NFT Staking

XPower uses two ERC-1155 NFT contracts to represent different stages of the staking lifecycle: XPowerNft (deposit receipt) and APowerNft (staking receipt).

XPowerNft — Deposit Receipt

When a user deposits XPOW, they receive an XPowerNft — a multi-token ERC-1155 that represents locked XPOW with specific tier parameters.

NFT ID Encoding

Each XPowerNft is identified by:

$$\text{nftId}=100\times \text{year}+\text{level}$$

Where year is the UTC calendar year (e.g., 2026) and level is the NFT tier.

Levels and Denominations

NFT levels must be multiples of 3: $\ell \in \{0,3,6,\dots ,99\}$. Each level determines:

• Denomination $D={10}^{\ell }$ — the XPOW value per NFT
• Maturity year $=\text{mintYear}+2^{\lfloor \ell /3\rfloor }-1$

Level	Denomination (XPOW)	Maturity Offset (years)
0	1	0
3	1,000	1
6	1,000,000	3
9	1,000,000,000	7
15	${10}^{15}$	31
30	${10}^{30}$	~1,024
60	${10}^{60}$	~1,000,000
99	${10}^{99}$	~8,600,000,000

Levels above ~75 are effectively permanent — their maturity dates exceed any practical timeframe.

Why Multiples of 3?

The level step of 3 enables the upgrade mechanism: 1,000 NFTs at level $\ell$ can be burned to mint 1 NFT at level $\ell +3$. This preserves the 10× denomination increase because ${10}^{\ell +3}=\mathrm{1,000}\times {10}^{\ell }$.

APowerNft — Staking Receipt

When a user stakes an XPowerNft in the NftTreasury, they receive an APowerNft — an ERC-1155 that tracks:

• Staked amount — the XPOW denomination represented
• Age accumulator — time-weighted presence in the staking pool
• Total shares — per-level aggregate, used for dynamic rate calculation

Age Tracking

The APowerNft maintains an _age accumulator that increases with each block while the NFT is staked. This age is the key input to the APB (Annual Percentage Bonus) calculation — older stakes earn higher rates.

Proportional Adjustments

When staking or unstaking part of a position:

• Staking — mints new APowerNft with age=0
• Partial unstaking — burns proportion of APowerNft, proportionally reduces age (proved to preserve average staking duration)
• Transfer — refreshClaimed() updates the total claimed counter, preventing double-claiming

Ownership

Only the NftTreasury contract can mint or burn APowerNfts — users cannot create them directly. This ensures every APowerNft corresponds to a legitimate staking position.

Upgrade Mechanism

Users can combine XPowerNfts into higher-level NFTs:

1. Own 1,000+ XPowerNfts at level $\ell$
2. Call XPowerNft.upgrade(account, ids, amounts)
3. 1,000 NFTs at level $\ell$ are burned
4. 1 NFT at level $\ell +3$ is minted

Tradeoff: Upgrading increases the denomination (and thus the reward rate) by 10×, but also extends the maturity lock period. You're trading liquidity for earning power.