📜 [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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コーヒーにピッタリのチーズケーキレシピのご紹介です♪
濃厚なベイクドチーズケーキの酸味と香ばしいキャラメルナッツを組み合わせ、芳醇なコーヒーの苦味が引き立つスイーツに仕上げました！
ぜひ、おうちで作ってみてくださいね。

このレシピにピッタリのオススメコーヒーはこちら ( https://www.maruyamacoffee.com/ec/feature/5529 )

キャラメルピーカンナッツチーズケーキ/Caramel-Pecan-Cheesecake

15cm丸型

材料：
ココアサンドクッキー　14枚
溶かしバター　30g
Aクリームチーズ　200g
A砂糖　30g
A卵　2個
A薄力粉（ふるっておく）大さじ3
ピーカンナッツ　30g
キャラメルソース　適量
チョコレートソース　適量

作り方：
1.オーブンを180度に予熱する。
2.オレオクッキーを保存袋に入れて麺棒で叩いて砕き、溶かしバターを入れて混ぜ合わせ、型に敷き詰める。
3.ボウルにAの材料を入れて混ぜ合わせ、(2)に流し入れ、ピーカンナッツをのせ、180度のオーブンで40分焼く。
4.(3)にキャラメルソースをかけ、お好みでチョコレートソースをかけたら、完成！

===

Caramel Pecan Cheesecake
Servings: 1 cake pan (15cm)

INGREDIENTS
14 chocolate sandwich cookies
30g melted butter

A
200g cream cheese
30g sugar
2 eggs
3 tablespoons flour (shifted)

30g pecan nuts
Some caramel sauce
Some chocolate sauce


PREPARATION
1. Preheat the oven to 180° (356°F).
2. Put cookies in a food storage bag and crush with a rolling pin. Mix with melted butter. Press crushed cookies down onto the bottom of a cake pan.
3. Mix all A ingredients in a bowl. Pour into the cake pan (2). Place some pecan nuts on top. Bake in the oven for 40 minutes.
4. Pour some caramel & chocolate sauce as you like.
5. Enjoy!

---

#TastyJapan #レシピ

MUSIC Licensed via Audio Network

Aisho Nakajima “Love me for me”

“Love me for me” on Spotify : https://open.spotify.com/artist/73TJEwnZTYd2p0qRbn2Pi3
“Love me for me” on Apple Music : https://music.apple.com/us/artist/aisho-nakajima/1503461433


Aisho’s Instagram : https://www.instagram.com/aishonakajima/?hl=en
Aisho’s Twitter : https://twitter.com/aishooo74?lang=en


Written by Aisho Nakajima
Recorded/ Mixed by Aisho Nakajima
Produced by ELI
Mastering by Wax Alchemy


Lyrics -
Ohh to be young, dead inside Tokyo city
Holding my tears while I walk down the streets
Ohh Im jealous, you got what I need
Thinking, When will I love me for me

Come and f**k the sadness out of me
Broken hearted bloody bitch like me
Can’t even feel my heartbeat

Dancing in acid rain, do you still feel my pain?
Haunted, my past that I wish could erase

Ohh to be young, dead inside Tokyo city
Holding my tears while I walk down the streets
Ohh Im jealous, you got what I need
Thinking, When will I love me for me

Pretty little birds crying
All my angels dying
I used to let everyone walk over me

Everybody pulling out their phones
Who you gonna do it all for?
The love you give others, not choosing yourself

I hear intuition screaming out my name
Maybe I’m over thinking, devils I’m my head

Demons in mind overtaking
Or am I hallucinating?

All on my neck till it breaks
Something shifted in my brain

Ohh to be young, dead inside tokyo city
Holding my tears while I walk down the streets
Ohh Im jealous, you got what I need
Thinking, When will I love me for me

Had to lose myself to find myself
I Begged for help
“Burn in hell”
Word for word
Hated myself
Gasping for air

Acceptance
That’s all I wanted
Depression
Somebody stop it
A broken star
Wrecking ball
I know it’s hard
Now Time to go up

x2

Ohh to be young, dead inside Tokyo city
Holding my tears while I walk down the streets
Ohh Im jealous, you got what I need
Dreaming, When will I love me for me

Hi there!

My name is Rosanna, or Rose, and I love singing more than anything. I’ve always been on & off with it because I thought I didn’t have what it takes to pursue my dreams/goals.

When I shift my thinking, I shifted my reality into believing I am what I am. We all have talents in which we must own up to and feel empowered rather than embarrassed by them.

Slowly, more and more, I will share more of the songs that I loved that shaped my whole singing life.

Thank you for tuning in to listen! I wish you all the best.

XoXo,
Rose

Find me on IG:
@iamrosannamun

Find me on TikTok:
@iamrosannamun

Business Inquiries:
rosannamun@gmail.com