pragma cashscript ^0.13.0;

// USD-pegged sale of a BCNR name certificate (CashTokens NFT).
//
// The certificate sits in this covenant while it is for sale. Anyone may
// buy it by paying the seller the BCH equivalent of `targetCents` at the
// oracle's current price; the seller may reclaim it at any time.
//
// Oracle message (40 bytes): timestamp as uint32 LE, price in USD cents per
// BCH as uint32 LE, then the 32-byte txid of the listing's own outpoint,
// signed by `oraclePk` (Schnorr data signature over the sha256 of the
// message, as checkDataSig expects).
//
// What the chain can and cannot enforce: script can require a transaction
// to be mined no EARLIER than a time (nLocktime), never no later, so a quote
// cannot expire on-chain. The [floorSats, ceilSats] band the seller chooses
// is therefore the real guarantee: whatever quote a buyer presents, the
// payout stays inside it. Binding the quote to the listing's outpoint means
// cancelling and relisting voids every quote ever issued. The locktime rule
// below still stops a quote from being used before the oracle issued it.
contract UsdListing(
    pubkey oraclePk,
    bytes20 sellerPkh,
    int targetCents,
    int floorSats,
    int ceilSats,
    int maxAgeSecs
) {
    // Buyer path. Output 0 pays the seller; output 1 carries the certificate
    // to wherever the buyer wants it. Remaining outputs are the buyer's.
    function buy(bytes oracleMsg, datasig oracleSig) {
        require(oracleMsg.length == 40);
        require(checkDataSig(oracleSig, oracleMsg, oraclePk));

        bytes tsBytes, bytes rest = oracleMsg.split(4);
        bytes priceBytes, bytes quotedTxid = rest.split(4);
        int ts = int(tsBytes);
        int priceCents = int(priceBytes);
        require(priceCents > 0);
        require(quotedTxid == tx.inputs[this.activeInputIndex].outpointTransactionHash);

        // Not before the quote: nLocktime is checked against median-time-past
        // (about an hour behind the clock), so the buyer sets it a couple of
        // hours behind the quote and the contract allows that slack.
        require(tx.locktime + 10800 >= ts);
        require(tx.locktime <= ts + maxAgeSecs);

        int need = targetCents * 100000000 / priceCents;
        if (need < floorSats) { need = floorSats; }
        if (need > ceilSats) { need = ceilSats; }

        // Seller is paid first, in plain BCH.
        require(tx.outputs[0].lockingBytecode == new LockingBytecodeP2PKH(sellerPkh));
        require(tx.outputs[0].value >= need);
        require(tx.outputs[0].tokenCategory == 0x);

        // The certificate leaves intact: same category, same commitment.
        require(tx.outputs[1].tokenCategory == tx.inputs[this.activeInputIndex].tokenCategory);
        require(tx.outputs[1].nftCommitment == tx.inputs[this.activeInputIndex].nftCommitment);
    }

    // Seller path: reclaim the certificate (cancel the listing, or re-list
    // at a new target).
    function cancel(pubkey pk, sig s) {
        require(hash160(pk) == sellerPkh);
        require(checkSig(s, pk));
    }
}