# deque

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Fast ring-buffer deque ([double-ended queue](https://en.wikipedia.org/wiki/Double-ended_queue)) implementation.

For a pictorial description, see the [Deque diagram](https://github.com/gammazero/deque/wiki)

## Installation

```
$ go get github.com/gammazero/deque
```

## Deque data structure

Deque generalizes a queue and a stack, to efficiently add and remove items at either end with O(1) performance. [Queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type)) (FIFO) operations are supported using `PushBack` and `PopFront`. [Stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)) (LIFO) operations are supported using `PushBack` and `PopBack`.

## Ring-buffer Performance

This deque implementation is optimized for CPU and GC performance. The circular buffer automatically re-sizes by powers of two, growing when additional capacity is needed and shrinking when only a quarter of the capacity is used, and uses bitwise arithmetic for all calculations. Since growth is by powers of two, adding elements will only cause O(log n) allocations. A base capacity can be set, with `SetBaseCap`, so that there is no resizing at or below that specified amount. The Deque can also be grown, using `Grow`, to ensure sufficient storage for n additional items, to prevent resizing when adding a number of itmes.

The ring-buffer implementation improves memory and time performance with fewer GC pauses, compared to implementations based on slices or linked lists. By wrapping around the buffer, previously used space is reused, making allocation unnecessary until all buffer capacity is used. The ring buffer implementation performs best when resizes are infrequest, as is the case when items moving in and out of the Deque are balanced or when the base capacity is large enough to rarely require a resize.

For maximum speed, this deque implementation leaves concurrency safety up to the application to provide, however the application chooses, if needed at all.

## Reading Empty Deque

Since it is OK for the deque to contain a `nil` value, it is necessary to either panic or return a second boolean value to indicate the deque is empty, when reading or removing an element. This deque panics when reading from an empty deque. This is a run-time check to help catch programming errors, which may be missed if a second return value is ignored. Simply check `Deque.Len()` before reading from the deque.

## Generics

Deque uses generics to create a Deque that contains items of the type specified. To create a Deque that holds a specific type, provide a type argument with the `Deque` variable declaration. For example:
```go
    stringDeque := new(deque.Deque[string])
    var intDeque deque.Deque[int]
```

Deque supports iterators that allow traversal or removal of items. When removing items this is useful to avoid intermediate resizes of the internal buffer. It also allows Deque to be used with the stdlib [`slices.Collect`](https://pkg.go.dev/slices#Collect), [`slices.Sorted`](https://pkg.go.dev/slices#Sorted), and others that take [`iter.Seq`](https://pkg.go.dev/iter#Seq) argumments.

## Example

```go
package main

import (
    "fmt"
    "github.com/gammazero/deque"
)

func main() {
    var q deque.Deque[string]
    q.PushBack("foo")
    q.PushBack("bar")
    q.PushBack("baz")

    fmt.Println(q.Len())   // Prints: 3
    fmt.Println(q.Front()) // Prints: foo
    fmt.Println(q.Back())  // Prints: baz

    q.PopFront() // remove "foo"
    q.PopBack()  // remove "baz"

    q.PushFront("hello")
    q.PushBack("world")

    // Consume deque and print elements.
    for q.Len() != 0 {
        fmt.Println(q.PopFront())
    }
}
```

## Uses

Deque can be used as both a:
- [Queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type)) using `PushBack` and `PopFront`
- [Stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)) using `PushBack` and `PopBack`
