# `gno.land/p/moul/deque`

A doubly-linked deque, optionally bounded, written for chain storage rather
than for RAM.

```go
d := deque.New()             // unbounded
d = deque.NewBounded(3)      // evicts from the front past 3 elements

d.PushBack("a", "b", "c")
d.PushFront("z")
first := d.PopFront()
last  := d.Last()
```

## Why not a slice

A slice-backed queue rewrites the whole backing array on append-and-grow and
shifts every element on pop-front. On gno that is not a constant factor, it is
the storage bill: the VM persists what changed, so touching N elements costs N.

Each operation here updates one or two node pointers and nothing else:

| operation | cost |
| --- | --- |
| `PushBack`, `PushFront`, `PopFront`, `PopBack` | O(1), 1-2 node writes |
| `First`, `Last`, `Size`, `IsEmpty` | O(1), no write |
| `Get(i)` | O(n) walk, no write |

Use it when you want a bounded log (the last N events), a FIFO work queue, or
any list where reads are "first / last / how many / iterate" and `Get(i)` is
rare. Use an avl tree instead when you need keyed lookup or ordering by
anything but insertion.

## Bounded mode

`NewBounded(n)` makes eviction automatic and symmetric: pushing past the limit
at the back drops from the front, and pushing past it at the front drops from
the back. `MaxSize()` returns 0 for unbounded, `IsBounded()` says which.

## Iteration

`All()` walks front to back, `Backward()` back to front. Both return a function
taking a `func(any) bool` callback; return `false` to stop early.

```go
d.All()(func(v any) bool {
	println(v)
	return true
})
```

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---

Part of **[moul/gno-contracts](https://github.com/moul/gno-contracts)** — moul's versioned gno.land contracts. See the repository for the full catalog, build/test tooling, and usage.

> ⚠️ **Disclaimer:** provided as-is, without warranty; not security-audited. Full disclaimer: [DISCLAIMER](https://github.com/moul/gno-contracts/blob/main/DISCLAIMER.md).

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