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36 lines
2.1 KiB
Markdown
36 lines
2.1 KiB
Markdown
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# `MinMultiset[T]` — A Lazy-Delete Min-Multiset Implementation
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A generic, type-safe multiset data structure in Go that maintains elements with multiplicity and provides efficient access to the current minimum. Built on top of a min-heap (`container/heap`) with **lazy deletion** to support efficient removals without rebuilding the heap.
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## Core Features
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- ✅ **Generic over comparable types** (`T`) with custom ordering via `less` comparator
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- ✅ **Multiset semantics**: supports multiple occurrences of the same value
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- ✅ **O(log n) insertion** (`Add`) and **amortized O(1)** minimum access
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- ✅ **Lazy deletion**: `RemoveOne` marks items for removal; physical cleanup occurs on next `Min()` call
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- ✅ **Size tracking**: logical size (`Len()`) excludes deleted items, even if still in heap
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- ✅ **Memory-efficient cleanup**: `shrink()` and `cleanTop()` prevent tombstone accumulation
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## API Summary
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| Method | Description |
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|--------|-------------|
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| `NewMinMultiset(less)` | Constructor; initializes heap, maps (`count`, `pending`), and sets ordering |
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| `Add(v)` | Inserts one occurrence of `v`; increments logical size & count map |
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| `RemoveOne(v)` | Removes *one* occurrence if present; returns success flag (`false` otherwise) |
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| `Min()` | Returns current minimum (or zero value + `ok=false`) after cleaning stale top entries |
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| `Len()` | Returns logical size (excludes pending deletions) |
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## Internal Mechanism
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- **`count[T]int`**: tracks how many times each value is *logically* present
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- **`pending[T]int`**: tracks how many times each value is *marked for removal*
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- **Heap invariant maintained only up to logical size** — stale entries are pruned lazily during `Min()` or after deletions
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- **No manual cleanup needed** — the structure self-balances incrementally
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## Use Cases
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Priority queues with deletable arbitrary elements (e.g., Dijkstra’s algorithm where distances are updated), sliding-window minima, event scheduling with cancellation.
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> ⚠️ Note: `less` must define a *strict total order* (transitive, antisymmetric, connected) for correctness.
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