f use checked_add instead of u128s

Author: valentinewallace

lightning/src/blinded_path/payment.rs

@@ -3,7 +3,6 @@
 //! [`BlindedPath`]: crate::blinded_path::BlindedPath
 
 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
-use core::convert::TryFrom;
 use crate::blinded_path::BlindedHop;
 use crate::blinded_path::utils;
 use crate::io;
@@ -165,21 +164,30 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 pub(super) fn compute_payinfo(
 	path: &[(PublicKey, BlindedPaymentTlvs)]
 ) -> Result<BlindedPayInfo, ()> {
-	let mut curr_base_fee: u128 = 0;
-	let mut curr_prop_mil: u128 = 0;
+	let mut curr_base_fee: u32 = 0;
+	let mut curr_prop_mil: u32 = 0;
 	for (_, payment_tlvs) in path.iter().rev().skip(1) {
-		let next_base_fee = payment_tlvs.fee_base_msat() as u128;
-		let next_prop_mil = payment_tlvs.fee_proportional_millionths() as u128;
-		curr_base_fee =
-			((next_base_fee * 1_000_000 + (curr_base_fee * (1_000_000 + next_prop_mil))) + 1_000_000 - 1)
-			 / 1_000_000;
-		curr_prop_mil =
-			(((curr_prop_mil + next_prop_mil) * 1_000_000 + curr_prop_mil * next_prop_mil) + 1_000_000 - 1)
-			 / 1_000_000;
+		let next_base_fee = payment_tlvs.fee_base_msat();
+		let next_prop_mil = payment_tlvs.fee_proportional_millionths();
+		// Use integer arithmetic to compute `ceil(a/b)` as `(a+b-1)/b`
+		// ((next_base_fee * 1_000_000 + (curr_base_fee * (1_000_000 + next_prop_mil))) + 1_000_000 - 1) / 1_000_000
+		curr_base_fee = next_prop_mil.checked_add(1_000_000)
+			.and_then(|f| f.checked_mul(curr_base_fee))
+			.and_then(|f| next_base_fee.checked_mul(1_000_000).and_then(|base| base.checked_add(f)))
+			.and_then(|f| f.checked_add(1_000_000 - 1))
+			.map(|f| f / 1_000_000)
+			.ok_or(())?;
+		// (((curr_prop_mil + next_prop_mil) * 1_000_000 + curr_prop_mil * next_prop_mil) + 1_000_000 - 1) / 1_000_000
+		curr_prop_mil = curr_prop_mil.checked_add(next_prop_mil)
+			.and_then(|f| f.checked_mul(1_000_000))
+			.and_then(|f| curr_prop_mil.checked_mul(next_prop_mil).and_then(|prop_mil| prop_mil.checked_add(f)))
+			.and_then(|f| f.checked_add(1_000_000 - 1))
+			.map(|f| f / 1_000_000)
+			.ok_or(())?;
 	}
 	Ok(BlindedPayInfo {
-		fee_base_msat: u32::try_from(curr_base_fee).map_err(|_| ())?,
-		fee_proportional_millionths: u32::try_from(curr_prop_mil).map_err(|_| ())?,
+		fee_base_msat: curr_base_fee,
+		fee_proportional_millionths: curr_prop_mil,
 		cltv_expiry_delta: path.iter().map(|(_, tlvs)| tlvs.cltv_expiry_delta())
 			.try_fold(0u16, |acc, delta| acc.checked_add(delta)).ok_or(())?,
 		htlc_minimum_msat: path.iter().map(|(_, tlvs)| tlvs.htlc_minimum_msat()).max().unwrap_or(0),