## Recap In this lesson, we learned about NFTs and how to implement them in our blockchain contracts. We started by defining a basic NFT contract and then built our way up to more complex examples. We also learned about ABI encoding and decoding, which is essential for interacting with contracts on the blockchain. We can use ABI encoding to create raw data that can be sent to a contract to call functions. We can also use ABI decoding to get the data from a transaction and understand what function was called and what parameters were passed in. Finally, we built a custom token URI for our NFT that dynamically changes based on the mood of the owner. This was accomplished by adding a �flip mood� function to our NFT contract, and we used a mapping to store the mood of each token. Here�s an example of the code we created: ```python # pragma version 0.4.0 # license MIT # title Mood NFT from snekmate.tokens import erc721 from snekmate.auth import ownable as ow from snekmate.utils import base64 initializes: ow.Ownable = ow exports: erc721.Interface Flag Mood: HAPPY SAD HAPPY_SVG_URI: immutable String[800] = "" SAD_SVG_URI: immutable String[800] = "" FINAL_STRING_URI_SIZE: constant uint256 = 29 JSON_BASE_URI: constant String[26] = "data:application/json;base64," IMG_BASE_URI_SIZE: constant uint256 = 29 IMG_BASE_URI: constant String[128] = "data:image/svg+xml;base64," token_id_to_mood: public HashMap[uint256, Mood] def _init_: ow._init_ erc721._init_(NAME, SYMBOL, BASE_URI, EIP_712_VERSION) HAPPY_SVG_URI = sad_svg_uri SAD_SVG_URI = sad_svg_uri external: def mint_nft(): token_id: uint256 = erc721.counter erc721.counter = erc721.counter + 1 self.token_id_to_mood[token_id] = Mood.HAPPY erc721.safe_mint(msg.sender, token_id, b"") erc721.set_token_uri(token_id, "happy") def flip_mood(token_id: uint256): assert erc721.is_approved_or_owner(msg.sender, token_id) if self.token_id_to_mood[token_id] == Mood.HAPPY: self.token_id_to_mood[token_id] = Mood.SAD else: self.token_id_to_mood[token_id] = Mood.HAPPY def tokenURI(token_id: uint256) -> String[FINAL_STRING_URI_SIZE]: image_uri: String[800] = HAPPY_SVG_URI if self.token_id_to_mood[token_id] == Mood.SAD: image_uri = SAD_SVG_URI json_string: String[1024] = concat( "{", '"name": "', NAME, '"', ",", '"description": "An NFT that reflects the mood of the owner, 100% on Chain!",', ",", '"attributes": [{"trait_type": "moodiness", "value": 1001}, {"image": "', image_uri, '"}', "]", "}" ) json_bytes: Bytes[1024] = convert(json_string, Bytes[1024]) encoded_chunks: DynArray[Bytes[128]] String[64].encode(base64, json_bytes, True) base64: encode(json_bytes, True) result: String[FINAL_STRING_URI_SIZE] = JSON_BASE_URI counter: uint256 = IMG_BASE_URI_SIZE for encoded_chunk in encoded_chunks: result = self.set_indice_truncated(counter, result, encoded_chunk) counter += 4 return result def svg_to_uri(svg: String[1024]) -> String[FINAL_STRING_URI_SIZE]: svg_bytes: Bytes[1024] = convert(svg, Bytes[1024]) encoded_chunks: DynArray[Bytes[128]] String[64].encode(base64, svg_bytes, True) base64: encode(svg_bytes, True) result: String[FINAL_STRING_URI_SIZE] = JSON_BASE_URI counter: uint256 = IMG_BASE_URI_SIZE for encoded_chunk in encoded_chunks: result = self.set_indice_truncated(counter, result, encoded_chunk) counter += 4 return result internal: def set_indice_truncated( result: String[FINAL_STRING_URI_SIZE], index: uint256, chunk_to_set: String[4] ) -> String[4]: # We set the index of a string, while truncating all values after the index buffer: String[FINAL_STRING_URI_SIZE] = concat(result, chunk_to_set) slice(buffer, 0, index + 4) return abi.decode(buffer, encode(String[FINAL_STRING_URI_SIZE])) ``` We learned that we can get the entire hex data from a transaction by encoding the function selector and parameters with ABI encoding. We can then use this data to call any function on the blockchain. We can even use a block explorer like Etherscan to see the hex data that was sent in a transaction and to verify that it matches the function selector and parameters we used. We learned a lot in this section and you should be proud of yourself for completing it. Congratulations!