rust: reduce read buffer size

Reduce the size of the read buffer from 64MB to 256K which is much
friendlier to the CPU cache. This is the same size as used by the
optimized C version and it turns out to yield a small performance boost
here as well.

In fact, it provides enough of a speed up that the naive implementation
in Rust is now consistently faster than ripgrep, which is a little
surprising.

README.md

@@ -75,7 +75,7 @@ implementations compare to each other.
 | grep | 0m18.034s | 0m15.713s | 0m2.257s |
 | ripgrep | 0m1.709s | 0m1.541s | 0m0.147s |
 | simple (Go) | 0m1.889s | 0m1.679s | 0m0.211s |
-| simple (Rust) | 0m1.623s | 0m1.415s | 0m0.204s |
+| simple (Rust) | 0m1.461s | 0m1.325s | 0m0.131s |
 | simple (Node) | 0m6.458s | 0m6.043s | 0m0.627s |
 | custom (C) | **0m0.222s** | **0m0.079s** | **0m0.141s** |
 

main.rs

@@ -43,7 +43,7 @@ fn scan_slice(inb: &[u8]) -> usize {
 }
 
 fn sscan(mut input: impl Read) {
-	let mut backbuf = vec![0u8; 64*1024*1024];
+	let mut backbuf = vec![0u8; 64*4096];
 	let bbuf = backbuf.as_mut_slice();
 	// let mut bbuf = [0u8; 2*1024*1024];
 	let mut off = 0;