std::rand: Add OSRng, ReaderRng wrappers around the OS RNG & generic Readers respectively.

The former reads from e.g. /dev/urandom, the latter just wraps any
std::rt::io::Reader into an interface that implements Rng.

This also adds Rng.fill_bytes for efficient implementations of the above
(reading 8 bytes at a time is inefficient when you can read 1000), and
removes the dependence on src/rt (i.e. rand_gen_seed) although this last
one requires implementing hand-seeding of the XorShiftRng used in the
scheduler on Linux/unixes, since OSRng relies on a scheduler existing to
be able to read from /dev/urandom.

src/libstd/rand/reader.rs

@@ -0,0 +1,94 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use rt::io::Reader;
+use rt::io::ReaderByteConversions;
+
+use rand::Rng;
+
+/// An RNG that reads random bytes straight from a `Reader`. This will
+/// work best with an infinite reader, but this is not required. The
+/// semantics of reading past the end of the reader are the same as
+/// those of the `read` method of the inner `Reader`.
+pub struct ReaderRng<R> {
+    priv reader: R
+}
+
+impl<R: Reader> ReaderRng<R> {
+    /// Create a new `ReaderRng` from a `Reader`.
+    pub fn new(r: R) -> ReaderRng<R> {
+        ReaderRng {
+            reader: r
+        }
+    }
+}
+
+impl<R: Reader> Rng for ReaderRng<R> {
+    fn next_u32(&mut self) -> u32 {
+        // XXX which is better: consistency between big/little-endian
+        // platforms, or speed.
+        if cfg!(target_endian="little") {
+            self.reader.read_le_u32_()
+        } else {
+            self.reader.read_be_u32_()
+        }
+    }
+    fn next_u64(&mut self) -> u64 {
+        if cfg!(target_endian="little") {
+            self.reader.read_le_u64_()
+        } else {
+            self.reader.read_be_u64_()
+        }
+    }
+    fn fill_bytes(&mut self, v: &mut [u8]) {
+        // XXX: check that we filled `v``
+        let _n = self.reader.read(v);
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use rt::io::mem::MemReader;
+    use cast;
+
+    #[test]
+    fn test_reader_rng_u64() {
+        // transmute from the target to avoid endianness concerns.
+        let v = ~[1u64, 2u64, 3u64];
+        let bytes: ~[u8] = unsafe {cast::transmute(v)};
+        let mut rng = ReaderRng::new(MemReader::new(bytes));
+
+        assert_eq!(rng.next_u64(), 1);
+        assert_eq!(rng.next_u64(), 2);
+        assert_eq!(rng.next_u64(), 3);
+    }
+    #[test]
+    fn test_reader_rng_u32() {
+        // transmute from the target to avoid endianness concerns.
+        let v = ~[1u32, 2u32, 3u32];
+        let bytes: ~[u8] = unsafe {cast::transmute(v)};
+        let mut rng = ReaderRng::new(MemReader::new(bytes));
+
+        assert_eq!(rng.next_u32(), 1);
+        assert_eq!(rng.next_u32(), 2);
+        assert_eq!(rng.next_u32(), 3);
+    }
+    #[test]
+    fn test_reader_rng_fill_bytes() {
+        let v = [1u8, 2, 3, 4, 5, 6, 7, 8];
+        let mut w = [0u8, .. 8];
+
+        let mut rng = ReaderRng::new(MemReader::new(v.to_owned()));
+        rng.fill_bytes(w);
+
+        assert_eq!(v, w);
+    }
+}