📜 [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

The Prayer Connection

It is easier to do nothing and passively let life throw you some moves before you react or respond to them.

Fewer people understand that through faith you can override what is happening in the natural through God’s power.

Through faith, you can reach out to receive what is eternal and invisible, and allow it to change your visible situation.

“He dreamed. Behold, a stairway set upon the earth, and its top reached to heaven. Behold, the angels of God ascending and descending on it. Behold, Yahweh stood above it, and said, “I am Yahweh, the God of Abraham your father, and the God of Isaac. The land whereon you lie, to you will I give it, and to your offspring.” (Genesis‬ ‭28:12-13‬ ‭WEB‬)

In Jacob’s dream, he saw angels ascending and descending on a stairway. The stairway connects Heaven and earth so that what is in Heaven can be brought to men here on earth.

When you pray, the angels assigned to you may bring your prayers in the form of golden bowls of fragrant incense up to Heaven, and there they receive God’s command on how to help you here on earth.

“Jesus saw Nathanael coming to him, and said about him, “Behold, an Israelite indeed, in whom is no deceit!” Nathanael said to him, “How do you know me?” Jesus answered him, “Before Philip called you, when you were under the fig tree, I saw you.” Nathanael answered him, “Rabbi, you are the Son of God! You are King of Israel!” Jesus answered him, “Because I told you, ‘I saw you underneath the fig tree,’ do you believe? You will see greater things than these!” He said to him, “Most certainly, I tell you, hereafter you will see heaven opened, and the angels of God ascending and descending on the Son of Man.”” (John‬ ‭1:47-51‬ ‭WEB‬‬)

Jesus received a vision of Nathaniel, and this caused Nathaniel to be so amazed. Afterwards, Jesus made reference to Jacob’s dream, suggesting that He Himself is the stairway that connects Heaven and earth.

Truly, Nathaniel went on to see Jesus doing many miracles—healing the sick, casting out demons, and much more. Jesus’ prayers were speedily brought up to the Father in Heaven, while visions and power were brought down to earth as answers to His prayers.

Thanks to Jesus’ finished work at the cross, we have been reconciled to God. The prayer line has been restored. Even we have become stairways that connect Heaven and earth. Angels ascend and descend on us when we pray.

Prayer is intentional and very powerful. Even if you cannot see what is going on, there are many angels gathered and at work, especially when believers pray.

“Then he said to me, Don’t be afraid, Daniel; for from the first day that you set your heart to understand, and to humble yourself before your God, your words were heard: and I have come for your words’ sake. But the prince of the kingdom of Persia withstood me twenty-one days; but, behold, Michael, one of the chief princes, came to help me: and I remained there with the kings of Persia.” (Daniel‬ ‭10:12-13‬ ‭WEB‬‬)

When Daniel first prayed for understanding from God, his prayers were already heard by Him. God sent the archangel Gabriel to deliver the prophetic word to Daniel, but the spiritual warfare was thick because the devil knew how important the message must have been.

The demon prince called the “prince of the kingdom of Persia” tried to prevent Gabriel from reaching Daniel, but he could not prevail when archangel Michael joined the fight.

Can you imagine that the demon had the ability to delay Gabriel for twenty-one days? Imagine if Daniel had stopped praying on the twentieth day.

Keep on praying until you receive the answers, and until the angel assigned to you comes back with the grace that you need.

There are things unseen that we may not understand fully, but trust in the power of prayer!

God has equipped you with mighty spiritual weapons in Christ so that you can emerge victorious in spiritual warfare. In “Silencing the Serpent”, you will discover what these weapons are and how to use them. Long-time Satanic bondages, yokes, and strongholds will be destroyed. I look forward to your reviews and feedback about this ebook: https://bit.ly/silence-the-serpent-now

ต้องอย่างนี้สิครับ!! แน่นมากกกก!!! ถ้าซื้อทุกเกม มีต้องขายไตแน่นอน กว่าจะตัดให้เหลือ 10 เกมได้...
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เกมอื่นๆ ที่วางจำหน่ายเดือน กันยายน 2564
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日本を代表する一流クリエイターとフランス/カナダの開発スタジオにより共同開発された王道RPG。
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タイトル：アストリア アセンディング/Astria Ascending
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ジャンル：JRPG, アクション
対応機種：PC, PlayStation 5, PlayStation 4, Xbox One, Xbox Series X/S, Nintendo Switch
開発元：Artisan Studio
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