cksum/crc32: add code docs

xform/src/crc32.rs

@@ -1,3 +1,13 @@
+//
+// Copyright (c) 2024 Jeff Garzik
+//
+// This file is part of the posixutils-rs project covered under
+// the MIT License.  For the full license text, please see the LICENSE
+// file in the root directory of this project.
+// SPDX-License-Identifier: MIT
+//
+
+// Data straight from POSIX documentation
 const CRCTAB: [u32; 256] = [
     0x00000000, 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b, 0x1a864db2, 0x1e475005,
     0x2608edb8, 0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
@@ -33,6 +43,9 @@ const CRCTAB: [u32; 256] = [
     0xafb010b1, 0xab710d06, 0xa6322bdf, 0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4,
 ];
 
+// Almost directly as presented in POSIX documentation
+
+// Update CRC with given data.
 pub fn update(crc_in: u32, buf: &[u8]) -> u32 {
     let mut s = crc_in;
 
@@ -44,6 +57,7 @@ pub fn update(crc_in: u32, buf: &[u8]) -> u32 {
     s
 }
 
+// Finalize crc with data size.
 pub fn finalize(crc_in: u32, n_in: usize) -> u32 {
     let mut s = crc_in;
     let mut n = n_in;