From e9adc5067bf66f4e0c9d96623eabe48acf76a437 Mon Sep 17 00:00:00 2001
From: Alex Lyon <arcterus@mail.com>
Date: Sat, 20 Mar 2021 02:33:04 -0700
Subject: [PATCH] cksum: generate CRC table in a const fn (#1744)

---
 src/uu/cksum/build.rs     | 46 ------------------
 src/uu/cksum/src/cksum.rs | 99 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 94 insertions(+), 51 deletions(-)
 delete mode 100644 src/uu/cksum/build.rs

diff --git a/src/uu/cksum/build.rs b/src/uu/cksum/build.rs
deleted file mode 100644
index a9edd0d592b..00000000000
--- a/src/uu/cksum/build.rs
+++ /dev/null
@@ -1,46 +0,0 @@
-// This file is part of the uutils coreutils package.
-//
-// (c) Alex Lyon <arcterus@mail.com>
-// (c) Michael Gehring <mg@ebfe.org>
-//
-// For the full copyright and license information, please view the LICENSE
-// file that was distributed with this source code.
-
-use std::env;
-use std::fs::File;
-use std::io::Write;
-use std::path::Path;
-
-const CRC_TABLE_LEN: usize = 256;
-
-fn main() {
-    let out_dir = env::var("OUT_DIR").unwrap();
-
-    let mut table = Vec::with_capacity(CRC_TABLE_LEN);
-    for num in 0..CRC_TABLE_LEN {
-        table.push(crc_entry(num as u8) as u32);
-    }
-    let file = File::create(&Path::new(&out_dir).join("crc_table.rs")).unwrap();
-    write!(
-        &file,
-        "#[allow(clippy::unreadable_literal)]\nconst CRC_TABLE: [u32; {}] = {:?};",
-        CRC_TABLE_LEN, table
-    )
-    .unwrap();
-}
-
-#[inline]
-fn crc_entry(input: u8) -> u32 {
-    let mut crc = (input as u32) << 24;
-
-    for _ in 0..8 {
-        if crc & 0x8000_0000 != 0 {
-            crc <<= 1;
-            crc ^= 0x04c1_1db7;
-        } else {
-            crc <<= 1;
-        }
-    }
-
-    crc
-}
diff --git a/src/uu/cksum/src/cksum.rs b/src/uu/cksum/src/cksum.rs
index f589e6f8161..e1c75fffc6c 100644
--- a/src/uu/cksum/src/cksum.rs
+++ b/src/uu/cksum/src/cksum.rs
@@ -14,11 +14,101 @@ use std::fs::File;
 use std::io::{self, stdin, BufReader, Read};
 use std::path::Path;
 
-include!(concat!(env!("OUT_DIR"), "/crc_table.rs"));
+// NOTE: CRC_TABLE_LEN *must* be <= 256 as we cast 0..CRC_TABLE_LEN to u8
+const CRC_TABLE_LEN: usize = 256;
+const CRC_TABLE: [u32; CRC_TABLE_LEN] = generate_crc_table();
+
+const SYNTAX: &str = "[OPTIONS] [FILE]...";
+const SUMMARY: &str = "Print CRC and size for each file";
+const LONG_HELP: &str = "";
+
+// this is basically a hack to get "loops" to work on Rust 1.33.  Once we update to Rust 1.46 or
+// greater, we can just use while loops
+macro_rules! unroll {
+    (256, |$i:ident| $s:expr) => {{
+        unroll!(@ 32, 0 * 32, $i, $s);
+        unroll!(@ 32, 1 * 32, $i, $s);
+        unroll!(@ 32, 2 * 32, $i, $s);
+        unroll!(@ 32, 3 * 32, $i, $s);
+        unroll!(@ 32, 4 * 32, $i, $s);
+        unroll!(@ 32, 5 * 32, $i, $s);
+        unroll!(@ 32, 6 * 32, $i, $s);
+        unroll!(@ 32, 7 * 32, $i, $s);
+    }};
+    (8, |$i:ident| $s:expr) => {{
+        unroll!(@ 8, 0, $i, $s);
+    }};
+
+    (@ 32, $start:expr, $i:ident, $s:expr) => {{
+        unroll!(@ 8, $start + 0 * 8, $i, $s);
+        unroll!(@ 8, $start + 1 * 8, $i, $s);
+        unroll!(@ 8, $start + 2 * 8, $i, $s);
+        unroll!(@ 8, $start + 3 * 8, $i, $s);
+    }};
+    (@ 8, $start:expr, $i:ident, $s:expr) => {{
+        unroll!(@ 4, $start, $i, $s);
+        unroll!(@ 4, $start + 4, $i, $s);
+    }};
+    (@ 4, $start:expr, $i:ident, $s:expr) => {{
+        unroll!(@ 2, $start, $i, $s);
+        unroll!(@ 2, $start + 2, $i, $s);
+    }};
+    (@ 2, $start:expr, $i:ident, $s:expr) => {{
+        unroll!(@ 1, $start, $i, $s);
+        unroll!(@ 1, $start + 1, $i, $s);
+    }};
+    (@ 1, $start:expr, $i:ident, $s:expr) => {{
+        let $i = $start;
+        let _ = $s;
+    }};
+}
+
+const fn generate_crc_table() -> [u32; CRC_TABLE_LEN] {
+    let mut table = [0; CRC_TABLE_LEN];
+
+    // NOTE: works on Rust 1.46
+    //let mut i = 0;
+    //while i < CRC_TABLE_LEN {
+    //    table[i] = crc_entry(i as u8) as u32;
+    //
+    //    i += 1;
+    //}
+    unroll!(256, |i| {
+        table[i] = crc_entry(i as u8) as u32;
+    });
+
+    table
+}
 
-static SYNTAX: &str = "[OPTIONS] [FILE]...";
-static SUMMARY: &str = "Print CRC and size for each file";
-static LONG_HELP: &str = "";
+const fn crc_entry(input: u8) -> u32 {
+    let mut crc = (input as u32) << 24;
+
+    // NOTE: this does not work on Rust 1.33, but *does* on 1.46
+    //let mut i = 0;
+    //while i < 8 {
+    //    if crc & 0x8000_0000 != 0 {
+    //        crc <<= 1;
+    //        crc ^= 0x04c1_1db7;
+    //    } else {
+    //        crc <<= 1;
+    //    }
+    //
+    //    i += 1;
+    //}
+    unroll!(8, |_i| {
+        let if_cond = crc & 0x8000_0000;
+        let if_body = (crc << 1) ^ 0x04c1_1db7;
+        let else_body = crc << 1;
+
+        // NOTE: i feel like this is easier to understand than emulating an if statement in bitwise
+        //       ops
+        let cond_table = [else_body, if_body];
+
+        crc = cond_table[(if_cond != 0) as usize];
+    });
+
+    crc
+}
 
 #[inline]
 fn crc_update(crc: u32, input: u8) -> u32 {
@@ -68,7 +158,6 @@ fn cksum(fname: &str) -> io::Result<(u32, usize)> {
             Err(err) => return Err(err),
         }
     }
-    //Ok((0 as u32,0 as usize))
 }
 
 pub fn uumain(args: impl uucore::Args) -> i32 {