improve mint fee calculation

routstr/wallet.py

@@ -1,5 +1,4 @@
 import asyncio
-import math
 import time
 from typing import TypedDict
 
@@ -60,6 +59,76 @@ async def send_token(amount: int, unit: str, mint_url: str | None = None) -> str
     return token
 
 
+async def _calculate_swap_amount(
+    amount_msat: int,
+    token_unit: str,
+    token_mint_url: str,
+    token_wallet: Wallet,
+    primary_wallet: Wallet,
+) -> int:
+    """
+    Calculate the amount to mint on the primary mint after accounting for
+    potential swap fees (melt fees) on the foreign mint.
+    """
+    if settings.primary_mint_unit == "sat":
+        receive_amount = amount_msat // 1000
+    else:
+        receive_amount = amount_msat
+
+    if token_mint_url == settings.primary_mint:
+        logger.info(
+            "swap_to_primary_mint: skipping fee estimation (same mint)",
+            extra={"minted_amount": receive_amount},
+        )
+        return int(receive_amount)
+
+    logger.info(
+        "swap_to_primary_mint: estimating fees",
+        extra={
+            "dummy_amount": receive_amount,
+            "unit": settings.primary_mint_unit,
+        },
+    )
+
+    try:
+        dummy_mint_quote = await primary_wallet.request_mint(receive_amount)
+        dummy_melt_quote = await token_wallet.melt_quote(dummy_mint_quote.request)
+
+        fee_reserve = dummy_melt_quote.fee_reserve
+        if token_unit == "sat":
+            fee_msat = fee_reserve * 1000
+        else:
+            fee_msat = fee_reserve
+
+        amount_msat_after_fee = amount_msat - fee_msat
+
+        if settings.primary_mint_unit == "sat":
+            minted_amount = int(amount_msat_after_fee // 1000)
+        else:
+            minted_amount = int(amount_msat_after_fee)
+
+        if minted_amount <= 0:
+            raise ValueError(f"Fees ({fee_reserve} {token_unit}) exceed token amount")
+
+        logger.info(
+            "swap_to_primary_mint: fee estimation result",
+            extra={
+                "token_amount_sat": amount_msat // 1000,
+                "estimated_fee_sat": fee_msat // 1000,
+                "minted_amount": minted_amount,
+                "minted_unit": settings.primary_mint_unit,
+            },
+        )
+        return minted_amount
+
+    except Exception as e:
+        logger.error(
+            "swap_to_primary_mint: fee estimation failed",
+            extra={"error": str(e)},
+        )
+        raise ValueError(f"Failed to estimate fees: {e}") from e
+
+
 async def swap_to_primary_mint(
     token_obj: Token, token_wallet: Wallet
 ) -> tuple[int, str, str]:
@@ -84,26 +153,14 @@ async def swap_to_primary_mint(
         amount_msat = token_amount
     else:
         raise ValueError("Invalid unit")
-    if token_obj.mint != settings.primary_mint:
-        estimated_fee_sat = math.ceil(max(amount_msat // 1000 * 0.01, 2)) + 1
-    else:
-        estimated_fee_sat = 0
-    amount_msat_after_fee = amount_msat - estimated_fee_sat * 1000
     primary_wallet = await get_wallet(settings.primary_mint, settings.primary_mint_unit)
 
-    if settings.primary_mint_unit == "sat":
+    minted_amount = await _calculate_swap_amount(
-        minted_amount = int(amount_msat_after_fee // 1000)
+        amount_msat,
-    else:
+        token_obj.unit,
-        minted_amount = int(amount_msat_after_fee)
+        token_obj.mint,
-
+        token_wallet,
-    logger.info(
+        primary_wallet,
-        "swap_to_primary_mint: fee estimation",
-        extra={
-            "token_amount_sat": amount_msat // 1000,
-            "estimated_fee_sat": estimated_fee_sat,
-            "minted_amount": minted_amount,
-            "minted_unit": settings.primary_mint_unit,
-        },
     )
 
     mint_quote = await primary_wallet.request_mint(minted_amount)

tests/unit/test_wallet.py

@@ -217,3 +217,77 @@ async def test_recieve_token_untrusted_mint() -> None:
                 assert amount == 900
                 assert unit == "sat"
                 assert mint == "http://mint:3338"
+
+
+@pytest.mark.asyncio
+async def test_swap_to_primary_mint_success() -> None:
+    """Test successful swap with dynamic fee calculation."""
+    from routstr.wallet import swap_to_primary_mint
+
+    mock_token = Mock()
+    mock_token.mint = "http://foreign:3338"
+    mock_token.unit = "sat"
+    mock_token.amount = 1000
+    mock_token.keysets = ["keyset1"]
+    mock_token.proofs = [{"amount": 1000}]
+
+    mock_token_wallet = Mock()
+    mock_token_wallet.load_mint = AsyncMock()
+    mock_token_wallet.load_proofs = AsyncMock()
+
+    mock_primary_wallet = Mock()
+    mock_primary_wallet.load_mint = AsyncMock()
+    mock_primary_wallet.load_proofs = AsyncMock()
+
+    # Mocks for the estimation phase
+    # 1. request_mint(dummy_amount=1000) -> invoice_dummy
+    # 2. melt_quote(invoice_dummy) -> fee=10
+
+    # Mocks for the execution phase
+    # 3. request_mint(minted_amount=990) -> invoice_real
+    # 4. melt_quote(invoice_real) -> amount=990, fee=10
+    # 5. melt() -> success
+    # 6. mint() -> success
+
+    mock_mint_quote_dummy = Mock(quote="dummy_quote", request="lnbc_dummy")
+    mock_mint_quote_real = Mock(quote="real_quote", request="lnbc_real")
+
+    # side_effect for request_mint to return dummy then real
+    mock_primary_wallet.request_mint = AsyncMock(
+        side_effect=[mock_mint_quote_dummy, mock_mint_quote_real]
+    )
+
+    mock_melt_quote_dummy = Mock(amount=1000, fee_reserve=10)
+    mock_melt_quote_real = Mock(amount=990, fee_reserve=10)
+
+    # side_effect for melt_quote
+    mock_token_wallet.melt_quote = AsyncMock(
+        side_effect=[mock_melt_quote_dummy, mock_melt_quote_real]
+    )
+
+    mock_token_wallet.melt = AsyncMock(return_value="melted_proofs")
+    mock_primary_wallet.mint = AsyncMock(return_value="minted_proofs")
+
+    from routstr.core.settings import settings
+
+    with patch.object(settings, "primary_mint", "http://primary:3338"):
+        with patch.object(settings, "primary_mint_unit", "sat"):
+            with patch("routstr.wallet.get_wallet", return_value=mock_primary_wallet):
+                amount, unit, mint = await swap_to_primary_mint(
+                    mock_token, mock_token_wallet
+                )
+
+                assert amount == 990  # 1000 - 10
+                assert unit == "sat"
+                assert mint == "http://primary:3338"
+
+                # Verify call order/counts
+                assert mock_primary_wallet.request_mint.call_count == 2
+                # First call with full amount for estimation
+                mock_primary_wallet.request_mint.assert_any_call(1000)
+                # Second call with calculated amount
+                mock_primary_wallet.request_mint.assert_any_call(990)
+
+                assert mock_token_wallet.melt_quote.call_count == 2
+                assert mock_token_wallet.melt.called
+                assert mock_primary_wallet.mint.called