Implement a SparseMap data structure.
This implements the classic Briggs/Torczon sparse set construct. Adapt it to our existing EntityRef infrastructure so we can use types keys instead of just integers like the original paper does. Also provide a SparseSet<T> type alias which implements a sparse set of entity refeences. This is actually closer to what the original paper describes.
This commit is contained in:
Jakob Stoklund Olesen
@@ -14,6 +14,7 @@ pub mod isa;
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pub mod cfg;
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pub mod dominator_tree;
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pub mod entity_map;
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pub mod sparse_map;
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pub mod settings;
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pub mod verifier;
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pub mod regalloc;
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297
lib/cretonne/src/sparse_map.rs
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297
lib/cretonne/src/sparse_map.rs
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@@ -0,0 +1,297 @@
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//! Sparse mapping of entity references to larger value types.
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//!
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//! This module provides a `SparseMap` data structure which implements a sparse mapping from an
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//! `EntityRef` key to a value type that may be on the larger side. This implementation is based on
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//! the paper:
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//!
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//! > Briggs, Torczon, *An efficient representation for sparse sets*,
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//! ACM Letters on Programming Languages and Systems, Volume 2, Issue 1-4, March-Dec. 1993.
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//!
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//! A `SparseMap<K, V>` map provides:
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//!
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//! - Memory usage equivalent to `EntityMap<K, u32>` + `Vec<V>`, so much smaller than
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//! `EntityMap<K, V>` for sparse mappings of larger `V` types.
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//! - Constant time lookup, slightly slower than `EntityMap`.
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//! - A very fast, constant time `clear()` operation.
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//! - Fast insert and erase operations.
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//! - Stable iteration that is as fast as a `Vec<V>`.
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//!
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//! # Compared to `EntityMap`
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//!
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//! When should we use a `SparseMap` instead of a secondary `EntityMap`? First of all, `SparseMap`
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//! does not provide the functionality of a primary `EntityMap` which can allocate and assign
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//! entity references to objects as they are pushed onto the map. It is only the secondary
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//! entity maps that can be replaced with a `SparseMap`.
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//!
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//! - A secondary entity map requires its values to implement `Default`, and it is a bit loose
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//! about creating new mappings to the default value. It doesn't distinguish clearly between an
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//! unmapped key and one that maps to the default value. `SparseMap` does not require `Default`
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//! values, and it tracks accurately if a key has been mapped or not.
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//! - Iterating over the contants of an `EntityMap` is linear in the size of the *key space*, while
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//! iterating over a `SparseMap` is linear in the number of elements in the mapping. This is an
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//! advantage precisely when the mapping is sparse.
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//! - `SparseMap::clear()` is constant time and super-fast. `EntityMap::clear()` is linear in the
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//! size of the key space. (Or, rather the required `resize()` call following the `clear()` is).
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//! - `SparseMap` requires the values to implement `SparseMapValue<K>` which means that they must
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//! contain their own key.
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use entity_map::{EntityRef, EntityMap};
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use std::mem;
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use std::u32;
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/// Trait for extracting keys from values stored in a `SparseMap`.
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///
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/// All values stored in a `SparseMap` must keep track of their own key in the map and implement
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/// this trait to provide access to the key.
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pub trait SparseMapValue<K> {
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/// Get the key of this sparse map value. This key is not alowed to change while the value
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/// is a member of the map.
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fn key(&self) -> K;
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}
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/// A sparse mapping of entity references.
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pub struct SparseMap<K, V>
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where K: EntityRef,
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V: SparseMapValue<K>
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{
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sparse: EntityMap<K, u32>,
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dense: Vec<V>,
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}
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impl<K, V> SparseMap<K, V>
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where K: EntityRef,
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V: SparseMapValue<K>
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{
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/// Create a new empty mapping.
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pub fn new() -> Self {
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SparseMap {
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sparse: EntityMap::new(),
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dense: Vec::new(),
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}
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}
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/// Returns the number of elements in the map.
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pub fn len(&self) -> usize {
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self.dense.len()
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}
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/// Returns true is the map contains no elements.
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pub fn is_empty(&self) -> bool {
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self.dense.is_empty()
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}
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/// Returns a reference to the value corresponding to the key.
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pub fn get(&self, key: K) -> Option<&V> {
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if let Some(idx) = self.sparse.get(key).cloned() {
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if let Some(entry) = self.dense.get(idx as usize) {
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if entry.key() == key {
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return Some(entry);
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}
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}
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}
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None
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}
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/// Returns a mutable reference to the value corresponding to the key.
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///
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/// Note that the returned value must not be mutated in a way that would change its key. This
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/// would invalidate the sparse set data structure.
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pub fn get_mut(&mut self, key: K) -> Option<&mut V> {
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if let Some(idx) = self.sparse.get(key).cloned() {
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if let Some(entry) = self.dense.get_mut(idx as usize) {
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if entry.key() == key {
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return Some(entry);
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}
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}
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}
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None
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}
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/// Return the index into `dense` of the value corresponding to `key`.
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fn index(&self, key: K) -> Option<usize> {
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if let Some(idx) = self.sparse.get(key).cloned() {
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let idx = idx as usize;
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if let Some(entry) = self.dense.get(idx) {
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if entry.key() == key {
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return Some(idx);
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}
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}
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}
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None
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}
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/// Insert a value into the map.
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///
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/// If the map did not have this key present, `None` is returned.
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///
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/// If the map did have this key present, the value is updated, and the old value is returned.
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///
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/// It is not necessary to provide a key since the value knows its own key already.
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pub fn insert(&mut self, value: V) -> Option<V> {
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let key = value.key();
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// Replace the existing entry for `key` if there is one.
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if let Some(entry) = self.get_mut(key) {
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return Some(mem::replace(entry, value));
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}
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// There was no previous entry for `key`. Add it to the end of `dense`.
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let idx = self.dense.len();
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assert!(idx <= u32::MAX as usize, "SparseMap overflow");
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self.dense.push(value);
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*self.sparse.ensure(key) = idx as u32;
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None
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}
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/// Remove a value from the map and return it.
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pub fn remove(&mut self, key: K) -> Option<V> {
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if let Some(idx) = self.index(key) {
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let back = self.dense.pop().unwrap();
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// Are we popping the back of `dense`?
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if idx == self.dense.len() {
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return Some(back);
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}
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// We're removing an element from the middle of `dense`.
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// Replace the element at `idx` with the back of `dense`.
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// Repair `sparse` first.
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self.sparse[back.key()] = idx as u32;
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return Some(mem::replace(&mut self.dense[idx], back));
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}
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// Nothing to remove.
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None
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}
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}
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/// Any `EntityRef` can be used as a sparse map value representing itself.
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impl<T> SparseMapValue<T> for T
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where T: EntityRef
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{
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fn key(&self) -> T {
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*self
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}
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}
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/// A sparse set of entity references.
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///
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/// Any type that implements `EntityRef` can be used as a sparse set value too.
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pub type SparseSet<T> = SparseMap<T, T>;
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#[cfg(test)]
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mod tests {
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use super::*;
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use entity_map::EntityRef;
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use ir::Inst;
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// Mock key-value object for testing.
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#[derive(PartialEq, Eq, Debug)]
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struct Obj(Inst, &'static str);
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impl SparseMapValue<Inst> for Obj {
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fn key(&self) -> Inst {
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self.0
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}
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}
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#[test]
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fn empty_immutable_map() {
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let i1 = Inst::new(1);
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let map: SparseMap<Inst, Obj> = SparseMap::new();
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assert!(map.is_empty());
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assert_eq!(map.len(), 0);
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assert_eq!(map.get(i1), None);
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}
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#[test]
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fn single_entry() {
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let i0 = Inst::new(0);
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let i1 = Inst::new(1);
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let i2 = Inst::new(2);
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let mut map = SparseMap::new();
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assert!(map.is_empty());
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assert_eq!(map.len(), 0);
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assert_eq!(map.get(i1), None);
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assert_eq!(map.get_mut(i1), None);
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assert_eq!(map.remove(i1), None);
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assert_eq!(map.insert(Obj(i1, "hi")), None);
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assert!(!map.is_empty());
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assert_eq!(map.len(), 1);
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assert_eq!(map.get(i0), None);
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assert_eq!(map.get(i1), Some(&Obj(i1, "hi")));
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assert_eq!(map.get(i2), None);
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assert_eq!(map.get_mut(i0), None);
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assert_eq!(map.get_mut(i1), Some(&mut Obj(i1, "hi")));
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assert_eq!(map.get_mut(i2), None);
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assert_eq!(map.remove(i0), None);
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assert_eq!(map.remove(i2), None);
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assert_eq!(map.remove(i1), Some(Obj(i1, "hi")));
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assert_eq!(map.len(), 0);
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assert_eq!(map.get(i1), None);
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assert_eq!(map.get_mut(i1), None);
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assert_eq!(map.remove(i0), None);
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assert_eq!(map.remove(i1), None);
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assert_eq!(map.remove(i2), None);
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}
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#[test]
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fn multiple_entries() {
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let i0 = Inst::new(0);
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let i1 = Inst::new(1);
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let i2 = Inst::new(2);
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let i3 = Inst::new(3);
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let mut map = SparseMap::new();
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assert_eq!(map.insert(Obj(i2, "foo")), None);
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assert_eq!(map.insert(Obj(i1, "bar")), None);
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assert_eq!(map.insert(Obj(i0, "baz")), None);
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assert_eq!(map.len(), 3);
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assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
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assert_eq!(map.get(i1), Some(&Obj(i1, "bar")));
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assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
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assert_eq!(map.get(i3), None);
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// Remove front object, causing back to be swapped down.
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assert_eq!(map.remove(i1), Some(Obj(i1, "bar")));
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assert_eq!(map.len(), 2);
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assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
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assert_eq!(map.get(i1), None);
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assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
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assert_eq!(map.get(i3), None);
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// Reinsert something at a previously used key.
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assert_eq!(map.insert(Obj(i1, "barbar")), None);
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assert_eq!(map.len(), 3);
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assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
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assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
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assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
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assert_eq!(map.get(i3), None);
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// Replace an entry.
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assert_eq!(map.insert(Obj(i0, "bazbaz")), Some(Obj(i0, "baz")));
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assert_eq!(map.len(), 3);
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assert_eq!(map.get(i0), Some(&Obj(i0, "bazbaz")));
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assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
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assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
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assert_eq!(map.get(i3), None);
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}
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#[test]
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fn entity_set() {
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let i0 = Inst::new(0);
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let i1 = Inst::new(1);
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let mut set = SparseSet::new();
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assert_eq!(set.insert(i0), None);
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assert_eq!(set.insert(i0), Some(i0));
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assert_eq!(set.insert(i1), None);
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assert_eq!(set.get(i0), Some(&i0));
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assert_eq!(set.get(i1), Some(&i1));
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}
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}
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