f no-std

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1022,6 +1022,12 @@ const PRECISION_LOWER_BOUND_DENOMINATOR: u64 = approx::LOWER_BITS_BOUND;
 const AMOUNT_PENALTY_DIVISOR: u64 = 1 << 20;
 const BASE_AMOUNT_PENALTY_DIVISOR: u64 = 1 << 30;
 
+/// Raises three `f64`s to the 3rd power, without `powi` because it requires `std` (dunno why).
+#[inline(always)]
+fn three_f64_pow_3(a: f64, b: f64, c: f64) -> (f64, f64, f64) {
+	(a * a * a, b * b * b, c * c * c)
+}
+
 /// Given liquidity bounds, calculates the success probability (in the form of a numerator and
 /// denominator) of an HTLC. This is a key assumption in our scoring models.
 ///
@@ -1058,9 +1064,9 @@ fn success_probability(
 			// calculate the cumulative density function between the min/max bounds trivially. Note
 			// that we don't bother to normalize the CDF to total to 1, as it will come out in the
 			// division of num / den.
-			let max_pow = (max - 0.5).powi(3);
-			let num = max_pow - (amount - 0.5).powi(3);
-			let den = max_pow - (min - 0.5).powi(3);
+			let (max_pow, amt_pow, min_pow) = three_f64_pow_3(max - 0.5, amount - 0.5, min - 0.5);
+			let num = max_pow - amt_pow;
+			let den = max_pow - min_pow;
 
 			// Because our numerator and denominator max out at 0.5^3 we need to multiply them by
 			// quite a large factor to get something useful (ideally in the 2^30 range).