Use reverse_bits intrinsic when running on nightly

Detect nightly and auto-enable reverse_bits

Author: thomwiggers

.travis.yml

@@ -1,13 +1,11 @@
 language: rust
 
-matrix:
-  include:
-    - rust: nightly
-      env: FEATURES=unsafe_internals
-    - rust: nightly
-      env: FEATURES=''
-    - rust: stable
-    - rust: beta
+rust:
+  - stable
+  - nightly
+env:
+  - FEATURES=''
+  - FEATURES='unsafe_internals'
 
 before_script:
   - rustup component add rustfmt-preview

Cargo.toml

@@ -10,6 +10,9 @@ readme = "README.md"
 license = "Apache-2.0 OR MIT"
 categories = ["data-structures"]
 
+[build-dependencies]
+rustc_version = "0.2"
+
 [dependencies]
 num-traits = "0.2.1"
 serde = { version="1.0", features=["derive"], optional=true }

benches/vob.rs

@@ -101,3 +101,12 @@ fn and(bench: &mut Bencher) {
         a.and(&b);
     });
 }
+
+#[bench]
+fn from_bytes(b: &mut Bencher) {
+    let mut rng = rand::thread_rng();
+    let mut source = [0u8; 1024];
+    rng.fill(&mut source);
+
+    b.iter(|| Vob::from_bytes(&source));
+}

build.rs

@@ -0,0 +1,13 @@
+extern crate rustc_version;
+
+use rustc_version::{version_meta, Channel};
+use std::env;
+
+fn main() {
+    // Set features depending on channel
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        println!("cargo:rustc-cfg=nightly");
+        // Nightly supports https://github.com/rust-lang/rust/issues/48763
+        println!("cargo:rustc-cfg=reverse_bits");
+    }
+}

src/lib.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(nightly, feature(reverse_bits))]
 // Copyright (c) 2018 King's College London created by the Software Development Team
 // <http://soft-dev.org/>
 //
@@ -162,6 +163,9 @@ impl Vob<usize> {
     /// Create a Vob from a `u8` slice. The most significant bit of each byte comes first in the
     /// resulting Vob.
     ///
+    /// If you are running nightly, this method will use the new `reverse_bits` intrinsic.
+    /// Set the `no_reverse_bits` feature to manually disable it.
+    ///
     /// # Examples
     ///
     /// ```
@@ -187,12 +191,21 @@ impl Vob<usize> {
                     continue;
                 }
                 let b = slice[off];
-                if b != 0 {
-                    let mut rb: u8 = 0; // the byte b with its bits in reverse order
-                    for k in 0..8 {
-                        rb |= ((b >> k) & 1) << (8 - k - 1);
+                #[cfg(not(reverse_bits))]
+                {
+                    if b != 0 {
+                        {
+                            let mut rb: u8 = 0; // the byte b with its bits in reverse order
+                            for k in 0..8 {
+                                rb |= ((b >> k) & 1) << (8 - k - 1);
+                            }
+                            w |= (rb as usize) << (j * 8);
+                        }
                     }
-                    w |= (rb as usize) << (j * 8);
+                }
+                #[cfg(reverse_bits)]
+                {
+                    w |= (b.reverse_bits() as usize) << (j * 8);
                 }
             }
             v.vec.push(w);