CDP / Collateralized Lending Pattern

{
  // CDP (Collateralized Debt Position) Box
  // R4: Owner public key
  // R5: Debt amount (in stable tokens)
  // R6: Minimum collateral ratio (e.g., 150 = 150%)
  // R7: Liquidation ratio (e.g., 120 = 120%)
  
  val owner = SELF.R4[SigmaProp].get
  val debtAmount = SELF.R5[Long].get
  val minRatio = SELF.R6[Int].get
  val liqRatio = SELF.R7[Int].get
  
  val collateralErg = SELF.value
  val stableTokenId = fromBase64("STABLE_TOKEN_ID")
  
  // Oracle price (nanoERG per stable token)
  val oracleBox = CONTEXT.dataInputs(0)
  val oracleNFT = fromBase64("ORACLE_NFT_ID")
  val validOracle = oracleBox.tokens(0)._1 == oracleNFT
  val ergPrice = oracleBox.R4[Long].get  // e.g., 2000000 = $2 per ERG
  
  // Calculate collateral ratio
  val collateralValue = collateralErg * ergPrice / 1000000000L
  val currentRatio = if (debtAmount > 0) collateralValue * 100 / debtAmount else 999
  
  // Owner operations (add collateral, repay debt, withdraw)
  val ownerAction = {
    val newCdp = OUTPUTS(0)
    val newDebt = newCdp.R5[Long].get
    val newCollateral = newCdp.value
    
    // Can add collateral freely
    val addingCollateral = newCollateral > collateralErg && newDebt == debtAmount
    
    // Can repay debt (must provide stable tokens)
    val repayingDebt = newDebt < debtAmount
    
    // Can withdraw if ratio stays above minimum
    val newRatio = newCollateral * ergPrice * 100 / (newDebt * 1000000000L)
    val safeWithdraw = newRatio >= minRatio
    
    owner && (addingCollateral || (repayingDebt && safeWithdraw))
  }
  
  // Liquidation (anyone can call if under-collateralized)
  val liquidation = {
    val underCollateralized = currentRatio < liqRatio
    
    // Liquidator must repay debt
    val debtRepaid = OUTPUTS.exists(o =>
      o.tokens.exists(t => t._1 == stableTokenId && t._2 >= debtAmount)
    )
    
    // Liquidator receives collateral at discount (e.g., 10%)
    val liquidatorBonus = 10  // 10% discount
    val liquidatorReceives = collateralErg * (100 - liquidatorBonus) / 100
    
    underCollateralized && debtRepaid
  }
  
  validOracle && (ownerAction || liquidation)
}