On-Chain Auction Pattern (English Auction on eUTXO)

{
  // Auction box holding the asset
  // R4: Minimum bid (nanoERG)
  // R5: Minimum increment
  // R6: Deadline (block height)
  // R7: Seller address
  // R8: Current highest bid amount
  // R9: Current highest bidder address
  
  val auctionNFT = SELF.tokens(0)._1  // Auction identifier
  val assetToken = SELF.tokens(1)     // Asset being auctioned
  
  val minBid = SELF.R4[Long].get
  val minIncrement = SELF.R5[Long].get
  val deadline = SELF.R6[Int].get
  val seller = SELF.R7[Coll[Byte]].get
  val currentBid = SELF.R8[Long].get
  val currentBidder = SELF.R9[Coll[Byte]].get
  
  val auctionActive = HEIGHT <= deadline
  val auctionEnded = HEIGHT > deadline
  
  // Place new bid (must be higher than current)
  val placeBid = {
    auctionActive &&
    
    val newAuction = OUTPUTS(0)
    val newBidAmount = newAuction.R8[Long].get
    val newBidder = newAuction.R9[Coll[Byte]].get
    
    // New bid must exceed current by minimum increment
    val validBidAmount = newBidAmount >= currentBid + minIncrement
    
    // Auction continues with updated bid
    val auctionContinues = 
      newAuction.tokens(0)._1 == auctionNFT &&
      newAuction.tokens(1) == assetToken &&
      newAuction.R4[Long].get == minBid &&
      newAuction.R5[Long].get == minIncrement &&
      newAuction.R6[Int].get == deadline &&
      newAuction.R7[Coll[Byte]].get == seller
    
    // Previous bidder gets refund (if there was one)
    val previousBidderRefunded = currentBid == 0 || OUTPUTS.exists(o =>
      o.propositionBytes == currentBidder &&
      o.value >= currentBid
    )
    
    validBidAmount && auctionContinues && previousBidderRefunded
  }
  
  // Settle auction (after deadline)
  val settleAuction = {
    auctionEnded && currentBid > 0 &&
    
    // Winner receives asset
    val winnerReceivesAsset = OUTPUTS.exists(o =>
      o.propositionBytes == currentBidder &&
      o.tokens.exists(t => t._1 == assetToken._1)
    )
    
    // Seller receives payment
    val sellerReceivesPayment = OUTPUTS.exists(o =>
      o.propositionBytes == seller &&
      o.value >= currentBid
    )
    
    winnerReceivesAsset && sellerReceivesPayment
  }
  
  // Cancel auction (seller, if no bids)
  val cancelAuction = {
    currentBid == 0 &&
    OUTPUTS.exists(o =>
      o.propositionBytes == seller &&
      o.tokens.exists(t => t._1 == assetToken._1)
    )
  }
  
  placeBid || settleAuction || cancelAuction
}