Loan: Interest calculation

[uzyn]

At the tick of every block, blockchain should keep track of the total amount of outstanding balance, with interest included.

Similar to DEX rewards, interest is calculated at every block. For performance reasons, it should be calculated lazily, i.e. at the tick of a block there is no need to calculate loan value for every single vaults.

Interest can come in 2 forms:

• Vault interest, based on loan scheme
• Loan token interest

See https://github.com/DeFiCh/pinkpaper/tree/main/loan#interest-rate

Suggested data implementation

DB keeps track of

• Loan tokens taken out
• Interest balance
• Interest balance calculated at block
• Interest increment per block per loan tokens
• Collateralization ratio
• Liquidation block (only need to trigger liquidation if block number is larger than this)

Calculation needs to be triggered on the following events:

• Oracle price updates. This should happen every 5-15 minutes and not on every block.
• Vault changes, e.g. more loan / more deposit.

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