Speed up sum by using reasonable read buffer sizes.

Use a 4K read buffer for each of the checksum functions, which seems
reasonable. This improves the performance of BSD checksums on
odyssey1024.txt from 399ms to 325ms on my laptop, and of SysV
checksums from 242ms to 67ms.

src/uu/sum/src/sum.rs

@@ -23,14 +23,19 @@ static USAGE: &str = "{} [OPTION]... [FILE]...";
 static SUMMARY: &str = "Checksum and count the blocks in a file.\n\
                         With no FILE, or when  FILE is -, read standard input.";
 
+// This can be replaced with usize::div_ceil once it is stabilized
+fn div_ceil(a: usize, b: usize) -> usize {
+    (a + b - 1) / b
+}
+
 fn bsd_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
-    let mut buf = [0; 1024];
-    let mut blocks_read = 0;
+    let mut buf = [0; 4096];
+    let mut bytes_read = 0;
     let mut checksum: u16 = 0;
     loop {
         match reader.read(&mut buf) {
             Ok(n) if n != 0 => {
-                blocks_read += 1;
+                bytes_read += n;
                 for &byte in buf[..n].iter() {
                     checksum = (checksum >> 1) + ((checksum & 1) << 15);
                     checksum = checksum.wrapping_add(u16::from(byte));
@@ -40,18 +45,19 @@ fn bsd_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
         }
     }
 
+    let blocks_read = div_ceil(bytes_read, 1024);
     (blocks_read, checksum)
 }
 
 fn sysv_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
-    let mut buf = [0; 512];
-    let mut blocks_read = 0;
+    let mut buf = [0; 4096];
+    let mut bytes_read = 0;
     let mut ret = 0u32;
 
     loop {
         match reader.read(&mut buf) {
             Ok(n) if n != 0 => {
-                blocks_read += 1;
+                bytes_read += n;
                 for &byte in buf[..n].iter() {
                     ret = ret.wrapping_add(u32::from(byte));
                 }
@@ -63,6 +69,7 @@ fn sysv_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
     ret = (ret & 0xffff) + (ret >> 16);
     ret = (ret & 0xffff) + (ret >> 16);
 
+    let blocks_read = div_ceil(bytes_read, 512);
     (blocks_read, ret as u16)
 }