external-api: add get_quote_amount helper

external-api/src/http/external_match.rs

@@ -185,6 +185,20 @@ impl ExternalOrder {
         let div = FixedPoint::floor_div_int(min_fill, price) + Scalar::one();
         scalar_to_u128(&div)
     }
+
+    /// Get the quote amount of the order implied by the external order
+    ///
+    /// The price here is expected to be decimal corrected; i.e. multiplied by
+    /// the decimal diff for the two tokens
+    pub fn get_quote_amount(&self, price: FixedPoint) -> Amount {
+        if self.quote_amount != 0 {
+            return self.quote_amount;
+        }
+
+        let base_amount_scalar = Scalar::from(self.base_amount);
+        let implied_quote_amount = price * base_amount_scalar;
+        scalar_to_u128(&implied_quote_amount.floor())
+    }
 }
 
 /// An atomic match settlement bundle, sent to the client so that they may