Port iter code from dhardy/master

Author: dhardy

src/iter.rs

@@ -0,0 +1,227 @@
+// 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.
+
+//! Iterators over `RngCore`
+
+use core::cmp::min;
+use core::usize;
+
+use RngCore;
+
+// TODO: in the future (see https://github.com/rust-lang/rfcs/issues/1403)
+// it may be possible to allow reborrows in user code; this would let us
+// replace `rng: &'a mut R` with `rng: R` in `Iter`, without having to create a
+// redundant reference when calling `Map::next`. In this case `Rng::iter` would
+// return `Iter<&mut Self>` and a separate constructor would be needed for
+// reference/copy types not needing an extra reference.
+
+/// An "iterator" over a random number generator; created by [`Rng::iter`].
+/// 
+/// This does not implement `std::iter::Iterator` since we cannot support
+/// `next()`: it makes no sense to return a copy of an RNG, and though in
+/// theory it should be possible to return `&mut RngCore`, `Iterator` does not
+/// allow the lifetime of the item returned by `next` to be linked to the
+/// iterator (or enclosed RNG). Instead, we support other selected operations
+/// such as `map` and `take` directly.
+/// 
+/// [`Rng::iter`]: ../trait.Rng.html#method.iter
+#[derive(Debug)]
+pub struct Iter<'a, R: RngCore + ?Sized + 'a> {
+    pub(crate) rng: &'a mut R,
+    pub(crate) len: Option<usize>,
+}
+
+impl<'a, R: RngCore + ?Sized + 'a> Iter<'a, R> {
+    pub(crate) fn new(rng: &'a mut R) -> Iter<'a, R> {
+        Iter { rng, len: None }
+    }
+}
+
+impl<'a, R: RngCore + ?Sized + 'a> Iter<'a, R> {
+    /// Restricts the number of generated items to at most `len`.
+    pub fn take(self, len: usize) -> Self {
+        Iter {
+            rng: self.rng,
+            len: Some(self.len.map_or(len, |old| min(old, len))),
+        }
+    }
+    
+    /// Takes a closure and creates an iterator which calls that closure on
+    /// each element.
+    /// 
+    /// ### Example
+    /// 
+    /// ```rust
+    /// use rand::{thread_rng, Rng};
+    /// use rand::distributions::Range;
+    /// 
+    /// let die_range = Range::new(1, 7);
+    /// let mut rng = thread_rng();
+    /// let mut die = rng.iter().map(|rng| rng.sample(die_range));
+    /// for _ in 0..3 {
+    ///     println!("Die roll: {}", die.next().unwrap());
+    /// }
+    /// ```
+    pub fn map<B, F>(self, f: F) -> Map<'a, R, B, F>
+        where F: FnMut(&mut R) -> B
+    {
+        Map {
+            rng: self.rng,
+            len: self.len,
+            f: f,
+        }
+    }
+    
+    /// Creates an iterator that works like map, but flattens nested structure.
+    /// 
+    /// The [`map`] adapter is very useful, but only when the closure argument
+    /// produces values. If it produces an iterator instead, there's an extra
+    /// layer of indirection. `flat_map()` will remove this extra layer on its
+    /// own.
+    /// 
+    /// ### Example
+    /// 
+    /// ```rust
+    /// use rand::{thread_rng, Rng};
+    /// use rand::distributions::Range;
+    /// 
+    /// let len_range = Range::new(1, 10);
+    /// let mut rng = thread_rng();
+    /// 
+    /// // Count from 1 to a number between 1 and 9 several times:
+    /// let mut iter = rng.iter().flat_map(|rng| 1..rng.sample(len_range)).take(20);
+    /// while let Some(n) = iter.next() {
+    ///     println!("{}", n);
+    /// }
+    /// ```
+    /// 
+    /// [`map`]: struct.Iter.html#method.map
+    pub fn flat_map<U, F>(self, f: F) -> FlatMap<'a, R, U, F>
+        where F: FnMut(&mut R) -> U, U: IntoIterator
+    {
+        FlatMap {
+            rng: self.rng,
+            len: self.len,
+            f: f,
+            frontiter: None,
+        }
+    }
+}
+
+/// Type created by [`Iter::map`](struct.Iter.html#method.map)
+#[derive(Debug)]
+pub struct Map<'a, R:?Sized+'a, B, F> where F: FnMut(&mut R) -> B {
+    rng: &'a mut R,
+    len: Option<usize>,
+    f: F,
+}
+impl<'a, R:?Sized+'a, B, F> Iterator for Map<'a, R, B, F>
+    where F: FnMut(&mut R) -> B
+{
+    type Item = B;
+    
+    fn next(&mut self) -> Option<B> {
+        match self.len {
+            Some(0) => return None,
+            Some(ref mut n) => { *n -= 1; }
+            None => {}
+        }
+        
+        Some((self.f)(self.rng))
+    }
+    
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // If len == None we have an infinite iterator; usize::MAX is nearest
+        // available lower bound. Probably this suffices to make the following equal:
+        // rng.iter().take(n).map(f).size_hint() == rng.iter().map(f).take(n).size_hint()
+        self.len.map_or((usize::MAX, None), |len| (len, Some(len)))
+    }
+}
+
+/// Type created by [`Iter::flat_map`](struct.Iter.html#method.flat_map)
+#[derive(Debug)]
+pub struct FlatMap<'a, R:?Sized+'a, U, F>
+    where F: FnMut(&mut R) -> U, U: IntoIterator
+{
+    rng: &'a mut R,
+    len: Option<usize>,
+    f: F,
+    frontiter: Option<U::IntoIter>,
+}
+impl<'a, R:?Sized+'a, U, F> Iterator for FlatMap<'a, R, U, F>
+    where F: FnMut(&mut R) -> U, U: IntoIterator
+{
+    type Item = <U as IntoIterator>::Item;
+    
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if let Some(ref mut inner) = self.frontiter {
+                if let Some(x) = inner.by_ref().next() {
+                    return Some(x)
+                }
+            }
+            
+            match self.len {
+                Some(0) => return None,
+                Some(ref mut n) => { *n -= 1; }
+                None => {}
+            }
+            
+            self.frontiter = Some(IntoIterator::into_iter((self.f)(self.rng)));
+        }
+    }
+    
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.len == Some(0) {
+            // No new iters, so we have frontiter or nothing
+            self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint())
+        } else {
+            // Can't compute an actual bound without producing the sub-iters,
+            // which we don't want to do. But we may have a lower bound.
+            let lb = self.frontiter.as_ref().map_or(0, |it| it.size_hint().0);
+            (lb, None)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use {Rng, RngCore};
+    use distributions::{Uniform};
+    #[cfg(all(not(feature="std"), feature="alloc"))] use alloc::{Vec, String};
+
+    #[test]
+    #[cfg(any(feature="std", feature="alloc"))]
+    fn test_iter() {
+        let mut rng = ::test::rng(160);
+        
+        let x: Vec<()> = rng.iter().take(10).map(|_| ()).collect();
+        assert_eq!(x.len(), 10);
+        let y: Vec<u32> = rng.iter().take(10).map(|rng| rng.sample(Uniform)).collect();
+        assert_eq!(y.len(), 10);
+        let z: Vec<u32> = rng.iter().take(10).flat_map(|rng|
+                vec![rng.sample(Uniform), rng.sample(Uniform)].into_iter()).collect();
+        assert_eq!(z.len(), 20);
+        let w: Vec<String> = rng.iter().take(10).flat_map(|_| vec![].into_iter()).collect();
+        assert_eq!(w.len(), 0);
+    }
+    
+    #[test]
+    fn test_dyn_dispatch() {
+        let mut rng = ::test::rng(161);
+        let mut r: &mut RngCore = &mut rng;
+        
+        let mut x = 0;
+        for n in r.iter().map(|rng| rng.next_u32()).take(2) {
+            x ^= n;
+        }
+        assert!(x != 0);
+    }
+}

src/lib.rs

@@ -252,7 +252,7 @@
 #![cfg_attr(all(target_arch = "wasm32", not(target_os = "emscripten")), recursion_limit="128")]
 
 #[cfg(feature="std")] extern crate std as core;
-#[cfg(all(feature = "alloc", not(feature="std")))] extern crate alloc;
+#[cfg(all(feature = "alloc", not(feature="std")))] #[macro_use] extern crate alloc;
 
 #[cfg(test)] #[cfg(feature="serde-1")] extern crate bincode;
 #[cfg(feature="serde-1")] extern crate serde;
@@ -296,6 +296,7 @@ use distributions::range::SampleRange;
 // public modules
 pub mod distributions;
 mod impls;
+pub mod iter;
 pub mod jitter;
 pub mod mock;
 #[cfg(feature="std")] pub mod os;
@@ -314,9 +315,9 @@ pub mod isaac {
 }
 
 // private modules
-mod le;
 #[cfg(feature="std")] mod entropy_rng;
 mod error;
+mod le;
 mod prng;
 #[cfg(feature="std")] mod thread_rng;
 
@@ -647,6 +648,35 @@ pub trait Rng: RngCore + Sized {
         n <= 1 || self.gen_range(0, n) == 0
     }
 
+    /// Construct an iterator on an `Rng`.
+    /// 
+    /// ### Example
+    /// 
+    /*
+    /// ```rust
+    /// use rand::{thread_rng, Rng};
+    /// use distributions::Range;
+    /// 
+    /// let die_range = Range::new(1, 7);
+    /// let mut die = thread_rng().iter().map(|rng| rng.sample(die_range));
+    /// for _ in 0..3 {
+    ///     println!("Die roll: {}", die.next());
+    /// }
+    /// ```
+    */
+    /* TODO: Alphanumeric
+    /// ```rust
+    /// use rand::{thread_rng, Rng};
+    /// use rand::distributions::Alphanumeric;
+    /// 
+    /// let mut rng = thread_rng();
+    /// let x: String = rng.iter().map(|rng| rng.sample(Alphanumeric)).take(6).collect();
+    /// ```
+    */
+    fn iter<'a>(&'a mut self) -> iter::Iter<'a, Self> {
+        iter::Iter::new(self)
+    }
+
     /// Return an iterator of random characters from the set A-Z,a-z,0-9.
     ///
     /// # Example