Token IDs

Because PURSE has been upgraded from an ERC20 to a 404 token, token transfers must now take into account what constitutes an ERC20 transfer or an ERC721 transfer when transferFrom is executed. Both ERC20 and ERC721 tokens have a transferFrom function which yields the same signature hash:

➜ keccak256("transferFrom(address,address,uint256)")
Type: bytes32
└ Data: 0x23b872dd7302113369cda2901243429419bec145408fa8b352b3dd92b66c680b

This means that on the level of the EVM, when PURSE is being transferred, calls to transferFrom will produce the same function selector of 0x23b872dd regardless of whether they are intended for an ERC20 or ERC721 transfer.

In order to allow PURSE to discern if a transferFrom should be an ERC20’s or ERC721’s implementation, the function was overridden to make that discernment based on its uint256 argument. The maximum value for the uint256 type is: $2^{256} - 1$, which is an extremely large number (to put this in perspective, there are approximately $10^{80}$ atoms in the observable universe and the max value of uint256 can be used to enumerate almost every known atom).

Most ERC20 transfers will not, realistically, exceed half of that maximum value, so the upper half of the range of uint256 remains to be utilized. Thus, PURSE reserves the lower half of $2^{256} - 1$ for ERC20’s transferFrom and the upper half of that number for PURSE NFT token ids.

This essentially means that when transferFrom is called, if the value of the uint256 argument is a number: $0 < x \leq 2^{255}$, it will be executed as a ERC20 transferFrom. And if the value of the uint256 argument is a number: $x > 2^{255}$ (or $x \geq 2^{255} \ + \ 1$), then transferFrom will be executed as ERC721’s implementation.

Note that even though transferFrom is used here, it is always recommended to use safeTransferFrom when transferring as ERC721 to mitigate any possibility of permanently locking the NFT in an unintended address. For more information on this, please refer to EIP721.

For clarity, $2^{255}$ in its full numerical representation, is: 57896044618658097711785492504343953926634992332820282019728792003956564819968

And this means that the very first PURSE NFT token id is: 57896044618658097711785492504343953926634992332820282019728792003956564819969