feat(collections): add SparseSet — bounded-universe sparse integer set with O(1) clear & dense iteration

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to Celerity are documented here. This project follows [Keep
 ### Added
 
 - **`Trie<TValue>`** in `Celerity.Collections` — an ordered prefix tree mapping `string` keys to values, filling a BCL gap (.NET ships no trie). `GetByPrefix` lists every entry whose key starts with a prefix in `O(prefix + matches)` and in ascending key order, and `TryGetLongestPrefix` finds the longest stored key that is a prefix of a query in `O(query)` — the autocomplete, longest-prefix-routing, and ordered-iteration workloads a `Dictionary<string, TValue>` can only answer with an `O(n)` scan plus a `StartsWith` per key. Exact `Add` / `TryGetValue` favour a `Dictionary`, so the trie earns its place on the prefix operations. Implements `IReadOnlyDictionary<string, TValue?>`; not thread-safe. Closes [#285](https://github.com/marius-bughiu/Celerity/issues/285).
+- **`SparseSet`** in `Celerity.Collections` — a bounded-universe `[0, Universe)` integer set (the Briggs–Torczon sparse set), filling a BCL gap. Where the set is cleared and rebuilt often — "visited" sets in graph traversal, ECS, sweep-line — it beats `HashSet<int>` with an `O(1)` `Clear` (the backing arrays are left untouched) and dense iteration over just the present elements. Costs `O(Universe)` memory, stores only values in `[0, Universe)`, and implements `ISet<int>`; an opt-in specialized type. Closes [#287](https://github.com/marius-bughiu/Celerity/issues/287).
 
 ### Fixed
 

README.md

@@ -62,8 +62,9 @@ Standalone libraries built **on top of** Celerity — each solves a real problem
 - `IntSet` / `LongSet` — `int` / `long`-keyed set specializations.
 - `SmallSet<T>` — flat-array, linear-scan set for the very-small (`n <= ~16`) case. No hasher; the default element is stored inline. The set counterpart of `SmallDictionary`.
 - `EnumSet<TEnum>` — bit-vector set for enum keys (the .NET `EnumSet`): membership is a single bit test and set algebra is word-wise bitwise ops, with no hashing or boxing. Enumerates in ascending underlying-value order.
+- `SparseSet` — bounded-universe integer set (Briggs–Torczon sparse set): `O(1)` `Clear` that leaves the backing arrays untouched, plus dense, cache-friendly iteration — for clear-and-rebuild "visited" sets over ids in `[0, N)` (graph traversal, ECS, sweep-line). Costs `O(Universe)` memory.
 
-The mutable sets (`CeleritySet`, `SwissSet`, `RobinHoodSet`, `HashCachingSet`, `IntSet`, `LongSet`, `SmallSet`, `EnumSet`) all implement **`ISet<T>`** — the full `HashSet<T>` set-algebra surface (`UnionWith` / `IntersectWith` / `ExceptWith` / `SymmetricExceptWith` and the `IsSubsetOf` / `IsSupersetOf` / `Overlaps` / `SetEquals` query family, plus `CopyTo`) with BCL semantics — so they drop in wherever a `HashSet<T>` is used.
+The mutable sets (`CeleritySet`, `SwissSet`, `RobinHoodSet`, `HashCachingSet`, `IntSet`, `LongSet`, `SmallSet`, `EnumSet`, `SparseSet`) all implement **`ISet<T>`** — the full `HashSet<T>` set-algebra surface (`UnionWith` / `IntersectWith` / `ExceptWith` / `SymmetricExceptWith` and the `IsSubsetOf` / `IsSupersetOf` / `Overlaps` / `SetEquals` query family, plus `CopyTo`) with BCL semantics — so they drop in wherever a `HashSet<T>` is used. (The bounded-domain sets, `EnumSet` and `SparseSet`, are the exception to "drop in anywhere": they store only values in their fixed domain, so a mutating op that must add an out-of-domain value throws.)
 
 **Caches**
 
@@ -222,7 +223,7 @@ Console.WriteLine(queued.ContainsKey(Priority.Normal)); // False — a single bi
 </details>
 
 <details>
-<summary><b>Sets</b> — IntSet, CeleritySet, SwissSet, RobinHoodSet, HashCachingSet, FrozenCeleritySet, SmallSet, EnumSet</summary>
+<summary><b>Sets</b> — IntSet, CeleritySet, SwissSet, RobinHoodSet, HashCachingSet, FrozenCeleritySet, SmallSet, EnumSet, SparseSet</summary>
 
 ```csharp
 var seen = new IntSet();
@@ -294,6 +295,15 @@ Console.WriteLine(granted.IsSupersetOf(required)); // False — word-wise subset
 granted.UnionWith(required);                        // one bitwise OR
 ```
 
+`SparseSet` is the bounded-universe integer set — the classic Briggs–Torczon sparse set (a dense value array + a sparse index array). Over a fixed universe `[0, Universe)` chosen at construction, `Add` / `Contains` / `Remove` are `O(1)` with no hashing, but the point of the type is what `HashSet<int>` can't match: `Clear()` is `O(1)` (it resets the count without scanning or clearing the backing arrays, versus zeroing the whole table) and iteration is a dense, contiguous scan over exactly the present elements. That is the winning shape for clear-and-rebuild "visited" sets — graph BFS/DFS, ECS entity membership, sweep-line — where the set is emptied every iteration. The cost is `O(Universe)` memory and non-negative-values-only: a value outside `[0, Universe)` throws on `Add` and reads as absent on `Contains` / `Remove`. It is an opt-in specialized type, not a `HashSet<int>` replacement — for an unbounded or huge-and-sparse key space, reach for `IntSet`.
+
+```csharp
+var visited = new SparseSet(nodeCount);   // universe = ids in [0, nodeCount)
+visited.Add(start);
+Console.WriteLine(visited.TryAdd(start)); // False — already seen, unchanged
+visited.Clear();                          // O(1) — ready for the next traversal
+```
+
 </details>
 
 <details>
@@ -495,6 +505,7 @@ Each type buys a different tradeoff. Find your workload below; if it isn't here,
 | Dictionary keyed by a small **enum** — config-by-enum, per-state data, enum→handler tables | `EnumMap<TEnum, TValue>` | Dense array indexed on the enum's underlying value (the .NET `EnumMap`): `this[key]` / `TryGetValue` / `Add` / `Remove` are a single direct array index — no hashing, no probing, no collisions — and a full sweep is a linear array walk. The dictionary counterpart of `EnumSet`; enumerates ascending by value. For enums whose members are small non-negative integers (the default); negative or sparse `[Flags]` enums are unsupported — use `CelerityDictionary<TEnum, TValue, THasher>` there. |
 | Tiny set (`n <= ~16`) that stays small — per-scope "seen" sets, small membership guards, deduping a handful of items | `SmallSet<T>` | The set counterpart of `SmallDictionary`: flat-array linear scan beats hashing at small `n`, no hasher to pick, the default element is stored inline. Implements `ISet<T>`. Degrades to `O(n)` for large sets, so only when instances stay small. |
 | Set of **enum** values — flag sets, permission sets, state sets over a small enum | `EnumSet<TEnum>` | Bit-vector set indexed on the enum's underlying value (the .NET `EnumSet`): `Add` / `Contains` / `Remove` are a single bit op — no hashing, no boxing — and set algebra between two `EnumSet`s is a word-wise bitwise `OR` / `AND` / `XOR`. Enumerates ascending by value; `All()` builds the full universe. For enums whose members are small non-negative integers (the default); negative or sparse `[Flags]` enums are unsupported — use `CeleritySet<TEnum, THasher>` there. |
+| Set of small **non-negative ints** over a bounded range that is **cleared & rebuilt often** — "visited" sets in graph BFS/DFS, ECS entity membership, sweep-line | `SparseSet` | Briggs–Torczon sparse set (dense value array + sparse index array): `O(1)` `Clear` that leaves the backing arrays untouched (vs `HashSet<int>` zeroing its table) and dense, cache-friendly iteration over just the present elements. `Add` / `Contains` / `Remove` are `O(1)`, no hashing. Costs `O(Universe)` memory and stores only values in `[0, Universe)`; for an unbounded or huge-and-sparse key space use `IntSet` / `HashSet<int>`. |
 | Set of `int` values | `IntSet` | Same fast path as `IntDictionary`, membership only. |
 | Set of `long` values | `LongSet` | 64-bit equivalent of `IntSet`; defaults to `Int64WangNaiveHasher`. |
 | Set of any other type | `CeleritySet<T, THasher>` | Same hasher choice as `CelerityDictionary`. |

docs/api/collections.md

@@ -1971,6 +1971,134 @@ foreach (var p in granted) { /* ascending: Read, Write, Execute */ }
 
 ---
 
+## SparseSet
+
+```csharp
+public class SparseSet : ISet<int>
+```
+
+A set of **non-negative integers over a bounded universe** `[0, Universe)`, backed by
+the classic **Briggs–Torczon sparse-set representation**: a *dense* array holding the
+present values contiguously, paired with a *sparse* array — indexed by value — that
+points each present value back at its slot in the dense array. Membership is the
+round-trip `sparse[v] < Count && dense[sparse[v]] == v`, which is correct even for a *stale*
+sparse entry — one left over from before a `Clear`, or the zero a never-written slot still
+holds. That single fact is what buys the type its two wins over `HashSet<int>`:
+
+- **`Clear()` is `O(1)`** — it resets the count *without scanning or clearing the
+  backing arrays*. `HashSet<int>.Clear()` is `O(capacity)` (it zeroes the whole entry table).
+  This is the headline: per-frame / per-query "visited" sets in graph traversal (BFS/DFS), ECS
+  entity membership, register-allocation liveness, and sweep-line algorithms clear on every
+  iteration.
+- **Dense, cache-friendly iteration** — present elements live contiguously in `[0, Count)`
+  of the dense array, so enumeration is a linear scan over exactly `Count` ints with no
+  empty-slot skipping.
+
+`Add` / `Contains` / `Remove` are each `O(1)` with **no hashing**, no probe chain, and no
+per-element allocation — a direct array index and the round-trip check. There is **no
+hasher** (and so no `THasher` type parameter).
+
+The trade-offs, stated honestly:
+
+- The sparse index array is **`O(Universe)` memory**, sized once at construction. The type
+  is worth it when the universe is bounded and the set is cleared / rebuilt / iterated
+  often — not as a general `HashSet<int>` replacement. For an unbounded or huge-and-sparse
+  key space, use [`IntSet`](#intset) / `HashSet<int>`.
+- It stores **only non-negative values below `Universe`**. A value outside `[0, Universe)`
+  is rejected by `Add` / `TryAdd` with `ArgumentOutOfRangeException`, and reported as absent
+  by `Contains` / `Remove` (the bounded-universe analogue of `EnumSet`).
+- `Remove` moves the last dense element into the vacated slot (an `O(1)` swap), so the
+  relative order of the surviving elements is not preserved. Enumeration order is
+  unspecified in general.
+
+It implements `ISet<int>` (and therefore `ICollection<int>` / `IEnumerable<int>`), ships an
+allocation-free struct enumerator, and accepts an `IEnumerable<int>` source at construction.
+
+### Constructors
+
+```csharp
+SparseSet(int universe)
+SparseSet(int universe, IEnumerable<int> source)
+```
+
+- `universe` is the **exclusive upper bound** of storable values; the set can hold any
+  non-negative integer strictly less than it. It sizes the sparse index array once, so
+  it is the dominant memory cost — choose it to match the actual value range. `0` creates
+  a set that can store nothing.
+- Throws `ArgumentOutOfRangeException` for a negative `universe`. There is **no
+  `loadFactor`** parameter.
+- The `source` constructor pre-sizes the dense array from an `ICollection<int>` source,
+  silently deduplicates (matching BCL `HashSet<int>(IEnumerable<int>)` semantics), throws
+  `ArgumentNullException` if `source` is `null` (the null check beats the universe
+  validation), and throws `ArgumentOutOfRangeException` if any source value is outside
+  `[0, universe)`.
+
+### Methods
+
+- `void Add(int item)` — throws `ArgumentException` on duplicate, `ArgumentOutOfRangeException` out of range.
+- `bool TryAdd(int item)` — `true` on success, `false` if already present; throws out of range.
+- `bool Contains(int item)` — `O(1)`; `false` for an out-of-range value.
+- `bool Remove(int item)` — `O(1)` swap-removal; `false` for an absent or out-of-range value.
+- `void Clear()` — **`O(1)`**, touches no memory; the set stays reusable.
+- `int EnsureCapacity(int capacity)` — grow the dense array to hold at least `capacity`
+  elements (clamped to `Universe`), returning the resulting dense-array length. Throws
+  `ArgumentOutOfRangeException` on a negative capacity.
+- `void TrimExcess()` / `void TrimExcess(int capacity)` — shrink the dense array to exactly
+  the current `Count` (or `capacity`). `TrimExcess(capacity)` throws if `capacity < Count`
+  or `capacity > Universe`. The sparse index array is unaffected.
+- `int Count { get; }`, `int Universe { get; }`
+- `Enumerator GetEnumerator()` — allocation-free struct enumerator over the dense array.
+- `void CopyTo(int[] array, int arrayIndex)` — matches `HashSet<int>.CopyTo` argument validation.
+
+### Set operations (`ISet<int>`)
+
+The full BCL `HashSet<int>` set-algebra surface is available and follows `HashSet<int>`
+semantics exactly within the universe (duplicate-tolerant `other`, self-aliasing
+`other == this`):
+
+- **Mutating:** `UnionWith`, `IntersectWith`, `ExceptWith`, `SymmetricExceptWith`.
+- **Query:** `IsSubsetOf`, `IsProperSubsetOf`, `IsSupersetOf`, `IsProperSupersetOf`, `Overlaps`, `SetEquals`.
+
+Each throws `ArgumentNullException` when `other` is `null`. The one bounded-universe caveat:
+a **mutating** operation that would *add* a value outside `[0, Universe)` (e.g. `UnionWith`
+with such an element) throws `ArgumentOutOfRangeException` rather than silently growing an
+unbounded set. Query operations tolerate out-of-range values in `other` (they simply read
+as absent). As with the other sets, `ISet<int>.Add(int)` returns `bool` (equivalent to
+`TryAdd`), the concrete `public void Add(int)` keeps its throw-on-duplicate behaviour, and
+`ICollection<int>.Add(int)` ignores duplicates.
+
+### Usage example
+
+```csharp
+using Celerity.Collections;
+
+// A BFS "visited" set over a graph whose nodes are ids in [0, nodeCount).
+var visited = new SparseSet(nodeCount);
+
+for (int start = 0; start < nodeCount; start++)
+{
+    visited.Clear(); // O(1) — no memory touched, ready for the next traversal
+    var queue = new Queue<int>();
+    queue.Enqueue(start);
+    visited.Add(start);
+
+    while (queue.Count > 0)
+    {
+        int node = queue.Dequeue();
+        foreach (int next in Neighbors(node))
+        {
+            if (visited.TryAdd(next)) // false if already seen
+                queue.Enqueue(next);
+        }
+    }
+
+    // Iterate exactly the reached nodes — a dense, contiguous scan.
+    Process(visited);
+}
+```
+
+---
+
 ## EnumMap&lt;TEnum, TValue&gt;
 
 ```csharp

src/Celerity.AotSmokeTest/Program.cs

@@ -544,6 +544,30 @@ void Check(bool condition, string message)
     Check(maxHeap.Dequeue() == "b", "IndexedPriorityQueue custom comparer (max-heap)");
 }
 
+// SparseSet — bounded-universe sparse integer set (Briggs–Torczon). Exercise add /
+// contains / swap-remove, the out-of-range rejection, the O(1) clear-then-reuse path
+// (which must reject stale sparse entries), and the dense-array enumerator.
+{
+    var ss = new SparseSet(64);
+    for (int i = 0; i < 10; i++) ss.Add(i);
+    Check(ss.Count == 10 && ss.Universe == 64 && ss.Contains(0) && ss.Contains(9), "SparseSet add + contains");
+    Check(!ss.TryAdd(5), "SparseSet.TryAdd duplicate");
+    Check(!ss.Contains(64) && !ss.Contains(-1), "SparseSet out-of-range reads absent");
+    Check(ss.Remove(5) && !ss.Contains(5) && ss.Contains(9), "SparseSet swap-remove keeps survivors");
+
+    ss.Clear();
+    Check(ss.Count == 0 && !ss.Contains(0) && !ss.Contains(9), "SparseSet O(1) clear rejects stale entries");
+    ss.Add(9); // 9 was present before Clear — must not false-positive until re-added
+    Check(ss.Count == 1 && ss.Contains(9) && !ss.Contains(0), "SparseSet reusable after clear");
+
+    var reached = new SparseSet(128, new[] { 3, 3, 7, 1, 7 }); // dedupes
+    var seen = new List<int>();
+    foreach (int x in reached) seen.Add(x);
+    Check(reached.Count == 3 && seen.Count == 3, "SparseSet source ctor dedupe + enumeration");
+    ((ISet<int>)reached).UnionWith(new[] { 1, 2 });
+    Check(reached.Count == 4 && reached.Contains(2), "SparseSet ISet<int> union within universe");
+}
+
 // SmallDictionary — flat-array, linear-scan dictionary (default key inline, no
 // hasher). Exercise the indexer, TryAdd/Add, TryGetValue, Remove, the swap-remove
 // path, the inline default/zero key, and the struct enumerator.

src/Celerity.Benchmarks/Program.cs

@@ -34,6 +34,7 @@ internal class Program
         typeof(LongSetBenchmark),
         typeof(SmallSetBenchmark),
         typeof(EnumSetBenchmark),
+        typeof(SparseSetBenchmark),
         typeof(BloomFilterBenchmark),
         typeof(CuckooFilterBenchmark),
         typeof(XorFilterBenchmark),