ripple/fossil/chunker: use const computation for DISCRIMINATOR

While `const fn` isn't permitted float computation, regular `const` is.
This deals with LLVM's reluctance to inline discriminator_from_average,
without forcing us to hardcode a magic number.

Change-Id: Ibdbfa4c733468517a2feff1ec0deedd1d9b70d47

1 files changed, 5 insertions, 18 deletions

diff --git a/ripple/fossil/src/chunker/mod.rs b/ripple/fossil/src/chunker/mod.rs
index 88414a2..59b3367 100644
--- a/ripple/fossil/src/chunker/mod.rs
+++ b/ripple/fossil/src/chunker/mod.rs
@@ -5,17 +5,15 @@ use std::mem;
 
 mod buz;
 
-const WINDOW_SIZE: usize = 48;
 pub const MIN_CHUNK_SIZE: usize = AVG_CHUNK_SIZE / 4;
 pub const AVG_CHUNK_SIZE: usize = 64 * 1024;
 pub const MAX_CHUNK_SIZE: usize = AVG_CHUNK_SIZE * 4;
-const DISCRIMINATOR: u32 = 0xc17f;
 
-#[cfg(test)]
-fn discriminator_from_average(avg: u64) -> u32 {
-	let avg = avg as f64;
+const WINDOW_SIZE: usize = 48;
+const DISCRIMINATOR: u32 = {
+	let avg = AVG_CHUNK_SIZE as f64;
 	(avg / (-1.42888852e-7 * avg + 1.33237515)) as u32
-}
+};
 
 pub struct Chunker<'a> {
 	buffer: &'a [u8],
@@ -88,10 +86,7 @@ impl<'a> Iterator for Chunker<'a> {
 #[cfg(test)]
 mod test {
 	use {
-		super::{
-			discriminator_from_average, Chunker, AVG_CHUNK_SIZE, DISCRIMINATOR, MAX_CHUNK_SIZE,
-			MIN_CHUNK_SIZE, WINDOW_SIZE,
-		},
+		super::{Chunker, MAX_CHUNK_SIZE, MIN_CHUNK_SIZE, WINDOW_SIZE},
 		std::io::Read,
 	};
 
@@ -239,12 +234,4 @@ mod test {
 			(2, Some(MAX_CHUNK_SIZE / MIN_CHUNK_SIZE + 1))
 		);
 	}
-
-	#[test]
-	fn discriminator() {
-		assert_eq!(
-			DISCRIMINATOR,
-			discriminator_from_average(AVG_CHUNK_SIZE as u64)
-		);
-	}
 }