From 24b61f51fe123dc687f3eb31da18d2b5e7c1b9e1 Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Mon, 21 Mar 2022 20:33:48 +0000
Subject: [PATCH 1/4] Implement more of the standard rust API
---
agb/src/hash_map.rs | 63 +++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 61 insertions(+), 2 deletions(-)
diff --git a/agb/src/hash_map.rs b/agb/src/hash_map.rs
index c6f9afe8..0e6baed8 100644
--- a/agb/src/hash_map.rs
+++ b/agb/src/hash_map.rs
@@ -36,6 +36,40 @@ impl<K, V> HashMap<K, V> {
self.nodes.len()
}
+ pub fn capacity(&self) -> usize {
+ self.nodes.capacity()
+ }
+
+ pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
+ self.iter().map(|(k, _)| k)
+ }
+
+ pub fn values(&self) -> impl Iterator<Item = &'_ V> {
+ self.iter().map(|(_, v)| v)
+ }
+
+ pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
+ self.iter_mut().map(|(_, v)| v)
+ }
+
+ pub fn clear(&mut self) {
+ self.nodes = NodeStorage::with_size(self.capacity());
+ }
+
+ pub fn iter(&self) -> impl Iterator<Item = (&'_ K, &'_ V)> {
+ self.nodes
+ .nodes
+ .iter()
+ .filter_map(|node| node.key_value_ref())
+ }
+
+ pub fn iter_mut(&mut self) -> impl Iterator<Item = (&'_ K, &'_ mut V)> {
+ self.nodes
+ .nodes
+ .iter_mut()
+ .filter_map(|node| node.key_value_mut())
+ }
+
pub fn is_empty(&self) -> bool {
self.len() == 0
}
@@ -85,12 +119,21 @@ where
self.nodes.nodes[location].value_mut().unwrap()
}
- pub fn get(&self, key: &K) -> Option<&V> {
+ pub fn contains_key(&self, k: &K) -> bool {
+ let hash = self.hash(k);
+ self.nodes.get_location(k, hash).is_some()
+ }
+
+ pub fn get_key_value(&self, key: &K) -> Option<(&K, &V)> {
let hash = self.hash(key);
self.nodes
.get_location(key, hash)
- .and_then(|location| self.nodes.nodes[location].value_ref())
+ .and_then(|location| self.nodes.nodes[location].key_value_ref())
+ }
+
+ pub fn get(&self, key: &K) -> Option<&V> {
+ self.get_key_value(key).map(|(_, v)| v)
}
pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
@@ -533,6 +576,22 @@ impl<K, V> Node<K, V> {
}
}
+ fn key_value_ref(&self) -> Option<(&K, &V)> {
+ if self.has_value() {
+ Some(unsafe { (self.key.assume_init_ref(), self.value.assume_init_ref()) })
+ } else {
+ None
+ }
+ }
+
+ fn key_value_mut(&mut self) -> Option<(&K, &mut V)> {
+ if self.has_value() {
+ Some(unsafe { (self.key.assume_init_ref(), self.value.assume_init_mut()) })
+ } else {
+ None
+ }
+ }
+
fn has_value(&self) -> bool {
self.distance_to_initial_bucket >= 0
}
From c7db20c1df81c1dfd711a2f467699d98db6014ac Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Mon, 21 Mar 2022 20:42:07 +0000
Subject: [PATCH 2/4] Implement insert correctly
---
agb/src/hash_map.rs | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)
diff --git a/agb/src/hash_map.rs b/agb/src/hash_map.rs
index 0e6baed8..8084a7cc 100644
--- a/agb/src/hash_map.rs
+++ b/agb/src/hash_map.rs
@@ -102,7 +102,23 @@ impl<K, V> HashMap<K, V>
where
K: Eq + Hash,
{
- pub fn insert(&mut self, key: K, value: V) -> &mut V {
+ pub fn insert(&mut self, key: K, value: V) -> Option<V> {
+ let hash = self.hash(&key);
+
+ if let Some(location) = self.nodes.get_location(&key, hash) {
+ Some(self.nodes.replace_at_location(location, key, value))
+ } else {
+ if self.nodes.capacity() * 85 / 100 <= self.len() {
+ self.resize(self.nodes.capacity() * 2);
+ }
+
+ self.nodes.insert_new(key, value, hash);
+
+ None
+ }
+ }
+
+ fn insert_and_get(&mut self, key: K, value: V) -> &'_ mut V {
let hash = self.hash(&key);
let location = if let Some(location) = self.nodes.get_location(&key, hash) {
@@ -254,7 +270,7 @@ impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
where
K: Hash + Eq,
{
- self.map.insert(self.key, value)
+ self.map.insert_and_get(self.key, value)
}
}
From 447554c295ca783bb67f6c5a8b5a0d3435a72fdb Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Mon, 21 Mar 2022 21:16:36 +0000
Subject: [PATCH 3/4] Add loads of doc comments
---
agb/src/hash_map.rs | 172 +++++++++++++++++++++++++++++++++++++++++---
1 file changed, 162 insertions(+), 10 deletions(-)
diff --git a/agb/src/hash_map.rs b/agb/src/hash_map.rs
index 8084a7cc..985f9422 100644
--- a/agb/src/hash_map.rs
+++ b/agb/src/hash_map.rs
@@ -11,51 +11,141 @@ use rustc_hash::FxHasher;
type HashType = u32;
-pub struct HashMap<K, V, H = BuildHasherDefault<FxHasher>>
-where
- H: BuildHasher,
-{
+// # Robin Hood Hash Tables
+//
+// The problem with regular hash tables where failing to find a slot for a specific
+// key will result in a linear search for the first free slot is that often these
+// slots can end up being quite far away from the original chosen location in fuller
+// hash tables. In Java, the hash table will resize when it is more than 2 thirds full
+// which is quite wasteful in terms of space. Robin Hood hash tables can be much
+// fuller before needing to resize and also keeps search times lower.
+//
+// The key concept is to keep the distance from the initial bucket chosen for a given
+// key to a minimum. We shall call this distance the "distance to the initial bucket"
+// or DIB for short. With each key - value pair, we store its DIB. When inserting
+// a value into the hashtable, we check to see if there is an element in the initial
+// bucket. If there is, we move onto the next value. Then, we check to see if there
+// is already a value there and if there is, we check its DIB. If our DIB is greater
+// than or equal to the DIB of the value that is already there, we swap the working
+// value and the current entry. This continues until an empty slot is found.
+//
+// Using this technique, the average DIB is kept fairly low which decreases search
+// times. As a simple search time optimisation, the maximum DIB is kept track of
+// and so we will only need to search as far as that in order to know whether or
+// not a given element is in the hash table.
+//
+// # Deletion
+//
+// Special mention is given to deletion. Unfortunately, the maximum DIB is not
+// kept track of after deletion, since we would not only need to keep track of
+// the maximum DIB but also the number of elements which have that maximum DIB.
+//
+// In order to delete an element, we search to see if it exists. If it does,
+// we remove that element and then iterate through the array from that point
+// and move each element back one space (updating its DIB). If the DIB of the
+// element we are trying to remove is 0, then we stop this algorithm.
+//
+// This means that deletion will lower the average DIB of the elements and
+// keep searching and insertion fast.
+//
+// # Rehashing
+//
+// Currently, no incremental rehashing takes place. Once the HashMap becomes
+// more than 85% full (this value may change when I do some benchmarking),
+// a new list is allocated with double the capacity and the entire node list
+// is migrated.
+
+/// A hash map implemented very simply using robin hood hashing.
+///
+/// HashMap uses FxHasher internally, which is a very fast hashing algorithm used
+/// by rustc and firefox in non-adversarial places. It is incredibly fast, and good
+/// enough for most cases.
+///
+/// It is required that the keys implement the [`Eq`] and [`Hash`] traits, although this
+/// can be frequently achieved by using `#[derive(PartialEq, Eq, Hash)]`. If you
+/// implement these yourself, it is important that the following property holds:
+///
+/// `k1 == k2 => hash(k1) == hash(k2)`
+///
+/// It is a logic error for the key to be modified in such a way that the key's hash, as
+/// determined by the [`Hash`] trait, or its equality as determined by the [`Eq`] trait,
+/// changes while it is in the map. The behaviour for such a logic error is not specified,
+/// but will not result in undefined behaviour. This could include panics, incorrect results,
+/// aborts, memory leaks and non-termination.
+///
+/// The API surface provided is incredibly similar to the
+/// [std::collections::HashMap](https://doc.rust-lang.org/std/collections/struct.HashMap.html)
+/// implementation with fewer guarantees, and better optimised for the GameBoy Advance.
+///
+/// [`Eq`]: https://doc.rust-lang.org/core/cmp/trait.Eq.html
+/// [`Hash`]: https://doc.rust-lang.org/core/hash/trait.Hash.html
+pub struct HashMap<K, V> {
nodes: NodeStorage<K, V>,
- hasher: H,
+ hasher: BuildHasherDefault<FxHasher>,
}
impl<K, V> HashMap<K, V> {
+ /// Creates a `HashMap`
pub fn new() -> Self {
- Self::with_capacity(16)
+ Self::with_size(16)
}
- pub fn with_capacity(capacity: usize) -> Self {
+ /// Creates an empty `HashMap` with specified internal size. The size must be a power of 2
+ pub fn with_size(size: usize) -> Self {
Self {
- nodes: NodeStorage::with_size(capacity),
+ nodes: NodeStorage::with_size(size),
hasher: Default::default(),
}
}
+ /// Creates an empty `HashMap` which can hold at least `capacity` elements before resizing. The actual
+ /// internal size may be larger as it must be a power of 2
+ pub fn with_capacity(capacity: usize) -> Self {
+ for i in 0..32 {
+ let attempted_size = 1usize << i;
+ if attempted_size * 85 / 100 > capacity {
+ return Self::with_size(attempted_size);
+ }
+ }
+
+ panic!(
+ "Failed to come up with a size which satisfies capacity {}",
+ capacity
+ );
+ }
+
+ /// Returns the number of elements in the map
pub fn len(&self) -> usize {
self.nodes.len()
}
+ /// Returns the number of elements the map can hold
pub fn capacity(&self) -> usize {
- self.nodes.capacity()
+ self.nodes.capacity() * 85 / 100
}
+ /// An iterator visiting all keys in an arbitrary order
pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
self.iter().map(|(k, _)| k)
}
+ /// An iterator visiting all values in an arbitrary order
pub fn values(&self) -> impl Iterator<Item = &'_ V> {
self.iter().map(|(_, v)| v)
}
+ /// An iterator visiting all values in an arbitrary order allowing for mutation
pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
self.iter_mut().map(|(_, v)| v)
}
+ /// Removes all elements from the map
pub fn clear(&mut self) {
self.nodes = NodeStorage::with_size(self.capacity());
}
+ /// An iterator visiting all key-value pairs in an arbitrary order
pub fn iter(&self) -> impl Iterator<Item = (&'_ K, &'_ V)> {
self.nodes
.nodes
@@ -63,6 +153,7 @@ impl<K, V> HashMap<K, V> {
.filter_map(|node| node.key_value_ref())
}
+ /// An iterator visiting all key-value pairs in an arbitrary order, with mutable references to the values
pub fn iter_mut(&mut self) -> impl Iterator<Item = (&'_ K, &'_ mut V)> {
self.nodes
.nodes
@@ -70,11 +161,12 @@ impl<K, V> HashMap<K, V> {
.filter_map(|node| node.key_value_mut())
}
+ /// Returns `true` if the map contains no elements
pub fn is_empty(&self) -> bool {
self.len() == 0
}
- pub fn resize(&mut self, new_size: usize) {
+ fn resize(&mut self, new_size: usize) {
assert!(
new_size >= self.nodes.capacity(),
"Can only increase the size of a hash map"
@@ -102,6 +194,13 @@ impl<K, V> HashMap<K, V>
where
K: Eq + Hash,
{
+ /// Inserts a key-value pair into the map.
+ ///
+ /// If the map did not have this key present, [`None`] is returned.
+ ///
+ /// If the map did have this key present, the value is updated and the old value
+ /// is returned. The key is not updated, which matters for types that can be `==`
+ /// without being identical.
pub fn insert(&mut self, key: K, value: V) -> Option<V> {
let hash = self.hash(&key);
@@ -135,11 +234,13 @@ where
self.nodes.nodes[location].value_mut().unwrap()
}
+ /// Returns `true` if the map contains a value for the specified key.
pub fn contains_key(&self, k: &K) -> bool {
let hash = self.hash(k);
self.nodes.get_location(k, hash).is_some()
}
+ /// Returns the key-value pair corresponding to the supplied key
pub fn get_key_value(&self, key: &K) -> Option<(&K, &V)> {
let hash = self.hash(key);
@@ -148,10 +249,14 @@ where
.and_then(|location| self.nodes.nodes[location].key_value_ref())
}
+ /// Returns a reference to the value corresponding to the key. Returns [`None`] if there is
+ /// no element in the map with the given key.
pub fn get(&self, key: &K) -> Option<&V> {
self.get_key_value(key).map(|(_, v)| v)
}
+ /// Returns a mutable reference to the value corresponding to the key. Return [`None`] if
+ /// there is no element in the map with the given key.
pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
let hash = self.hash(key);
@@ -162,6 +267,8 @@ where
}
}
+ /// Removes the given key from the map. Returns the current value if it existed, or [`None`]
+ /// if it did not.
pub fn remove(&mut self, key: &K) -> Option<V> {
let hash = self.hash(key);
@@ -215,6 +322,7 @@ impl<'a, K, V> IntoIterator for &'a HashMap<K, V> {
}
}
+/// A view into an occupied entry in a `HashMap`. This is part of the [`Entry`] enum.
pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
key: K,
map: &'a mut HashMap<K, V>,
@@ -222,50 +330,71 @@ pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
}
impl<'a, K: 'a, V: 'a> OccupiedEntry<'a, K, V> {
+ /// Gets a reference to the key in the entry.
pub fn key(&self) -> &K {
&self.key
}
+ /// Take the ownership of the key and value from the map.
pub fn remove_entry(self) -> (K, V) {
let old_value = self.map.nodes.remove_from_location(self.location);
(self.key, old_value)
}
+ /// Gets a reference to the value in the entry.
pub fn get(&self) -> &V {
self.map.nodes.nodes[self.location].value_ref().unwrap()
}
+ /// Gets a mutable reference to the value in the entry.
+ ///
+ /// If you need a reference to the `OccupiedEntry` which may outlive the destruction
+ /// of the `Entry` value, see [`into_mut`].
+ ///
+ /// [`into_mut`]: Self::into_mut
pub fn get_mut(&mut self) -> &mut V {
self.map.nodes.nodes[self.location].value_mut().unwrap()
}
+ /// Converts the `OccupiedEntry` into a mutable reference to the value in the entry with
+ /// a lifetime bound to the map itself.
+ ///
+ /// If you need multiple references to the `OccupiedEntry`, see [`get_mut`].
+ ///
+ /// [`get_mut`]: Self::get_mut
pub fn into_mut(self) -> &'a mut V {
self.map.nodes.nodes[self.location].value_mut().unwrap()
}
+ /// Sets the value of the entry and returns the entry's old value.
pub fn insert(&mut self, value: V) -> V {
self.map.nodes.nodes[self.location].replace_value(value)
}
+ /// Takes the value out of the entry and returns it.
pub fn remove(self) -> V {
self.map.nodes.remove_from_location(self.location)
}
}
+/// A view into a vacant entry in a `HashMap`. It is part of the [`Entry`] enum.
pub struct VacantEntry<'a, K: 'a, V: 'a> {
key: K,
map: &'a mut HashMap<K, V>,
}
impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
+ /// Gets a reference to the key that would be used when inserting a value through `VacantEntry`
pub fn key(&self) -> &K {
&self.key
}
+ /// Take ownership of the key
pub fn into_key(self) -> K {
self.key
}
+ /// Sets the value of the entry with the `VacantEntry`'s key and returns a mutable reference to it.
pub fn insert(self, value: V) -> &'a mut V
where
K: Hash + Eq,
@@ -274,8 +403,15 @@ impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
}
}
+/// A view into a single entry in a map, which may be vacant or occupied.
+///
+/// This is constructed using the [`entry`] method on [`HashMap`]
+///
+/// [`entry`]: HashMap::entry()
pub enum Entry<'a, K: 'a, V: 'a> {
+ /// An occupied entry
Occupied(OccupiedEntry<'a, K, V>),
+ /// A vacant entry
Vacant(VacantEntry<'a, K, V>),
}
@@ -283,6 +419,8 @@ impl<'a, K, V> Entry<'a, K, V>
where
K: Hash + Eq,
{
+ /// Ensures a value is in the entry by inserting the given value, and returns a mutable
+ /// reference to the value in the entry.
pub fn or_insert(self, value: V) -> &'a mut V {
match self {
Entry::Occupied(e) => e.into_mut(),
@@ -290,6 +428,8 @@ where
}
}
+ /// Ensures a value is in the entry by inserting the result of the function if empty, and
+ /// returns a mutable reference to the value in the entry.
pub fn or_insert_with<F>(self, f: F) -> &'a mut V
where
F: FnOnce() -> V,
@@ -300,6 +440,12 @@ where
}
}
+ /// Ensures a value is in the entry by inserting the result of the function if empty, and
+ /// returns a mutable reference to the value in the entry. This method allows for key-derived
+ /// values for insertion by providing the function with a reference to the key.
+ ///
+ /// The reference to the moved key is provided so that cloning or copying the key is unnecessary,
+ /// unlike with `.or_insert_with(|| ...)`.
pub fn or_insert_with_key<F>(self, f: F) -> &'a mut V
where
F: FnOnce(&K) -> V,
@@ -313,6 +459,8 @@ where
}
}
+ /// Provides in-place mutable access to an occupied entry before any potential inserts
+ /// into the map.
pub fn and_modify<F>(self, f: F) -> Self
where
F: FnOnce(&mut V),
@@ -326,6 +474,8 @@ where
}
}
+ /// Ensures a value is in th entry by inserting the default value if empty. Returns a
+ /// mutable reference to the value in the entry.
pub fn or_default(self) -> &'a mut V
where
V: Default,
@@ -336,6 +486,7 @@ where
}
}
+ /// Returns a reference to this entry's key.
pub fn key(&self) -> &K {
match self {
Entry::Occupied(e) => &e.key,
@@ -348,6 +499,7 @@ impl<'a, K, V> HashMap<K, V>
where
K: Hash + Eq,
{
+ /// Gets the given key's corresponding entry in the map for in-place manipulation.
pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
let hash = self.hash(&key);
let location = self.nodes.get_location(&key, hash);
From e6fb67503b12adb0a158235619492612dc63176b Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Mon, 21 Mar 2022 21:18:56 +0000
Subject: [PATCH 4/4] Add a comment explaining where the docs came from
---
agb/src/hash_map.rs | 2 ++
agb/src/lib.rs | 2 +-
2 files changed, 3 insertions(+), 1 deletion(-)
diff --git a/agb/src/hash_map.rs b/agb/src/hash_map.rs
index 985f9422..a5183c72 100644
--- a/agb/src/hash_map.rs
+++ b/agb/src/hash_map.rs
@@ -1,3 +1,5 @@
+//! A lot of the documentation for this module was copied straight out of the rust
+//! standard library. The implementation however is not.
use alloc::vec::Vec;
use core::{
hash::{BuildHasher, BuildHasherDefault, Hash, Hasher},
diff --git a/agb/src/lib.rs b/agb/src/lib.rs
index efc06cbf..2b947a47 100644
--- a/agb/src/lib.rs
+++ b/agb/src/lib.rs
@@ -152,7 +152,7 @@ mod memory_mapped;
pub mod mgba;
/// Implementation of fixnums for working with non-integer values.
pub use agb_fixnum as fixnum;
-/// Contains an implementation of a hashmap which suits the gameboy advance's hardware
+/// Contains an implementation of a hashmap which suits the gameboy advance's hardware.
pub mod hash_map;
mod single;
/// Implements sound output.