f - Use u16 values in look-up table

Author: jkczyz

lightning/src/routing/scoring.rs

@@ -806,7 +806,7 @@ mod approx {
 
 	/// Look-up table for `log10(x) * 1000` where row `i` is used for each `x` having `i` as the
 	/// most significant bit. The next 4 bits of `x`, if applicable, are used for the second index.
-	const LOG10_TIMES_1K: [[u64; LOWER_BITS_BOUND as usize]; BITS as usize] = [
+	const LOG10_TIMES_1K: [[u16; LOWER_BITS_BOUND as usize]; BITS as usize] = [
 		[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
 		[301, 301, 301, 301, 301, 301, 301, 301, 477, 477, 477, 477, 477, 477, 477, 477],
 		[602, 602, 602, 602, 699, 699, 699, 699, 778, 778, 778, 778, 845, 845, 845, 845],
@@ -877,11 +877,11 @@ mod approx {
 	#[inline]
 	pub fn negative_log10_times_1k(numerator: u64, denominator: u64) -> u64 {
 		// Multiply the -1 through to avoid needing to use signed numbers.
-		log10_times_1k(denominator) - log10_times_1k(numerator)
+		(log10_times_1k(denominator) - log10_times_1k(numerator)) as u64
 	}
 
 	#[inline]
-	fn log10_times_1k(x: u64) -> u64 {
+	fn log10_times_1k(x: u64) -> u16 {
 		debug_assert_ne!(x, 0);
 		let most_significant_bit = HIGHEST_BIT - x.leading_zeros();
 		let lower_bits = (x >> most_significant_bit.saturating_sub(LOWER_BITS)) & LOWER_BITMASK;
@@ -897,7 +897,7 @@ mod approx {
 			for msb in 0..BITS {
 				for i in 0..LOWER_BITS_BOUND {
 					let x = ((LOWER_BITS_BOUND + i) << (HIGHEST_BIT - LOWER_BITS)) >> (HIGHEST_BIT - msb);
-					let log10_times_1k = ((x as f64).log10() * 1000.0).round() as u64;
+					let log10_times_1k = ((x as f64).log10() * 1000.0).round() as u16;
 					assert_eq!(log10_times_1k, LOG10_TIMES_1K[msb as usize][i as usize]);
 
 					if i % LOWER_BITS_BOUND == 0 {