Variables

Noteworthy 721J Variables

These variables demonstrate advanced gas optimization techniques used in the 721J smart contract.

Gas Optimization Strategy

Ethereum stores data in 256-bit chunks. Every on-chain storage operation costs gas, so efficient data packing is crucial for cost-effective contracts.

Digit Packing: Multiple values stored in a single uint256 by assigning specific digit ranges to different data types. This reduces storage slots and gas costs.

Struct Packing: Multiple smaller variables combined into a single 256-bit struct when their combined size equals exactly 256 bits.

Core Variables

_songURIs

mapping(uint256 => string) private _songURIs;

Purpose: Maps song metadata URIs using a packed key for efficient lookups.

Key Packing (input): The uint256 key combines songId and generation:

• Digits 1-18: songGeneration (max value: (10^18) - 1)

• Digits 19+: songId (calculated as songId × 10^18)

Usage: To retrieve a URI, pack the songId and generation into a single uint256, then use it as the mapping key. The stored value is simply the metadata string.

_tokenIdInfo

mapping(uint256 => tokenInfo) private _tokenIdInfo;

Purpose: Stores song and rarity data for each token.

Structure: Uses tokenInfo struct with two 128-bit values (song + generation = 256 bits total).

Promo System Variables

_addressClaim

mapping(address => uint256) private _addressClaim;

Purpose: Contract-wide promotion data per address.

SongOut Packing:

• Digits 1-18: generationOut

• Digits 19-36: songIdOut (×10^18)

• Digits 37+: promoPercentOut (×10^36)

_addressClaimSpecific

mapping(address => mapping(uint256 => uint256)) private _addressClaimSpecific;

Purpose: Granular promotions requiring specific song/rarity combinations.

SongIn Packing (requirement):

• Digits 1-18: generationIn

• Digits 19+: songIdIn (×10^18)

SongOut Packing (reward): Same structure as _addressClaim