Add le module, to be moved to rand_core later

Copied from dhardy/master, minus single-read functions, with extra tests

Author: dhardy

src/le.rs

@@ -0,0 +1,73 @@
+// Copyright 2017-2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// https://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Little-Endian utilities
+//! 
+//! Little-Endian order has been chosen for internal usage; this makes some
+//! useful functions available.
+
+// TODO: eventually these should be exported somehow
+#![allow(unused)]
+
+use core::{mem, ptr};
+
+macro_rules! read_slice {
+    ($src:expr, $dst:expr, $size:expr, $which:ident) => {{
+        assert_eq!($src.len(), $size * $dst.len());
+
+        unsafe {
+            ptr::copy_nonoverlapping(
+                $src.as_ptr(),
+                $dst.as_mut_ptr() as *mut u8,
+                $src.len());
+        }
+        for v in $dst.iter_mut() {
+            *v = v.$which();
+        }
+    }};
+}
+
+/// Reads unsigned 32 bit integers from `src` into `dst`.
+/// Borrowed from the `byteorder` crate.
+#[inline]
+pub fn read_u32_into(src: &[u8], dst: &mut [u32]) {
+    read_slice!(src, dst, 4, to_le);
+}
+
+/// Reads unsigned 64 bit integers from `src` into `dst`.
+/// Borrowed from the `byteorder` crate.
+#[inline]
+pub fn read_u64_into(src: &[u8], dst: &mut [u64]) {
+    read_slice!(src, dst, 8, to_le);
+}
+
+#[test]
+fn test_read() {
+    let bytes = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16];
+    
+    let mut buf = [0u32; 4];
+    read_u32_into(&bytes, &mut buf);
+    assert_eq!(buf[0], 0x04030201);
+    assert_eq!(buf[3], 0x100F0E0D);
+    
+    let mut buf = [0u32; 3];
+    read_u32_into(&bytes[1..13], &mut buf);  // unaligned
+    assert_eq!(buf[0], 0x05040302);
+    assert_eq!(buf[2], 0x0D0C0B0A);
+    
+    let mut buf = [0u64; 2];
+    read_u64_into(&bytes, &mut buf);
+    assert_eq!(buf[0], 0x0807060504030201);
+    assert_eq!(buf[1], 0x100F0E0D0C0B0A09);
+    
+    let mut buf = [0u64; 1];
+    read_u64_into(&bytes[7..15], &mut buf); // unaligned
+    assert_eq!(buf[0], 0x0F0E0D0C0B0A0908);
+}

src/lib.rs

@@ -298,6 +298,7 @@ pub mod isaac {
 }
 
 // private modules
+mod le;
 mod error;
 mod rand_impls;
 mod prng;