f matt different math

Author: valentinewallace

lightning/src/blinded_path/payment.rs

@@ -174,12 +174,12 @@ pub(super) fn compute_payinfo(
 			.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)))
+		// ceil(((curr_prop_mil + 1_000_000) * (next_prop_mil + 1_000_000)) / 1_000_000) - 1_000_000
+		curr_prop_mil = curr_prop_mil.checked_add(1_000_000)
+			.and_then(|f1| next_prop_mil.checked_add(1_000_000).and_then(|f2| f2.checked_mul(f1)))
 			.and_then(|f| f.checked_add(1_000_000 - 1))
 			.map(|f| f / 1_000_000)
+			.and_then(|f| f.checked_sub(1_000_000))
 			.ok_or(())?;
 
 		cltv_expiry_delta = cltv_expiry_delta.checked_add(tlvs.payment_relay.cltv_expiry_delta).ok_or(())?;