This document specifies compression of uint256 to smaller data structures like uint64, uint96, uint128, for optimizing costs for storage. The smaller data structure (represented as cintx) is divided into two parts, in the first one we store significant bits and in the other number of left shifts needed on the significant bits to decompress. This document also includes two specifications for decompression due to the nature of compression being lossy, i.e. it causes underflow.
uint256 which takes up one entire slot.In this specification, the structure for representing a compressed value is represented using cintx, where x is the number of bits taken by the entire compressed value. On the implementation level, an uintx can be used for storing a cintx value.
The rightmost, or least significant, 8 bits in cintx are reserved for storing the shift and the rest available bits are used to store the significant bits starting from the first 1 bit in uintx.
The rightmost, or least significant, 7 bits in cintx are reserved for storing the shift and the rest available bits are used to store the significant bits starting from the first one bit in uintx.
In the following code example,
uint7is used just for representation purposes only, but it should be noted that uints in Solidity are in multiples of 8.
Examples:
Two decompression methods are defined: a normal decompress and a decompressRoundingUp.
The significant bits in the cintx are moved to a uint256 space and shifted left by shift.
NOTE: In the following example, cint16 is used for visual demonstration purposes. But it should be noted that it is definitely not safe for storing money amounts because its significant bits capacity is 8, while at least 50 bits are required for storing money amounts.
The significant bits in the cintx are moved to a uint256 space and shifted left by shift and the least significant shift bits are 1s.
This specification is to be used by a new smart contract for managing its internal state so that any state mutating calls to it can be cheaper. These compressed values on a smart contract's state are something that should not be exposed to the external world (other smart contracts or clients). A smart contract should expose a decompressed value if needed.
significant bits are stored in the most significant part of cintx while shift bits in the least significant part, to help prevent obvious dev mistakes. For e.g. a number smaller than 256-1 its compressed cint64 value would be itself if the arrangement were to be opposite than specified. If a developer forgets to uncompress a value before using it, this case would still pass if the compressed value is the same as decompressed value.cint64 doesn't render gas savings automatically. The solidity compiler needs to pack more data into the same storage slot.uint120 and 7 up to uint248).There are no known backward-incompatible issues.
On the implementation level uint64 may be used directly, or with custom types introduced in 0.8.9.
The above gist has library CInt64 that contains demonstrative logic for compression, decompression, and arithmetic for cint64. The gist also has an example contract that uses the library for demonstration purposes.
The CInt64 format is intended only for storage, while dev should convert it to uint256 form using suitable logic (decompress or decompressRoundingUp) to perform any arithmetic on it.
The following security considerations are discussed:
cint64cintxuint256s.When a value is compressed, it causes underflow, i.e. some less significant bits are sacrificed. This results in a cintx value whose decompressed value is less than or equal to the actual uint256 value.
Let's consider we have a value of the order 2m (less than 2m and greater than or equal to 2m-1).
For all values such that 2m - 1 - (2m-56 - 1) <= value <= 2m - 1, the compressed value cvalue is 2m - 1 - (2m-56 - 1).
The maximum error is 2m-56 - 1, approximating it to decimal: 10n-17 (log2(56) is 17). Here n is number of decimal digits + 1.
For e.g. compressing a value of the order $1,000,000,000,000 (or 1T or 1012) to cint64, the maximum error turns out to be 1012+1-17 = $10-4 = $0.0001. This means the precision after 4 decimal places is lost, or we can say that the uncompressed value is at maximum $0.0001 smaller. Similarly, if someone is storing $1,000,000 into cint64, the uncompressed value would be at maximum $0.0000000001 smaller. In comparison, the storage costs are almost $0.8 to initialize and $0.2 to update (20 gwei, 2000 ETHUSD).
Note that compression makes the value slightly smaller (underflow). But we also have another operation that also does that. In integer math, the division is a lossy operation (causing underflow). For instance,
The result of the division operation is not always exact, but it's smaller than the actual value, in some cases as in the above example. Though, most engineers try to reduce this effect by doing all the divisions at the end.
The division operation has been in use in the wild, and plenty of lossy integer divisions have taken place, causing DeFi users to get very very slightly less withdrawal amounts, which they don't even notice. If been careful, then the risk is very negligible. Compression is similar, in the sense that it is also a division by 2shift. If been careful with this too, the effects are minimized.
In general, one should follow the rule:
amount.decompress()).amount.decompressUp()).The above ensures that smart contract does not loose money due to the compression, it is the user who receives less funds or pays more funds. The extent of rounding is something that is negligible enough for the user. Also just to mention, this rounding up and down pattern is observed in many projects including UniswapV3.
cintxThis is an example where dev errors while using compression can be a problem.
Usual user amounts mostly have an max entropy of 50, i.e. 1015 (or 250) values in use, that is the reason why we find uint56 enough for storing significant bits. However, let's see an example:
The above code results in a serious precision loss. sharesC has an entropy of 50, as well as priceC also has an entropy of 50. When we multiply them, we get a value that contains entropies of both, and hence, an entropy of 100. After multiplication is done, cmul compresses the value, which drops the entropy of amountC to 56 (as we have uint56 there to store significant bits).
To prevent entropy/precision from dropping, we get out from compression.
Compression is only useful when writing to storage while doing arithmetic with them should be done very carefully.
uint256s.Compressed Integers is intended to only compress money amount. Technically there are about 1077 values that a uint256 can store but most of those values have a flat distribution i.e. the probability is 0 or extremely negligible. (What is a probability that a user would be depositing 1000T DAI or 1T ETH to a contract? In normal circumstances it doesn't happen, unless someone has full access to the mint function). Only the amounts that people work with have a non-zero distribution ($0.001 DAI to $1T or 1015 to 1030 in uint256). 50 bits are enough to represent this information, just to round it we use 56 bits for precision.
Using the same method for compressing something else which have a completely different probability distribution will likely result in a problem. It's best to just not compress if you're not sure about the distribution of values your uint256 is going to take. And also, for things you think you are sure about using compression for, it's better to give more thought if compression can result in edge cases (e.g. in previous multiplication example).
Since we have a dynamic uint8 shift value that can move around. So even if you wanted to represent 1 Million SHIBA INU tokens or 0.0002 WBTC (both $10 as of this writing), cint64 will pick its top 56 significant bits which will take care of the value representation.
It can be a problem for volatile tokens if the coin is extremely volatile wrt user's native currency. Imagine a very unlikely case where a coin goes 256x up (price went up by 1016 lol). In such cases uint56 might not be enough as even its least significant bit is very valuable. If such insanely volatile tokens are to be stored, you should store more significant bits, i.e. using cint96 or cint128.
cint64 has 56 bits for storing significant, when only 50 were required. Hence there are 6 extra bits, which means that it is fine if the $ value of the cryptocurrency stored in cint64 increases by 26 or 64x. If the value goes down it's not a problem.
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