Replace Transaction::fee with a FeeCalculator

sdk/src/fee_calculator.rs

@@ -0,0 +1,48 @@
+use crate::message::Message;
+
+#[derive(Default)]
+pub struct FeeCalculator {
+    pub lamports_per_signature: u64,
+}
+
+impl FeeCalculator {
+    pub fn new(lamports_per_signature: u64) -> Self {
+        Self {
+            lamports_per_signature,
+        }
+    }
+
+    pub fn calculate_fee(&self, message: &Message) -> u64 {
+        self.lamports_per_signature * u64::from(message.num_required_signatures)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::pubkey::Pubkey;
+    use crate::system_instruction::SystemInstruction;
+
+    #[test]
+    fn test_fee_calculator_calculate_fee() {
+        // Default: no fee.
+        let message = Message::new(vec![]);
+        assert_eq!(FeeCalculator::default().calculate_fee(&message), 0);
+
+        // No signature, no fee.
+        assert_eq!(FeeCalculator::new(1).calculate_fee(&message), 0);
+
+        // One signature, a fee.
+        let pubkey0 = Pubkey::new(&[0; 32]);
+        let pubkey1 = Pubkey::new(&[1; 32]);
+        let ix0 = SystemInstruction::new_move(&pubkey0, &pubkey1, 1);
+        let message = Message::new(vec![ix0]);
+        assert_eq!(FeeCalculator::new(2).calculate_fee(&message), 2);
+
+        // Two signatures, double the fee.
+        let ix0 = SystemInstruction::new_move(&pubkey0, &pubkey1, 1);
+        let ix1 = SystemInstruction::new_move(&pubkey1, &pubkey0, 1);
+        let message = Message::new(vec![ix0, ix1]);
+        assert_eq!(FeeCalculator::new(2).calculate_fee(&message), 4);
+    }
+}