fix calculation and add check (#145)

* fix calculation and add check

* modify

modules/cdp_engine/src/lib.rs

@@ -608,9 +608,11 @@ impl<T: Trait> Module<T> {
 
 		// forth: handle bad debt and collateral
 		if !supply_amount.is_zero() 				// supply_amount must not be zero
-		&& collateral_balance >= supply_amount		// make sure supply can afford target debit amount
+		&& collateral_balance >= supply_amount		// ensure have sufficient collateral
 		&& slippage_limit > Ratio::from_natural(0)	// slippage_limit must be greater than zero
 		&& slippage.map_or(false, |s| s <= slippage_limit)
+		&& T::Dex::get_target_amount(currency_id, stable_currency_id, supply_amount) >= target
+		// ensure supply can afford target
 		{
 			// directly exchange with DEX
 			// deposit supply_amount collateral to cdp treasury

modules/dex/src/lib.rs

@@ -242,10 +242,12 @@ impl<T: Trait> Module<T> {
 			Rate::from_natural(1)
 				.checked_sub(&T::GetExchangeFee::get())
 				.and_then(|n| Ratio::from_natural(1).checked_div(&n))
+				.and_then(|n| Ratio::from_parts(1).checked_add(&n)) // add Ratio::from_parts(1) to correct the possible losses caused by discarding the remainder in inner division
 				.and_then(|n| n.checked_mul_int(&target_amount))
 				.and_then(|n| n.checked_add(&1.into())) // add 1 to correct the possible losses caused by discarding the remainder in division
 				.and_then(|n| target_pool.checked_sub(&n))
 				.and_then(|n| Some(Ratio::from_rational(supply_pool, n)))
+				.and_then(|n| Ratio::from_parts(1).checked_add(&n)) // add Ratio::from_parts(1) to correct the possible losses caused by discarding the remainder in inner division
 				.and_then(|n| n.checked_mul_int(&target_pool))
 				.and_then(|n| n.checked_add(&1.into())) // add 1 to correct the possible losses caused by discarding the remainder in division
 				.and_then(|n| n.checked_sub(&supply_pool))
@@ -480,6 +482,14 @@ impl<T: Trait> Module<T> {
 }
 
 impl<T: Trait> DexManager<T::AccountId, CurrencyIdOf<T>, BalanceOf<T>> for Module<T> {
+	fn get_target_amount(
+		supply_currency_id: CurrencyIdOf<T>,
+		target_currency_id: CurrencyIdOf<T>,
+		supply_currency_amount: BalanceOf<T>,
+	) -> BalanceOf<T> {
+		Self::get_target_amount_available(supply_currency_id, target_currency_id, supply_currency_amount)
+	}
+
 	fn get_supply_amount(
 		supply_currency_id: CurrencyIdOf<T>,
 		target_currency_id: CurrencyIdOf<T>,

modules/support/src/lib.rs

@@ -49,6 +49,12 @@ pub trait AuctionManagerExtended<AccountId>: AuctionManager<AccountId> {
 }
 
 pub trait DexManager<AccountId, CurrencyId, Balance> {
+	fn get_target_amount(
+		supply_currency_id: CurrencyId,
+		target_currency_id: CurrencyId,
+		supply_currency_amount: Balance,
+	) -> Balance;
+
 	fn get_supply_amount(
 		supply_currency_id: CurrencyId,
 		target_currency_id: CurrencyId,
@@ -72,12 +78,20 @@ impl<AccountId, CurrencyId, Balance> DexManager<AccountId, CurrencyId, Balance>
 where
 	Balance: Default,
 {
+	fn get_target_amount(
+		_supply_currency_id: CurrencyId,
+		_target_currency_id: CurrencyId,
+		_supply_currency_amount: Balance,
+	) -> Balance {
+		Default::default()
+	}
+
 	fn get_supply_amount(
 		_supply_currency_id: CurrencyId,
 		_target_currency_id: CurrencyId,
 		_target_currency_amount: Balance,
 	) -> Balance {
-		Balance::default()
+		Default::default()
 	}
 
 	fn exchange_currency(