removeTokenFromOwnerEnumeration

removeTokenFromOwnerEnumeration is an internal function used by the ERC721Enumerable extension to maintain per-owner token lists when a token is removed from an owner's account. It updates the internal ownership enumeration structures to reflect that a given tokenId is no longer owned by the given address.

It relies on the data structures _ownedTokens, a mapping from an owner address to an array of token IDs owned by that address, and _ownedTokensIndex, a mapping from tokenId to its index within that owner's array. The function ensures constant-time updates and keeps the owner's token list gap-free.

Algorithm: It computes lastTokenIndex as balanceOf(from) - 1 and reads tokenIndex from _ownedTokensIndex[tokenId]. If tokenIndex is not equal to lastTokenIndex, it swaps the last token in the owner's list into the position of the token being removed and updates the swapped token's index accordingly. It then deletes the mapping entry for the removed token and clears the last slot in the owner's array.

removeTokenFromOwnerEnumeration is private and is typically invoked by the token transfer or burn process, e.g., during _beforeTokenTransfer, when from is not the zero address. This ensures that the owner's enumeration remains consistent after a token leaves the owner's balance. A corresponding add function updates an owner’s list when tokens are received.

See also: _removeTokenFromAllTokensEnumeration and _addTokenToOwnerEnumeration.