feat(swap): implement 0 dexfee for kmd trading pairs

mm2src/coins/eth.rs

@@ -5891,11 +5891,24 @@ impl MmCoin for EthCoin {
         dex_fee_amount: DexFee,
         stage: FeeApproxStage,
     ) -> TradePreimageResult<TradeFee> {
-        let dex_fee_amount = wei_from_big_decimal(&dex_fee_amount.fee_amount().into(), self.decimals)?;
+        let fee_coin = match &self.coin_type {
+            EthCoinType::Eth => self.ticker.to_owned(),
+            EthCoinType::Erc20 { platform, .. } => platform.to_owned(),
+            EthCoinType::Nft { .. } => return MmError::err(TradePreimageError::NftProtocolNotSupported),
+        };
+        if dex_fee_amount.no_fee() {
+            return Ok(TradeFee {
+                coin: fee_coin,
+                amount: MmNumber::default(),
+                paid_from_trading_vol: false,
+            });
+        }
 
+        let dex_fee_amount = wei_from_big_decimal(&dex_fee_amount.fee_amount().into(), self.decimals)?;
         // pass the dummy params
         let to_addr = addr_from_raw_pubkey(&DEX_FEE_ADDR_RAW_PUBKEY)
             .expect("addr_from_raw_pubkey should never fail with DEX_FEE_ADDR_RAW_PUBKEY");
+        let my_address = self.derivation_method.single_addr_or_err().await?;
         let (eth_value, data, call_addr, fee_coin) = match &self.coin_type {
             EthCoinType::Eth => (dex_fee_amount, Vec::new(), &to_addr, &self.ticker),
             EthCoinType::Erc20 { platform, token_addr } => {
@@ -5905,8 +5918,6 @@ impl MmCoin for EthCoin {
             },
             EthCoinType::Nft { .. } => return MmError::err(TradePreimageError::NftProtocolNotSupported),
         };
-
-        let my_address = self.derivation_method.single_addr_or_err().await?;
         let fee_policy_for_estimate = get_swap_fee_policy_for_estimate(self.get_swap_transaction_fee_policy());
         let pay_for_gas_option = self.get_swap_pay_for_gas_option(fee_policy_for_estimate).await?;
         let pay_for_gas_option = increase_gas_price_by_stage(pay_for_gas_option, &stage);

mm2src/coins/lp_coins.rs

@@ -3624,6 +3624,8 @@ pub enum DexFee {
         fee_amount: MmNumber,
         burn_amount: MmNumber,
     },
+    /// Zero dex fee, exclusive to KMD only.
+    Zero,
 }
 
 impl DexFee {
@@ -3640,13 +3642,14 @@ impl DexFee {
         match self {
             DexFee::Standard(t) => t.clone(),
             DexFee::WithBurn { fee_amount, .. } => fee_amount.clone(),
+            DexFee::Zero => MmNumber::default(),
         }
     }
 
     /// Gets the burn amount associated with the dex fee, if applicable.
     pub fn burn_amount(&self) -> Option<MmNumber> {
         match self {
-            DexFee::Standard(_) => None,
+            DexFee::Standard(_) | DexFee::Zero => None,
             DexFee::WithBurn { burn_amount, .. } => Some(burn_amount.clone()),
         }
     }
@@ -3659,6 +3662,7 @@ impl DexFee {
                 fee_amount,
                 burn_amount,
             } => fee_amount + burn_amount,
+            DexFee::Zero => MmNumber::default(),
         }
     }
 
@@ -3676,6 +3680,9 @@ impl DexFee {
             Ok(None)
         }
     }
+
+    /// Check and return true if DexFee is not required to trade otherwise return false.
+    pub fn no_fee(&self) -> bool { matches!(self, Self::Zero) }
 }
 
 pub struct CoinsContext {

mm2src/coins/qrc20.rs

@@ -1418,6 +1418,14 @@ impl MmCoin for Qrc20Coin {
         dex_fee_amount: DexFee,
         stage: FeeApproxStage,
     ) -> TradePreimageResult<TradeFee> {
+        if dex_fee_amount.no_fee() {
+            return Ok(TradeFee {
+                coin: self.platform.clone(),
+                amount: MmNumber::default(),
+                paid_from_trading_vol: false,
+            });
+        }
+
         let amount = wei_from_big_decimal(&dex_fee_amount.fee_amount().into(), self.utxo.decimals)?;
 
         // pass the dummy params

mm2src/coins/tendermint/tendermint_coin.rs

@@ -2478,6 +2478,14 @@ impl MmCoin for TendermintCoin {
         dex_fee_amount: DexFee,
         _stage: FeeApproxStage,
     ) -> TradePreimageResult<TradeFee> {
+        if dex_fee_amount.no_fee() {
+            return Ok(TradeFee {
+                coin: self.ticker.clone(),
+                amount: MmNumber::default(),
+                paid_from_trading_vol: false,
+            });
+        }
+
         self.get_fee_to_send_taker_fee_for_denom(self.ticker.clone(), self.denom.clone(), self.decimals, dex_fee_amount)
             .await
     }

mm2src/coins/utxo/slp.rs

@@ -1844,6 +1844,14 @@ impl MmCoin for SlpToken {
         dex_fee_amount: DexFee,
         stage: FeeApproxStage,
     ) -> TradePreimageResult<TradeFee> {
+        if dex_fee_amount.no_fee() {
+            return Ok(TradeFee {
+                coin: self.platform_coin.ticker().into(),
+                amount: MmNumber::default(),
+                paid_from_trading_vol: false,
+            });
+        }
+
         let slp_amount = sat_from_big_decimal(&dex_fee_amount.fee_amount().into(), self.decimals())?;
         // can use dummy P2PKH script_pubkey here
         let script_pubkey = ScriptBuilder::build_p2pkh(&H160::default().into()).into();
@@ -1861,6 +1869,7 @@ impl MmCoin for SlpToken {
             &stage,
         )
         .await?;
+
         Ok(TradeFee {
             coin: self.platform_coin.ticker().into(),
             amount: fee.into(),

mm2src/coins/utxo/utxo_common.rs

@@ -1243,39 +1243,50 @@ async fn gen_taker_payment_spend_preimage<T: UtxoCommonOps>(
     args: &GenTakerPaymentSpendArgs<'_, T>,
     n_time: NTimeSetting,
 ) -> GenPreimageResInner {
-    let dex_fee_address = address_from_raw_pubkey(
-        args.dex_fee_pub,
-        coin.as_ref().conf.address_prefixes.clone(),
-        coin.as_ref().conf.checksum_type,
-        coin.as_ref().conf.bech32_hrp.clone(),
-        coin.addr_format().clone(),
-    )
-    .map_to_mm(|e| TxGenError::AddressDerivation(format!("Failed to derive dex_fee_address: {}", e)))?;
-
-    let mut outputs = generate_taker_fee_tx_outputs(coin.as_ref().decimals, dex_fee_address.hash(), args.dex_fee)?;
-    if let DexFee::WithBurn { .. } = args.dex_fee {
-        let script = output_script(args.maker_address).map_to_mm(|e| {
-            TxGenError::Other(format!(
-                "Couldn't generate output script for maker address {}, error {}",
-                args.maker_address, e
-            ))
-        })?;
-        let tx_fee = coin
-            .get_htlc_spend_fee(DEFAULT_SWAP_TX_SPEND_SIZE, &FeeApproxStage::WithoutApprox)
-            .await?;
-        let maker_value = args
-            .taker_tx
-            .first_output()
-            .map_to_mm(|e| TxGenError::PrevTxIsNotValid(e.to_string()))?
-            .value
-            - outputs[0].value
-            - outputs[1].value
-            - tx_fee;
-        outputs.push(TransactionOutput {
-            value: maker_value,
-            script_pubkey: script.to_bytes(),
-        })
-    }
+    let outputs = match args.dex_fee {
+        DexFee::Zero => vec![],
+        dex_fee => {
+            let dex_fee_address = address_from_raw_pubkey(
+                args.dex_fee_pub,
+                coin.as_ref().conf.address_prefixes.clone(),
+                coin.as_ref().conf.checksum_type,
+                coin.as_ref().conf.bech32_hrp.clone(),
+                coin.addr_format().clone(),
+            )
+            .map_to_mm(|e| TxGenError::AddressDerivation(format!("Failed to derive dex_fee_address: {}", e)))?;
+
+            let mut fee_outputs =
+                generate_taker_fee_tx_outputs(coin.as_ref().decimals, dex_fee_address.hash(), dex_fee)?;
+
+            if let DexFee::WithBurn { .. } = dex_fee {
+                let script = output_script(args.maker_address).map_to_mm(|e| {
+                    TxGenError::Other(format!(
+                        "Couldn't generate output script for maker address {}, error {}",
+                        args.maker_address, e
+                    ))
+                })?;
+
+                let tx_fee = coin
+                    .get_htlc_spend_fee(DEFAULT_SWAP_TX_SPEND_SIZE, &FeeApproxStage::WithoutApprox)
+                    .await?;
+
+                let total_fee_value: u64 = fee_outputs.iter().map(|out| out.value).sum();
+                let first_output_value = args
+                    .taker_tx
+                    .first_output()
+                    .map_to_mm(|e| TxGenError::PrevTxIsNotValid(e.to_string()))?
+                    .value;
+
+                let maker_value = first_output_value - total_fee_value - tx_fee;
+                fee_outputs.push(TransactionOutput {
+                    value: maker_value,
+                    script_pubkey: script.to_bytes(),
+                });
+            }
+
+            fee_outputs
+        },
+    };
 
     p2sh_spending_tx_preimage(
         coin,
@@ -1306,7 +1317,7 @@ pub async fn gen_and_sign_taker_payment_spend_preimage<T: UtxoCommonOps>(
 
     let sig_hash_type = match args.dex_fee {
         DexFee::Standard(_) => SIGHASH_SINGLE,
-        DexFee::WithBurn { .. } => SIGHASH_ALL,
+        DexFee::WithBurn { .. } | DexFee::Zero => SIGHASH_ALL,
     };
 
     let signature = calc_and_sign_sighash(
@@ -1349,7 +1360,7 @@ pub async fn validate_taker_payment_spend_preimage<T: UtxoCommonOps + SwapOps>(
 
     let sig_hash_type = match gen_args.dex_fee {
         DexFee::Standard(_) => SIGHASH_SINGLE,
-        DexFee::WithBurn { .. } => SIGHASH_ALL,
+        DexFee::WithBurn { .. } | DexFee::Zero => SIGHASH_ALL,
     };
 
     let sig_hash = signature_hash_to_sign(
@@ -1433,7 +1444,7 @@ pub async fn sign_and_broadcast_taker_payment_spend<T: UtxoCommonOps>(
     let mut taker_signature_with_sighash = preimage.signature.to_vec();
     let taker_sig_hash = match gen_args.dex_fee {
         DexFee::Standard(_) => (SIGHASH_SINGLE | coin.as_ref().conf.fork_id) as u8,
-        DexFee::WithBurn { .. } => (SIGHASH_ALL | coin.as_ref().conf.fork_id) as u8,
+        DexFee::WithBurn { .. } | DexFee::Zero => (SIGHASH_ALL | coin.as_ref().conf.fork_id) as u8,
     };
 
     taker_signature_with_sighash.push(taker_sig_hash);
@@ -1486,6 +1497,11 @@ fn generate_taker_fee_tx_outputs(
     address_hash: &AddressHashEnum,
     dex_fee: &DexFee,
 ) -> Result<Vec<TransactionOutput>, MmError<NumConversError>> {
+    // Don't add outputs for Zero DexFee.
+    if dex_fee.no_fee() {
+        return Ok(vec![]);
+    }
+
     let fee_amount = dex_fee.fee_uamount(decimals)?;
 
     let mut outputs = vec![TransactionOutput {
@@ -3973,6 +3989,14 @@ pub async fn get_fee_to_send_taker_fee<T>(
 where
     T: MarketCoinOps + UtxoCommonOps,
 {
+    if dex_fee.no_fee() {
+        return Ok(TradeFee {
+            coin: coin.ticker().to_owned(),
+            amount: MmNumber::default(),
+            paid_from_trading_vol: false,
+        });
+    }
+
     let decimals = coin.as_ref().decimals;
 
     let outputs = generate_taker_fee_tx_outputs(decimals, &AddressHashEnum::default_address_hash(), &dex_fee)?;

mm2src/coins/z_coin.rs

@@ -1750,9 +1750,17 @@ impl MmCoin for ZCoin {
 
     async fn get_fee_to_send_taker_fee(
         &self,
-        _dex_fee_amount: DexFee,
+        dex_fee_amount: DexFee,
         _stage: FeeApproxStage,
     ) -> TradePreimageResult<TradeFee> {
+        if dex_fee_amount.no_fee() {
+            return Ok(TradeFee {
+                coin: self.ticker().to_owned(),
+                amount: MmNumber::default(),
+                paid_from_trading_vol: false,
+            });
+        }
+
         Ok(TradeFee {
             coin: self.ticker().to_owned(),
             amount: self.get_one_kbyte_tx_fee().await?.into(),