Add optional support for SmallVec as underlying peekable queue (#8)

Current version does not enable smallvec by default because it currently still requires a nightly compiler.
With the recent stabilization of liballoc, smallvec may add support for the stable compiler in combination with no_std soon =).

.ci/test.sh

@@ -0,0 +1,2 @@
+cargo build --verbose --all
+cargo test --verbose --all

.ci/test_nightly.sh

@@ -0,0 +1,3 @@
+.ci/test.sh
+cargo +nightly build --verbose --all --features smallvec
+cargo +nightly test --verbose --all --features smallvec

.travis.yml

@@ -1,14 +1,27 @@
 language: rust
-rust:
-  - stable
-  - nightly
-os:
-  - linux
-  - windows
+before_install:
+  - if [ $TRAVIS_OS_NAME=linux ]; then chmod +x -R .ci; fi
+
 matrix:
   allow_failures:
-    - rust: nightly
+    - env: ALLOW_FAILURE=1
   fast_finish: true
-script:
-  - cargo build --verbose --all
-  - cargo test --verbose --all
+  include:
+    - name: "stable->linux"
+      rust: stable
+      os: linux
+      script: .ci/test.sh
+    - name: "stable->windows"
+      rust: stable
+      os: windows
+      script: .ci/test.sh
+    - name : "nightly->linux"
+      rust: nightly
+      os: linux
+      env: ALLOW_FAILURE=1
+      script: .ci/test_nightly.sh
+    - name: "nightly->windows"
+      rust: nightly
+      os: windows
+      env: ALLOW_FAILURE=1
+      script: .ci/test_nightly.sh

Cargo.toml

@@ -17,4 +17,13 @@ keywords = ["peek", "lookahead", "iterator", "peekable", "look-ahead"]
 categories = ["no-std"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-[dependencies]
+
+[features]
+default = []
+
+# smallvec with no-std requires nightly.
+# see https://github.com/servo/rust-smallvec/issues/160
+[dependencies.smallvec]
+version = "0.6.10"
+optional = true
+default-features = false

src/lib.rs

@@ -8,6 +8,17 @@
 //! allows you to peek at the next element and no further. This crate aims to take that limitation
 //! away.
 //!
+//! **Peekable queue:**
+//!
+//! To enable peeking at multiple elements ahead of consuming a next element, the iterator uses a
+//! traversable queue which holds the elements which you can peek at, but have not been
+//! consumed (yet).
+//! By default the underlying data structure of this queue is a Vec. By enabling the `smallvec`
+//! feature, you can opt-in to use SmallVec as the underlying queue data structure. SmallVec uses
+//! the stack for a limited amount of elements and will only allocate on the heap if this maximum
+//! amount of elements is reached. SmallVec support for `no_std` is experimental and currently
+//! [requires] a nightly compiler.
+//!
 //! **Usage example:**
 //!
 //! ```rust
@@ -65,15 +76,21 @@
 //!
 //! [`Peekable`]: https://doc.rust-lang.org/core/iter/struct.Peekable.html
 //! [`PeekView::peek`]: trait.PeekView.html#tymethod.peek
+//! [requires]: https://github.com/servo/rust-smallvec/issues/160
 
 /// We do need to allocate to save and store elements which are in the future compared to our last
 /// iterator element. Perhaps in the future we could optimize this slightly by using the stack for
 /// a limited amount of elements.
 extern crate alloc;
 
-/// We use a Vec to queue iterator elements.
+/// We use a Vec to queue iterator elements if the smallvec feature is disabled.
+#[cfg(not(feature = "smallvec"))]
 use alloc::vec::Vec;
 
+/// If the smallvec feature is enabled, we use a SmallVec to queue iterator elements instead of a Vec.
+#[cfg(feature = "smallvec")]
+use smallvec::SmallVec;
+
 /// Trait which allows one to create an iterator which allows us to peek multiple times forward.
 pub trait CreatePeekMoreIterator: Iterator + Sized {
     /// Create an iterator where we can look (peek) forward multiple times from an existing iterator.
@@ -84,12 +101,23 @@ impl<I: Iterator> CreatePeekMoreIterator for I {
     fn peekmore(self) -> PeekMoreIterator<I> {
         PeekMoreIterator {
             iterator: self,
+
+            #[cfg(not(feature = "smallvec"))]
             queue: Vec::new(),
+
+            #[cfg(feature = "smallvec")]
+            queue: SmallVec::new(),
+
             needle: 0usize,
         }
     }
 }
 
+/// Default stack size for SmallVec.
+/// Admittedly the current size is chosen quite arbitrarily.
+#[cfg(feature = "smallvec")]
+const DEFAULT_STACK_SIZE: usize = 256;
+
 /// This iterator makes it possible to peek multiple times without consuming a value.
 /// In reality the underlying iterator will be consumed, but the values will be stored in a local
 /// vector. This vector allows us to shift to visible element (the 'view').
@@ -102,7 +130,10 @@ pub struct PeekMoreIterator<I: Iterator> {
     /// The queue represent the items of our iterator which have not been consumed, but have been
     /// prepared to view ('peek') without consuming them. Once an element is consumed, we can no longer
     /// view an item in the queue.
+    #[cfg(not(feature = "smallvec"))]
     queue: Vec<Option<I::Item>>,
+    #[cfg(feature = "smallvec")]
+    queue: SmallVec<[Option<I::Item>; DEFAULT_STACK_SIZE]>,
 
     /// The needle helps us determine which item we currently have in view.
     /// Within these docs, it is also sometimes known as the peek view, peek view handle and peek view
@@ -484,7 +515,7 @@ mod tests {
         assert_eq!(v2, Some(&&2));
 
         let _ = iter.reset_view();
-        let v1again = iter.peek();;
+        let v1again = iter.peek();
         assert_eq!(v1again, Some(&&1));
 
         let v2again = iter.peek_next();