From d5c5967e33ad0a6a9371bf24d71de2e4a3a1144f Mon Sep 17 00:00:00 2001
From: Daniel Bradley <daniel.bradley@telefonica.com>
Date: Thu, 7 May 2020 14:46:35 +0100
Subject: [PATCH] feat(iterables): add pairwise

Add new pairwise function and tests.
---
 src/iterables.ts      | 18 ++++++++++++++++++
 test/iterable.test.ts | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/src/iterables.ts b/src/iterables.ts
index 34efaa2..ff3eeb0 100644
--- a/src/iterables.ts
+++ b/src/iterables.ts
@@ -659,6 +659,24 @@ export function* reverse<T>(source: Iterable<T>): Iterable<T> {
   }
 }
 
+/**
+ * Returns an interable of each element in the input sequence and its predecessor,
+ * with the exception of the first element which is only returned as the predecessor of the second element.
+ * @param source The input collection
+ */
+export function* pairwise<T>(source: Iterable<T>): Iterable<[T, T]> {
+  let prev: T | undefined = undefined
+  let started = false
+  for (const item of source) {
+    if (!started) {
+      started = true
+    } else {
+      yield [prev!, item]
+    }
+    prev = item
+  }
+}
+
 /**
  * Returns the sum of the values in the collection.
  * @param source The input collection.
diff --git a/test/iterable.test.ts b/test/iterable.test.ts
index f54f873..af51476 100644
--- a/test/iterable.test.ts
+++ b/test/iterable.test.ts
@@ -716,6 +716,43 @@ describe('take', () => {
   })
 })
 
+describe('pairwise', () => {
+  test('empty', () => {
+    expect(
+      pipe(Iterables.init({ count: 0 }))
+        .then(Iterables.pairwise)
+        .then(Iterables.toArray).result
+    ).toEqual([])
+  })
+  test('single item', () => {
+    expect(
+      pipe(
+        (function* () {
+          yield 1
+        })()
+      )
+        .then(Iterables.pairwise)
+        .then(Iterables.toArray).result
+    ).toEqual([])
+  })
+  test('multiple items', () => {
+    expect(
+      pipe(
+        (function* () {
+          yield 1
+          yield 2
+          yield 3
+        })()
+      )
+        .then(Iterables.pairwise)
+        .then(Iterables.toArray).result
+    ).toEqual([
+      [1, 2],
+      [2, 3],
+    ])
+  })
+})
+
 describe('length', () => {
   it('can return zero length', () => {
     expect(pipe(Iterables.init({ count: 0 })).then(Iterables.length).result).toEqual(0)