MULTIARTIST_NFT_CHARITY_AUCTION

type token_id is nat;
type auction_id is nat;
type bid is record
  bidder : address;
  amount : tez;
end;

type auction is record
  token : token_id;
  highest_bid : bid;
  end_time : timestamp;
end;

type token is record
  id : token_id;
  owner : address;
  artist : address;
  image_url : string;
  metadata : map(string, string); // Additional metadata about the token
  royalties : nat; // The royalty percentage for the artist, represented as an integer between 0 and 100
end;

type storage is record
  tokens : map(token_id, token);
  auctions : map(auction_id, auction);
  minter : address; // The address that's allowed to mint new tokens
end;

type mint_params is token;

type transfer_params is record
  token : token_id;
  to : address;
end;

type start_auction_params is record
  token : token_id;
  start_price : tez;
  end_time : timestamp;
end;

type bid_params is record
  auction : auction_id;
  amount : tez;
end;

type end_auction_params is auction_id;

type action is
  | Mint of mint_params

  type end_auction_params is auction_id;

function end_auction (const eap : end_auction_params; const storage : storage) : (list(operation), storage) is
  block {
    const auction_opt : option(auction) = Map.find_opt(eap, storage.auctions);
    case auction_opt of
      None -> failwith("Auction does not exist")
    | Some(auction) ->
        if now < auction.end_time
        then failwith("Auction has not ended yet")
        else skip;
        const token_opt : option(token) = Map.find_opt(auction.token, storage.tokens);
        case token_opt of
          None -> failwith("Token does not exist")
        | Some(token) ->
            if sender <> token.owner
            then failwith("Only the owner can end the auction")
            else skip;
            const new_token : token = { token with owner = auction.highest_bid.bidder };
            const tokens : map(token_id, token) = Map.update(auction.token, Some(new_token), storage.tokens);
            const auctions : map(auction_id, auction) = Map.remove(eap, storage.auctions);
            const op : operation = Tezos.transaction(auction.highest_bid.amount, 0tz, token.owner);
        end
  } with (list [op], { storage with tokens = tokens; auctions = auctions })

  | Transfer of transfer_params
  | StartAuction of start_auction_params
  | Bid on bid_params
  | EndAuction of end_auction_params;

function mint (const mp : mint_params; const storage : storage) : storage is
  block {
    if sender <> storage.minter
    then failwith("Only the minter can mint new tokens")
    else skip;
    if Map.mem(mp.id, storage.tokens)
    then failwith("Token already exists")
    else skip;
    if mp.royalties < 0n or mp.royalties > 100n
    then failwith("Royalties must be between 0 and 100")
    else skip;
    const tokens : map(token_id, token) = Map.update(mp.id, Some(mp), storage.tokens);
  } with { storage with tokens = tokens }

function transfer (const tp : transfer_params; const storage : storage) : storage is
  block {
    const token_opt : option(token) = Map.find_opt(tp.token, storage.tokens);
    case token_opt of
      None -> failwith("Token does not exist")
    | Some(token) ->
        if sender <> token.owner
        then failwith("Only the owner can transfer the token")
        else skip;
        const new_token : token = { token with owner = tp.to };
        const tokens : map(token_id, token) = Map.update(tp.token, Some(new_token), storage.tokens);
    end
  } with { storage with tokens = tokens }

function start_auction (const sap : start_auction_params; const storage : storage) : storage is
  block {
    const token_opt : option(token) = Map.find_opt(sap.token, storage.tokens);
    case token_opt of
      None -> failwith("Token does not exist")
    | Some(token) ->
        if sender <> token.owner
        then failwith("Only the owner can start an auction")
        else skip;
        const auction : auction = { token = sap.token; highest_bid = { bidder = (null : option(address)); amount = 0tz }; end_time = sap.end_time };
        const auctions : map(auction_id, auction) = Map.update(sap.token, Some(auction), storage.auctions);
    end
  } with { storage with auctions = auctions }

function bid (const bp : bid_params; const storage : storage) : storage is
  block {
    const auction_opt : option(auction) = Map.find_opt(bp.auction, storage.auctions);
    case auction_opt of
      None -> failwith("Auction does not exist")
    | Some(auction) ->
        if now >= auction.end_time
        then failwith("Auction has ended")
        else skip;
        if bp.amount <= auction.highest_bid.amount
        then failwith("Bid must be higher than the current highest bid")
        else skip;
        const new_auction : auction = { auction with highest_bid = { bidder = Some(sender); amount = bp.amount } };
        const auctions : map(auction_id, auction) = Map.update(bp.auction, Some(new_auction), storage.auctions);
    end
  } with { storage with auctions = auctions }

function main (const action : action; const storage : storage) : (list(operation), storage) is
  case action of
    Mint(mp) -> (nil : list(operation)), mint(mp, storage)
  | Transfer(tp) -> (nil : list(operation)), transfer(tp, storage)
  | StartAuction(sap) -> (nil : list(operation)), start_auction(sap, storage)
  | Bid(bp) -> (nil : list(operation)), bid(bp, storage)
  | EndAuction(eap) -> end_auction(eap, storage)
  end;