Better hashing

Badeend.ValueCollections/ValueList.cs

@@ -66,6 +66,8 @@ public static ValueListBuilder<T> Builder<T>(ReadOnlySpan<T> items)
 [CollectionBuilder(typeof(ValueList), nameof(ValueList.Create))]
 public sealed class ValueList<T> : IReadOnlyList<T>, IList<T>, IEquatable<ValueList<T>>
 {
+	private const int UninitializedHashCode = 0;
+
 	/// <summary>
 	/// Get a new empty list.
 	///
@@ -79,6 +81,11 @@ public sealed class ValueList<T> : IReadOnlyList<T>, IList<T>, IEquatable<ValueL
 	private readonly T[] items;
 	private readonly int count;
 
+	/// <summary>
+	/// Warning! This class promises to be thread-safe, yet this is a mutable field.
+	/// </summary>
+	private int hashCode = UninitializedHashCode;
+
 	internal T[] Items
 	{
 		[MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -289,7 +296,39 @@ void ICollection<T>.CopyTo(T[] array, int arrayIndex)
 	}
 
 	/// <inheritdoc/>
-	public sealed override int GetHashCode() => this.AsValueSlice().GetHashCode();
+	public sealed override int GetHashCode()
+	{
+		var hashCode = Volatile.Read(ref this.hashCode);
+		if (hashCode != UninitializedHashCode)
+		{
+			return hashCode;
+		}
+
+		hashCode = this.ComputeHashCode();
+		Volatile.Write(ref this.hashCode, hashCode);
+		return hashCode;
+	}
+
+	private int ComputeHashCode()
+	{
+		var hasher = new HashCode();
+		hasher.Add(typeof(ValueList<T>));
+		hasher.Add(this.Count);
+
+		foreach (var item in this)
+		{
+			hasher.Add(item);
+		}
+
+		var hashCode = hasher.ToHashCode();
+		if (hashCode == UninitializedHashCode)
+		{
+			// Never return 0, as that is our placeholder value.
+			hashCode = 1;
+		}
+
+		return hashCode;
+	}
 
 	/// <summary>
 	/// Returns <see langword="true"/> when the two lists have identical length

Badeend.ValueCollections/ValueSet.cs

@@ -328,7 +328,7 @@ private int ComputeHashCode()
 		}
 
 		var hasher = new HashCode();
-		hasher.Add(typeof(T));
+		hasher.Add(typeof(ValueSet<T>));
 		hasher.Add(this.Count);
 		hasher.AddUnordered(ref contentHasher);
 

Badeend.ValueCollections/ValueSlice.cs

@@ -378,22 +378,16 @@ public readonly ref readonly T Current
 	/// <inheritdoc/>
 	public override int GetHashCode()
 	{
-		int hashCode = typeof(T).GetHashCode();
-		hashCode = (hashCode * -1521134295) + this.length;
+		var hasher = new HashCode();
+		hasher.Add(typeof(ValueSlice<T>));
+		hasher.Add(this.Length);
 
-		// Include first item
-		if (this.length >= 1)
-		{
-			hashCode = (hashCode * -1521134295) + (this[0]?.GetHashCode() ?? 0);
-		}
-
-		// Include last item
-		if (this.length >= 2)
+		foreach (var item in this)
 		{
-			hashCode = (hashCode * -1521134295) + (this[this.length - 1]?.GetHashCode() ?? 0);
+			hasher.Add(item);
 		}
 
-		return hashCode;
+		return hasher.ToHashCode();
 	}
 
 	/// <summary>