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Make hashmap iterators implement ExactSize #19327

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Make hashmap iterators implement ExactSize #19327

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3669c57
Make hashmap iterators implement ExactSize
Nov 26, 2014
60c29c4
fixed reverse iteration logic
Nov 26, 2014
5cfaedd
fix the build
Nov 26, 2014
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29 changes: 28 additions & 1 deletion src/libstd/collections/hash/map.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,7 @@ use cmp::{max, Eq, Equiv, PartialEq};
use default::Default;
use fmt::{mod, Show};
use hash::{Hash, Hasher, RandomSipHasher};
use iter::{mod, Iterator, FromIterator, Extend};
use iter::{mod, Iterator, DoubleEndedIterator, ExactSize, FromIterator, Extend};
use kinds::Sized;
use mem::{mod, replace};
use num::UnsignedInt;
Expand Down Expand Up @@ -1325,6 +1325,15 @@ impl<'a, K, V> Iterator<(&'a K, &'a V)> for Entries<'a, K, V> {
}
}

impl<'a, K, V> DoubleEndedIterator<(&'a K, &'a V)> for Entries<'a, K, V> {
#[inline]
fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
self.inner.next_back()
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Hashmap is fundamentally unordered. Semantically, what does it even mean to iterate backwards? Couldn't this just call next and call it a day? Or am I missing a usecase?

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I think it'd be surprising if forward iteration and backward iteration returned the same series of elements. Anyhow, I only implemented DoubleEndedIterator because it's needed for ExactSize for some reason.

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Yes, I'm fine with impl'ing DoubleEnded and ExactSize for usability with that tooling. I don't however think people should be able to rely on the order of elements when you call next/next_back between iterations. I think it's fine to have next_back be a synonym for next here.

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I think that would go against the spirit of DoubleEndedIterator. We should at least be doing something "backwards-ish" or at least as backwards as makes sense for a hashtable. People might have a usecase for it and I feel like we shouldn't just lie in the interface.

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I can see the argument either way, honestly. Plus we can explicitly document that behaviour can't be relied on for different sequences of next/next_back. Which is backwards-compatible to "undo". I'm not a huge fan of duplicating a bunch on unsafe code in the already-beefy rawtable. Making next=next_back seems like a simple way to do that.

Interested in getting some other opinions though.

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Me too. I wish someone "owned" this code. :)

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That's probably @pczarn if anyone. Interested in @aturon's API-thoughts though.

}
}

impl<'a, K, V> ExactSize<(&'a K, &'a V)> for Entries<'a, K, V> {}

impl<'a, K, V> Iterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
#[inline]
fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
Expand All @@ -1336,6 +1345,15 @@ impl<'a, K, V> Iterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
}
}

impl<'a, K, V> DoubleEndedIterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
#[inline]
fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
self.inner.next_back()
}
}

impl<'a, K, V> ExactSize<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {}

impl<K, V> Iterator<(K, V)> for MoveEntries<K, V> {
#[inline]
fn next(&mut self) -> Option<(K, V)> {
Expand All @@ -1347,6 +1365,15 @@ impl<K, V> Iterator<(K, V)> for MoveEntries<K, V> {
}
}

impl<K, V> DoubleEndedIterator<(K, V)> for MoveEntries<K, V> {
#[inline]
fn next_back(&mut self) -> Option<(K, V)> {
self.inner.next_back()
}
}

impl<K, V> ExactSize<(K, V)> for MoveEntries<K, V> {}

impl<'a, K, V> OccupiedEntry<'a, K, V> {
/// Gets a reference to the value in the entry
pub fn get(&self) -> &V {
Expand Down
103 changes: 91 additions & 12 deletions src/libstd/collections/hash/table.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@ pub use self::BucketState::*;
use clone::Clone;
use cmp;
use hash::{Hash, Hasher};
use iter::{Iterator, count};
use iter::{Iterator, DoubleEndedIterator, ExactSize, count};
use kinds::{Sized, marker};
use mem::{min_align_of, size_of};
use mem;
Expand Down Expand Up @@ -620,6 +620,25 @@ impl<K, V> RawTable<K, V> {
}
}

fn one_past_last_bucket_raw(&self) -> RawBucket<K, V> {
let hashes_size = self.capacity * size_of::<u64>();
let keys_size = self.capacity * size_of::<K>();
let vals_size = self.capacity * size_of::<V>();

let buffer = self.hashes as *mut u8;
let (keys_offset, vals_offset) = calculate_offsets(hashes_size,
keys_size, min_align_of::<K>(),
min_align_of::<V>());

unsafe {
RawBucket {
hash: self.hashes.offset(hashes_size as int),
key: buffer.offset(keys_offset as int + keys_size as int) as *mut K,
val: buffer.offset(vals_offset as int + vals_size as int) as *mut V
}
}
}

/// Creates a new raw table from a given capacity. All buckets are
/// initially empty.
#[allow(experimental)]
Expand All @@ -644,10 +663,8 @@ impl<K, V> RawTable<K, V> {

fn raw_buckets(&self) -> RawBuckets<K, V> {
RawBuckets {
raw: self.first_bucket_raw(),
hashes_end: unsafe {
self.hashes.offset(self.capacity as int)
},
start: self.first_bucket_raw(),
end: self.one_past_last_bucket_raw(),
marker: marker::ContravariantLifetime,
}
}
Expand All @@ -667,12 +684,12 @@ impl<K, V> RawTable<K, V> {
}

pub fn into_iter(self) -> MoveEntries<K, V> {
let RawBuckets { raw, hashes_end, .. } = self.raw_buckets();
let RawBuckets { start, end, .. } = self.raw_buckets();
// Replace the marker regardless of lifetime bounds on parameters.
MoveEntries {
iter: RawBuckets {
raw: raw,
hashes_end: hashes_end,
start: start,
end: end,
marker: marker::ContravariantLifetime,
},
table: self,
Expand All @@ -695,18 +712,18 @@ impl<K, V> RawTable<K, V> {
/// A raw iterator. The basis for some other iterators in this module. Although
/// this interface is safe, it's not used outside this module.
struct RawBuckets<'a, K, V> {
raw: RawBucket<K, V>,
hashes_end: *mut u64,
start: RawBucket<K, V>,
end: RawBucket<K, V>, // points one after the end.
marker: marker::ContravariantLifetime<'a>,
}

impl<'a, K, V> Iterator<RawBucket<K, V>> for RawBuckets<'a, K, V> {
fn next(&mut self) -> Option<RawBucket<K, V>> {
while self.raw.hash != self.hashes_end {
while self.start.hash != self.end.hash {
unsafe {
// We are swapping out the pointer to a bucket and replacing
// it with the pointer to the next one.
let prev = ptr::replace(&mut self.raw, self.raw.offset(1));
let prev = ptr::replace(&mut self.start, self.start.offset(1));
if *prev.hash != EMPTY_BUCKET {
return Some(prev);
}
Expand All @@ -717,6 +734,21 @@ impl<'a, K, V> Iterator<RawBucket<K, V>> for RawBuckets<'a, K, V> {
}
}

impl<'a, K, V> DoubleEndedIterator<RawBucket<K, V>> for RawBuckets<'a, K, V> {
fn next_back(&mut self) -> Option<RawBucket<K, V>> {
while self.start.hash != self.end.hash {
unsafe {
let next = ptr::replace(&mut self.end, self.end.offset(-1));
if *next.hash != EMPTY_BUCKET {
return Some(next);
}
}
}

None
}
}

/// An iterator that moves out buckets in reverse order. It leaves the table
/// in an inconsistent state and should only be used for dropping
/// the table's remaining entries. It's used in the implementation of Drop.
Expand Down Expand Up @@ -785,6 +817,20 @@ impl<'a, K, V> Iterator<(&'a K, &'a V)> for Entries<'a, K, V> {
}
}

impl<'a, K, V> DoubleEndedIterator<(&'a K, &'a V)> for Entries<'a, K, V> {
fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
self.iter.next_back().map(|bucket| {
self.elems_left += 1;
unsafe {
(&*bucket.key,
&*bucket.val)
}
})
}
}

impl<'a, K, V> ExactSize<(&'a K, &'a V)> for Entries<'a, K, V> {}

impl<'a, K, V> Iterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
self.iter.next().map(|bucket| {
Expand All @@ -801,6 +847,20 @@ impl<'a, K, V> Iterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
}
}

impl<'a, K, V> DoubleEndedIterator<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {
fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
self.iter.next().map(|bucket| {
self.elems_left += 1;
unsafe {
(&*bucket.key,
&mut *bucket.val)
}
})
}
}

impl<'a, K, V> ExactSize<(&'a K, &'a mut V)> for MutEntries<'a, K, V> {}

impl<K, V> Iterator<(SafeHash, K, V)> for MoveEntries<K, V> {
fn next(&mut self) -> Option<(SafeHash, K, V)> {
self.iter.next().map(|bucket| {
Expand All @@ -823,6 +883,25 @@ impl<K, V> Iterator<(SafeHash, K, V)> for MoveEntries<K, V> {
}
}

impl<K, V> DoubleEndedIterator<(SafeHash, K, V)> for MoveEntries<K, V> {
fn next_back(&mut self) -> Option<(SafeHash, K, V)> {
self.iter.next().map(|bucket| {
self.table.size += 1;
unsafe {
(
SafeHash {
hash: *bucket.hash,
},
ptr::read(bucket.key as *const K),
ptr::read(bucket.val as *const V)
)
}
})
}
}

impl<K, V> ExactSize<(SafeHash, K, V)> for MoveEntries<K, V> {}

impl<K: Clone, V: Clone> Clone for RawTable<K, V> {
fn clone(&self) -> RawTable<K, V> {
unsafe {
Expand Down