f clean up weight calculation

Author: TheBlueMatt

lightning/src/util/transaction_utils.rs

@@ -50,7 +50,8 @@ pub(crate) fn maybe_add_change_output(tx: &mut Transaction, input_value: u64, wi
 		value: 0,
 	};
 	let change_len = change_output.consensus_encode(&mut sink()).unwrap();
-	let mut weight_with_change: i64 = tx.get_weight() as i64 + WITNESS_FLAG_BYTES + witness_max_weight as i64 + change_len as i64 * 4;
+	let starting_weight = tx.get_weight() + WITNESS_FLAG_BYTES as usize + witness_max_weight;
+	let mut weight_with_change: i64 = starting_weight as i64 + change_len as i64 * 4;
 	// Include any extra bytes required to push an extra output.
 	weight_with_change += (VarInt(tx.output.len() as u64 + 1).len() - VarInt(tx.output.len() as u64).len()) as i64 * 4;
 	// When calculating weight, add two for the flag bytes
@@ -59,10 +60,10 @@ pub(crate) fn maybe_add_change_output(tx: &mut Transaction, input_value: u64, wi
 		change_output.value = change_value as u64;
 		tx.output.push(change_output);
 		Ok(weight_with_change as usize)
-	} else if (input_value - output_value) as i64 - (tx.get_weight() as i64 + 2 + witness_max_weight as i64) * feerate_sat_per_1000_weight as i64 / 1000 < 0 {
+	} else if (input_value - output_value) as i64 - (starting_weight as i64) * feerate_sat_per_1000_weight as i64 / 1000 < 0 {
 		Err(())
 	} else {
-		Ok(tx.get_weight() + WITNESS_FLAG_BYTES as usize + witness_max_weight)
+		Ok(starting_weight)
 	}
 }