Generic Stack (VBA, Array-Backed)

A lightweight LIFO stack implemented on top of a dynamic array with doubling growth and lazy shrink.
Designed for speed and memory efficiency versus a Collection-backed stack, while preserving a clean API.

📁 Files

File	Description
Stack.cls	Source file with Rubberduck annotations ('@Description, '@DefaultMember, '@Enumerator)
Stack_WithAttributes.cls	Ready-to-import version with VB attributes baked in — no Rubberduck required

Both files are identical in behaviour. Import Stack_WithAttributes.cls if you are not using Rubberduck.

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📦 Features

• Fast push/pop using a dynamic array with exponential growth (GrowthFactor = 2)
• Lower memory use than Collection (≈2× less per item on x64; extra headspace applies)
• Shrink logic: reclaims capacity when occupancy drops below ShrinkTrigger (25%)
• Enumeration: For Each (top → bottom) via a snapshot Collection
• Utility export: Items([base]) returns a 0- or 1-based array copy
• Pure VBA, no external references

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⚙️ Public Interface

Member	Type	Description
Push(Item)	Sub	Adds an item at the top.
Pop()	Function	Returns and removes the top item. Raises error 5 if empty.
Peek (Default)	Property	Returns the top item without removing it. Raises error 5 if empty.
Count	Property	Number of items currently stored.
IsEmpty	Property	True if empty, else False.
Clear	Sub	Clears the stack and resets capacity to InitialSize.
Items([base])	Function	Copies items into Variant(); order is top → bottom; arr(base) = top.
For Each	Enumerator	Iterates top → bottom on a snapshot. (Mutation during iteration is safe but iterates the old snapshot.)

Errors

• Empty stack on Peek/Pop raises vbErrorInvalidProcedureCall (=5) with source "Stack.Peek" / "Stack.Pop".

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🚀 Quick Start

Dim s As New Stack

s.Push "alpha"
s.Push "beta"
Debug.Print s.Peek        ' -> beta
Debug.Print s.Pop         ' -> beta (removed)
Debug.Print s.Pop         ' -> alpha (removed)
Debug.Print s.IsEmpty     ' -> True

⚡ Performance Notes

The array-based implementation provides constant-time O(1) amortized Push and Pop operations,
while outperforming the Collection-based version in speed and memory use.

Timings (ms) per Push + Pop cycle

Count	Array	Collection
10	0.00030	0.00053
100	0.00030	0.00053
1 000	0.00029	0.00052
10 000	0.00028	0.00052
100 000	0.00029	0.00052

Observations

• The array version is roughly 40% faster than the Collection version across all stack sizes.
• It uses about half the memory per stored item on x64 systems (excluding the headspace buffer).
• The doubling growth strategy (GrowthFactor = 2) keeps ReDim Preserve calls rare and amortized O(1).
• Shrink fires only when occupancy drops below ShrinkTrigger (25%), preventing thrashing on alternating push/pop.
• The collection version stays stable at very large sizes but pays a constant late-binding penalty internally.

Practical Guidance

• For computational stacks, recursion simulations, or evaluation engines prefer the array stack for speed and memory efficiency.
• For unbounded or long-lived stacks (e.g., message buffers), the collection stack may offer simpler code.

All timings were measured in VBA 7 (x64) on Windows 11 using a high-resolution Stopwatch based on QueryPerformanceCounter.

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