AutomaticTokenBridge gas dropoff percentage of max change and decimal scaling fix

Previously the AutomaticTokenBridge route calculated the gas dropoff amount
as a percentage of the input amount. However this has been changed to
be a percentage of the maxSwapAmount as it's an easier to use interface
for users of the library (e.g. Connect).

Also fixed an issue where the native gas wasn't scaled correctly
when the source and destination chains have different decimals
for the input token.

Author: kev1n-peters

connect/src/protocols/tokenBridge/tokenTransfer.ts

@@ -658,7 +658,18 @@ export namespace TokenTransfer {
       dstDecimals,
     );
 
-    const dstNativeGasAmountRequested = transfer.nativeGas ?? 0n;
+    // nativeGas is in source chain decimals
+    const srcNativeGasAmountRequested = transfer.nativeGas ?? 0n;
+    // convert to destination chain decimals
+    const dstNativeGasAmountRequested = amount.units(
+      amount.scale(
+        amount.truncate(
+          amount.fromBaseUnits(srcNativeGasAmountRequested, srcDecimals),
+          TokenTransfer.MAX_DECIMALS,
+        ),
+        dstDecimals,
+      ),
+    );
 
     // The expected destination gas can be pulled from the destination token bridge
     let destinationNativeGas = 0n;

connect/src/routes/tokenBridge/automatic.ts

@@ -135,9 +135,9 @@ export class AutomaticTokenBridgeRoute<N extends Network>
       if (options.nativeGas && (options.nativeGas > 1.0 || options.nativeGas < 0.0))
         throw new Error("Native gas must be between 0.0 and 1.0 (0% and 100%)");
 
-      // If destination is native, max out the nativeGas requested
+      // native gas drop-off when the native token is the destination should be 0
       const { destination } = request;
-      if (isNative(destination.id.address) && options.nativeGas === 0.0) options.nativeGas = 1.0;
+      if (isNative(destination.id.address) && options.nativeGas === 0.0) options.nativeGas = 0;
 
       const updatedParams = { ...params, options };
       const validatedParams: Vp = {
@@ -177,18 +177,33 @@ export class AutomaticTokenBridgeRoute<N extends Network>
 
     const redeemableAmount = amount.units(amt) - fee;
 
-    // Determine nativeGas
-    let nativeGasAmount = 0n;
+    let srcNativeGasAmount = amount.fromBaseUnits(0n, request.source.decimals);
     if (params.options && params.options.nativeGas > 0) {
+      const dtb = await request.toChain.getAutomaticTokenBridge();
+      // the maxSwapAmount is in destination chain decimals
+      const maxSwapAmount = await dtb.maxSwapAmount(request.destination.id.address);
       const scale = 10000;
-      const scaledGas = BigInt(params.options.nativeGas * scale);
-      nativeGasAmount = (redeemableAmount * scaledGas) / BigInt(scale);
+      const scaledGasPercent = BigInt(params.options.nativeGas * scale);
+      const dstNativeGasUnits = (maxSwapAmount * scaledGasPercent) / BigInt(scale);
+      const dstNativeGasAmount = amount.fromBaseUnits(
+        dstNativeGasUnits,
+        request.destination.decimals,
+      );
+      // convert the native gas amount to source chain decimals
+      srcNativeGasAmount = amount.scale(
+        amount.truncate(dstNativeGasAmount, TokenTransfer.MAX_DECIMALS),
+        request.source.decimals,
+      );
+      // can't request more gas than the redeemable amount
+      if (amount.units(srcNativeGasAmount) > redeemableAmount) {
+        srcNativeGasAmount = amount.fromBaseUnits(redeemableAmount, request.source.decimals);
+      }
     }
 
     return {
       fee: amount.fromBaseUnits(fee, request.source.decimals),
       amount: amt,
-      nativeGasAmount: amount.fromBaseUnits(nativeGasAmount, request.source.decimals),
+      nativeGasAmount: srcNativeGasAmount,
     };
   }