Why does Base64.encode() not reset memory after being called? - memory-management

Question

Ok, so this is my very first question on stack exchange and I am new to solidity.
I am trying to create an ERC 721 contract that emits a url containing the address and (hashed) tokenId of minted NFTs. I noticed that the URLs emitted become larger every time the mint function is called.
I narrowed down the problem to Base64.encode(), but am stuck. I searched but can't seem to find an answer, possibly because I do not know what to search for.
Take the contract below. It emits a custom URL that includes a data field created through Base64.encode(). I would expect the emitted URL to be the same length every time, as memory is wiped between calls.
But... each time the Mint function is called, the data field includes data from previous calls.
The first time I call Mint() the emitted reads as follows: "someURL.com?MHg5ZDgzZTE0MDMzMDc1OGE4ZmZkMDdmOGJkNzNlODZlYmNhOGE1Njky"
The second time I call Mint() the emitted is longer:
"someURL.com?MHg5ZDgzZTE0MDMzMDc1OGE4ZmZkMDdmOGJkNzNlODZlYmNhOGE1Njky?MHg5ZDgzZTE0MDMzMDc1OGE4ZmZkMDdmOGJkNzNlODZlYmNhOGE1Njky"
And on it goes evertime I call Mint(). Memory seems to be retained. Why? What am I missing here?
Many many thanks for any help or pointers!
pragma solidity ^0.8.4;
import "#openzeppelin/contracts/utils/Strings.sol";
import "#openzeppelin/contracts/utils/Base64.sol";
contract URLMemory {
using Strings for uint256;
string customURL = "someURL.com";
event CustomURLIssued(string customURL);
function issueCustomURL() public returns (string memory){
customURL = string(
abi.encodePacked(
customURL,
"?",
Base64.encode( abi.encodePacked( Strings.toHexString(address(this)) )
)
));
return customURL;
}
function Mint() public {
emit CustomURLIssued(issueCustomURL());
}
}

Answer 1

customURL = string(
abi.encodePacked(
customURL,
"?",
Base64.encode(abi.encodePacked(Strings.toHexString(address(this))))
)
);
This snippet assigns a new value to customURL, and the new value consists of following parts:
the previous value of customURL
? string literal
and the encoded address
So each time you execute this snippet, it effectively appends the customURL with the new values.
Simplified example when we omit the Base64 encoding.
customURL = string(
abi.encodePacked(
customURL,
"?"
)
);
Initial customURL value = A
After first run it becomes A?. Second run A??. Third run A???. And so on.
If you want to keep the someURL.com prefix and only add the ? and encoded value for each token, you can just return the newly generated value without assigning it back to customURL.
function issueCustomURL() public view returns (string memory){
return string(
abi.encodePacked(
customURL,
"?",
Base64.encode( abi.encodePacked( Strings.toHexString(address(this)) )
)
));
}
Note: If you upgrade to Solidity 0.8.12 or later, you can use native function string.concat() instead of connecting the strings with abi.encodePacked().
string.concat(customURL, "?", encoded)