feat(arrays): add pairwise

Add new pairwise function and tests.

src/arrays.ts

@@ -900,6 +900,19 @@ export function reverse<T>(source: ReadonlyArray<T>): T[] {
   return Array.from(source).reverse()
 }
 
+/**
+ * Returns an array of each element in the input collection 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: ReadonlyArray<T>): [T, T][] {
+  const pairs: [T, T][] = []
+  for (let index = 1; index < source.length; index++) {
+    pairs.push([source[index - 1], source[index]])
+  }
+  return pairs
+}
+
 /**
  * Returns the sum of the values in the collection.
  * @param source The input collection.

test/arrays.test.ts

@@ -518,6 +518,21 @@ describe('init', () => {
   })
 })
 
+describe('pairwise', () => {
+  test('empty', () => {
+    expect(Arrays.pairwise([])).toEqual([])
+  })
+  test('single item', () => {
+    expect(Arrays.pairwise([1])).toEqual([])
+  })
+  test('multiple items', () => {
+    expect(Arrays.pairwise([1, 2, 3])).toEqual([
+      [1, 2],
+      [2, 3],
+    ])
+  })
+})
+
 describe('length', () => {
   test('zero length', () => {
     expect(Arrays.length([])).toEqual(0)