yeah Yeah
Gold Benz used Benz Yeah Foohhhh

＊Hook
My old Benz
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Expand my world
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Act character
Keep my promotion
Changed name for SHO
（SHO⭕️ S.H.O❌）

Verse
Running golden temple
Pour green curry on Coriander eat in the car
Check the leather seat immediately after eating to ensure they’re clean
Good lyrics come together because the Benz speakers are drip
My Lucky color is gold bling bling Good
Won’t stop don’t end can’t stop the time
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Drive safely
Bling bling bling bling bling bring my Benz is the shiny car they talk about it
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My car take me to great places where I can hang out good people
Forget everything refresh go Straight a head full throttle
Simple elegance is sometimes best no dout

＊Hook
My old Benz
Keep on riding my Used Benz
Gold Benz everyone’s talking about it
Gold Benz turning heads for different sense
I’m different man
Different in Japan
Expand my world
My shining benz
I really love it
Act character
Keep my promotion
Changed name for SHO
（SHO⭕️ S.H.O❌）

Verse
Not an S600 or 550
S350 just fine
Reliable comfortable affordable
All-in-all $22 thousand
I’m not thinking about the next car
I don’t make boo koo bucks
Breaking my limit
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I released FOCUS album to show my stay of mind
Pointed front light
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I love latest W221 model
With the 21 inches Carlsson rim
Everyone appreciate Gold Benz
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Celebrities treasure benz gang taste body line
Old S class is amazing
Pairs well in the back seat Champagne glass

＊Hook
My old Benz
Keep on riding my Used Benz
Gold Benz everyone’s talking about it
Gold Benz turning heads for different sense
I’m different man
Different in Japan
Expand my world
My shining benz
I really love it
Act character
Keep my promotion
Changed name for SHO
（SHO⭕️ S.H.O❌）

My old Benz
Keep on riding my Used Benz
Gold Benz everyone’s talking about it
Gold Benz turning heads for different sense
I’m different man
Different in Japan
Expand my world
My shining benz
I really love it
Act character
Keep my promotion
Changed name for SHO
（SHO⭕️ S.H.O❌）

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SHOの出演依頼は [email protected] までお気軽にメールください。（イベント、テレビ、雑誌、ラジオ、インターネットメディアなどなどブッキングをお待ちしております）

#SHO #GOLDBENZ #Shorts

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