fix(tokens): fall back to the house sell fee on the buy leg (#1305)

Author: bmc08gt

apps/flipcash/shared/tokens/src/main/kotlin/com/flipcash/app/tokens/ui/SwapViewModel.kt

@@ -50,6 +50,7 @@ import com.getcode.opencode.model.core.errors.SwapError
 import com.getcode.opencode.model.financial.Currency
 import com.getcode.opencode.model.financial.CurrencyCode
 import com.getcode.opencode.model.financial.Fiat
+import com.getcode.opencode.model.financial.HOUSE_SELL_FEE_BPS
 import com.getcode.opencode.model.financial.Limits
 import com.getcode.opencode.model.financial.LocalFiat
 import com.getcode.opencode.model.financial.Rate
@@ -63,6 +64,7 @@ import com.getcode.opencode.model.financial.max
 import com.getcode.opencode.model.financial.min
 import com.getcode.opencode.model.financial.minus
 import com.getcode.opencode.model.financial.plus
+import com.getcode.opencode.model.financial.sellFeeBpsOrHouseRate
 import com.getcode.opencode.model.financial.toFiat
 import com.getcode.opencode.model.financial.usdf
 import com.getcode.opencode.model.transactions.SwapState
@@ -483,8 +485,8 @@ class SwapViewModel @Inject constructor(
         get() {
             val purpose = stateFlow.value.purpose as? SwapPurpose.Convert ?: return 0
             if (purpose.mint == Mint.usdf) return DEFAULT_CONVERT_FEE_BPS
-            return stateFlow.value.tokenWithBalance?.token?.launchpadMetadata?.sellFeeBps
-                ?: DEFAULT_CONVERT_FEE_BPS
+            return stateFlow.value.tokenWithBalance?.token?.launchpadMetadata
+                .sellFeeBpsOrHouseRate
         }
 
     private val feeAmount: Fiat
@@ -559,12 +561,13 @@ class SwapViewModel @Inject constructor(
     /**
      * The fee a buy funded by [fundingToken] charges, as bps and whether it rides on top of the
      * entered amount. A v2 Get from Dollars pays the flat house rate on top (no pool to skim);
-     * every other currency has its pool's sell fee grossed up into the debit; v1's reserves buy is
+     * every other currency has its pool's sell fee grossed up into the debit (falling back to the
+     * house rate when the pool doesn't declare one, as [convertFeeBps] does); v1's reserves buy is
      * free.
      */
     private fun buyFeeFor(fundingToken: Token): Pair<Int, Boolean> = when {
         fundingToken.address != Mint.usdf ->
-            (fundingToken.launchpadMetadata?.sellFeeBps ?: 0) to false
+            fundingToken.launchpadMetadata.sellFeeBpsOrHouseRate to false
         stateFlow.value.isGet -> DEFAULT_CONVERT_FEE_BPS to true
         else -> 0 to false
     }
@@ -1315,7 +1318,7 @@ class SwapViewModel @Inject constructor(
                         }
                     } else {
                         enteredFiat.grossingUpLaunchpadSellFee(
-                            bps = fundingToken.launchpadMetadata?.sellFeeBps ?: 0,
+                            bps = fundingToken.launchpadMetadata.sellFeeBpsOrHouseRate,
                         )
                     },
                     token = fundingToken,
@@ -1891,7 +1894,7 @@ class SwapViewModel @Inject constructor(
          * House rate for a conversion when the source pool doesn't charge its own sell fee — 1%,
          * matching the launchpad default.
          */
-        private const val DEFAULT_CONVERT_FEE_BPS = 100
+        private const val DEFAULT_CONVERT_FEE_BPS = HOUSE_SELL_FEE_BPS
 
         val updateStateForEvent: (Event) -> ((State) -> State) = { event ->
             when (event) {

services/opencode/src/main/kotlin/com/getcode/opencode/model/financial/LaunchpadSellFee.kt

@@ -58,3 +58,18 @@ fun Fiat.spendableUnderSellFeeOnTop(bps: Int): Fiat {
     val cappedBps = bps.coerceIn(0, MAX_FEE_BPS)
     return this / (1.0 + cappedBps / MAX_FEE_BPS.toDouble())
 }
+
+/**
+ * The flat house sell fee, in basis points, charged wherever a pool doesn't declare one of its own.
+ */
+const val HOUSE_SELL_FEE_BPS = 100
+
+/**
+ * This pool's declared sell fee, or the house rate when there is no pool to declare one.
+ *
+ * Erring high is the safe direction: the fee is grossed up into the debit, so falling back to zero
+ * prices a debit the funding balance can't actually cover. iOS assumes the house rate on every leg
+ * for the same reason.
+ */
+val LaunchpadMetadata?.sellFeeBpsOrHouseRate: Int
+    get() = this?.sellFeeBps ?: HOUSE_SELL_FEE_BPS

services/opencode/src/test/kotlin/com/getcode/opencode/model/financial/LaunchpadSellFeeTest.kt

@@ -2,6 +2,7 @@ package com.getcode.opencode.model.financial
 
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import com.getcode.opencode.tests.generateRandomPublicKeyForTest
 import kotlin.test.assertFalse
 
 /**
@@ -110,4 +111,37 @@ class LaunchpadSellFeeTest {
         // Clamped to 10_000 (100%): half the balance covers a fee equal to the entry.
         assertEquals("$10.00", balance.spendableUnderSellFeeOnTop(bps = 12_000).formatted())
     }
+
+    // region House-rate fallback
+
+    @Test
+    fun `a pool that declares its own sell fee uses it`() {
+        assertEquals(250, pool(sellFeeBps = 250).sellFeeBpsOrHouseRate)
+    }
+
+    @Test
+    fun `a token with no pool falls back to the house rate`() {
+        // Erring high is the safe direction — the fee is grossed up into the debit, so a zero
+        // fallback prices a debit the funding balance can't actually cover. iOS assumes the house
+        // rate on every leg for the same reason.
+        assertEquals(HOUSE_SELL_FEE_BPS, null.sellFeeBpsOrHouseRate)
+    }
+
+    private fun pool(sellFeeBps: Int): LaunchpadMetadata {
+        val key = generateRandomPublicKeyForTest()
+        return LaunchpadMetadata(
+            currencyConfig = key,
+            liquidityPool = key,
+            seed = key,
+            authority = key,
+            mintVault = key,
+            coreMintVault = key,
+            currentCirculatingSupplyQuarks = 1_000_000L,
+            sellFeeBps = sellFeeBps,
+            price = Fiat(fiat = 1.0),
+            marketCap = Fiat(fiat = 1000.0),
+        )
+    }
+
+    // endregion
 }