std: Implement RandomAccessIterator for iterator adaptors

Implement RAI where possible for iterator adaptors such as Map,
Enumerate, Skip, Take, Zip, Cycle (all of the requiring that the adapted
iterator also implements RAI).

src/libstd/iterator.rs

@@ -811,6 +811,30 @@ impl<A, T: Clone + Iterator<A>> Iterator<A> for Cycle<T> {
     }
 }
 
+impl<A, T: Clone + RandomAccessIterator<A>> RandomAccessIterator<A> for Cycle<T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        if self.orig.indexable() > 0 {
+            uint::max_value
+        } else {
+            0
+        }
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<A> {
+        let liter = self.iter.indexable();
+        let lorig = self.orig.indexable();
+        if lorig == 0 {
+            None
+        } else if index < liter {
+            self.iter.idx(index)
+        } else {
+            self.orig.idx((index - liter) % lorig)
+        }
+    }
+}
+
 /// An iterator which strings two iterators together
 #[deriving(Clone)]
 pub struct Chain<T, U> {
@@ -924,20 +948,44 @@ impl<A, B, T: Iterator<A>, U: Iterator<B>> Iterator<(A, B)> for Zip<T, U> {
     }
 }
 
+impl<A, B, T: RandomAccessIterator<A>, U: RandomAccessIterator<B>>
+RandomAccessIterator<(A, B)> for Zip<T, U> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        cmp::min(self.a.indexable(), self.b.indexable())
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<(A, B)> {
+        match (self.a.idx(index), self.b.idx(index)) {
+            (Some(x), Some(y)) => Some((x, y)),
+            _ => None
+        }
+    }
+}
+
 /// An iterator which maps the values of `iter` with `f`
 pub struct Map<'self, A, B, T> {
     priv iter: T,
     priv f: &'self fn(A) -> B
 }
 
-impl<'self, A, B, T: Iterator<A>> Iterator<B> for Map<'self, A, B, T> {
+impl<'self, A, B, T> Map<'self, A, B, T> {
     #[inline]
-    fn next(&mut self) -> Option<B> {
-        match self.iter.next() {
+    fn do_map(&self, elt: Option<A>) -> Option<B> {
+        match elt {
             Some(a) => Some((self.f)(a)),
             _ => None
         }
     }
+}
+
+impl<'self, A, B, T: Iterator<A>> Iterator<B> for Map<'self, A, B, T> {
+    #[inline]
+    fn next(&mut self) -> Option<B> {
+        let next = self.iter.next();
+        self.do_map(next)
+    }
 
     #[inline]
     fn size_hint(&self) -> (uint, Option<uint>) {
@@ -949,10 +997,21 @@ impl<'self, A, B, T: DoubleEndedIterator<A>> DoubleEndedIterator<B>
 for Map<'self, A, B, T> {
     #[inline]
     fn next_back(&mut self) -> Option<B> {
-        match self.iter.next_back() {
-            Some(a) => Some((self.f)(a)),
-            _ => None
-        }
+        let next = self.iter.next_back();
+        self.do_map(next)
+    }
+}
+
+impl<'self, A, B, T: RandomAccessIterator<A>> RandomAccessIterator<B>
+for Map<'self, A, B, T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        self.iter.indexable()
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<B> {
+        self.do_map(self.iter.idx(index))
     }
 }
 
@@ -1069,6 +1128,21 @@ impl<A, T: Iterator<A>> Iterator<(uint, A)> for Enumerate<T> {
     }
 }
 
+impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<(uint, A)> for Enumerate<T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        self.iter.indexable()
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<(uint, A)> {
+        match self.iter.idx(index) {
+            Some(a) => Some((self.count + index, a)),
+            _ => None,
+        }
+    }
+}
+
 /// An iterator which rejects elements while `predicate` is true
 pub struct SkipWhile<'self, A, T> {
     priv iter: T,
@@ -1189,6 +1263,27 @@ impl<A, T: Iterator<A>> Iterator<A> for Skip<T> {
     }
 }
 
+impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Skip<T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        let N = self.iter.indexable();
+        if N < self.n {
+            0
+        } else {
+            N - self.n
+        }
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<A> {
+        if index >= self.indexable() {
+            None
+        } else {
+            self.iter.idx(index + self.n)
+        }
+    }
+}
+
 /// An iterator which only iterates over the first `n` iterations of `iter`.
 #[deriving(Clone)]
 pub struct Take<T> {
@@ -1223,6 +1318,23 @@ impl<A, T: Iterator<A>> Iterator<A> for Take<T> {
     }
 }
 
+impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Take<T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        cmp::min(self.iter.indexable(), self.n)
+    }
+
+    #[inline]
+    fn idx(&self, index: uint) -> Option<A> {
+        if index >= self.n {
+            None
+        } else {
+            self.iter.idx(index)
+        }
+    }
+}
+
+
 /// An iterator to maintain state while iterating another iterator
 pub struct Scan<'self, A, B, T, St> {
     priv iter: T,
@@ -1311,17 +1423,23 @@ pub struct Peek<'self, A, T> {
     priv f: &'self fn(&A)
 }
 
-impl<'self, A, T: Iterator<A>> Iterator<A> for Peek<'self, A, T> {
+impl<'self, A, T> Peek<'self, A, T> {
     #[inline]
-    fn next(&mut self) -> Option<A> {
-        let next = self.iter.next();
-
-        match next {
+    fn do_peek(&self, elt: Option<A>) -> Option<A> {
+        match elt {
             Some(ref a) => (self.f)(a),
             None => ()
         }
 
-        next
+        elt
+    }
+}
+
+impl<'self, A, T: Iterator<A>> Iterator<A> for Peek<'self, A, T> {
+    #[inline]
+    fn next(&mut self) -> Option<A> {
+        let next = self.iter.next();
+        self.do_peek(next)
     }
 
     #[inline]
@@ -1334,13 +1452,19 @@ impl<'self, A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A> for Peek<'self,
     #[inline]
     fn next_back(&mut self) -> Option<A> {
         let next = self.iter.next_back();
+        self.do_peek(next)
+    }
+}
 
-        match next {
-            Some(ref a) => (self.f)(a),
-            None => ()
-        }
+impl<'self, A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Peek<'self, A, T> {
+    #[inline]
+    fn indexable(&self) -> uint {
+        self.iter.indexable()
+    }
 
-        next
+    #[inline]
+    fn idx(&self, index: uint) -> Option<A> {
+        self.do_peek(self.iter.idx(index))
     }
 }