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# AdjacentPairs
* Author(s): [László Teveli](https://github.com/tevelee)
[[Source](https://github.com/apple/swift-async-algorithms/blob/main/Sources/AsyncAlgorithms/AsyncAdjacentPairsSequence.swift) |
[Tests](https://github.com/apple/swift-async-algorithms/blob/main/Tests/AsyncAlgorithmsTests/TestAdjacentPairs.swift)]
The `adjacentPairs()` API serve the purpose of collecting adjacent values. This operation is available for any `AsyncSequence` by calling the `adjacentPairs()` method.
```swift
extension AsyncSequence {
public func adjacentPairs() -> AsyncAdjacentPairsSequence<Self>
}
```
## Detailed Design
The `adjacentPairs()` algorithm produces elements of tuple (size of 2), containing a pair of the original `Element` type.
The interface for this algorithm is available on all `AsyncSequence` types. The returned `AsyncAdjacentPairsSequence` conditionally conforms to `Sendable`.
Its iterator keeps track of the previous element returned in the `next()` function and updates it in every turn.
```swift
for await (first, second) in (1...5).async.adjacentPairs() {
print("First: \(first), Second: \(second)")
}
// First: 1, Second: 2
// First: 2, Second: 3
// First: 3, Second: 4
// First: 4, Second: 5
```
It composes well with the [Dictionary.init(_:uniquingKeysWith:)](https://github.com/apple/swift-async-algorithms/blob/main/Guides/Collections.md) API that deals with `AsyncSequence` of tuples.
```swift
Dictionary(uniqueKeysWithValues: url.lines.adjacentPairs())
```
## Alternatives Considered
This functionality is often written as a `zip` of a sequence together with itself, dropping its first element (`zip(source, source.dropFirst())`).
It's such a dominant use-case, the [swift-algorithms](https://github.com/apple/swift-algorithms) package also [introduced](https://github.com/apple/swift-algorithms/pull/119) it to its collection of algorithms.
## Credits/Inspiration
The synchronous counterpart in [swift-algorithms](https://github.com/apple/swift-algorithms/blob/main/Guides/AdjacentPairs.md).
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