Fix wiggle sort (#991)

Co-authored-by: Antonia Strack <antonia.strack@thm.mni.de>

Author: BranAndSceolan

Sorts/SimplifiedWiggleSort.js

@@ -0,0 +1,36 @@
+/*
+ * Wiggle sort sorts the array into a wave like array.
+ * An array ‘arr[0..n-1]’ is sorted in wave form if arr[0] <= arr[1] >= arr[2] <= arr[3] >= arr[4] <= …..
+ * KEEP IN MIND: there are also more strict definitions of wiggle sort which use
+ * the rule arr[0] < arr[1] > arr[2] < arr[3] > arr[4] < … but this function
+ * allows for equality of values next to each other.
+ */
+import { quickSelectSearch } from '../Search/QuickSelectSearch.js'
+
+export const simplifiedWiggleSort = function (arr) {
+  // find Median using QuickSelect
+  let median = quickSelectSearch(arr, Math.floor(arr.length / 2.0))
+  median = median[Math.floor(arr.length / 2.0)]
+
+  const sorted = new Array(arr.length)
+
+  let smallerThanMedianIndx = 0
+  let greaterThanMedianIndx = arr.length - 1 - (arr.length % 2)
+
+  for (let i = 0; i < arr.length; i++) {
+    if (arr[i] > median) {
+      sorted[greaterThanMedianIndx] = arr[i]
+      greaterThanMedianIndx -= 2
+    } else {
+      if (smallerThanMedianIndx < arr.length) {
+        sorted[smallerThanMedianIndx] = arr[i]
+        smallerThanMedianIndx += 2
+      } else {
+        sorted[greaterThanMedianIndx] = arr[i]
+        greaterThanMedianIndx -= 2
+      }
+    }
+  }
+
+  return sorted
+}

Sorts/WiggleSort.js

@@ -0,0 +1,24 @@
+import { simplifiedWiggleSort } from '../SimplifiedWiggleSort.js'
+
+describe('simplified wiggle sort', () => {
+  test('simplified wiggle sort for chars', () => {
+    const src = ['a', 'b', 'c']
+    expect(simplifiedWiggleSort(src)).toEqual(['a', 'c', 'b'])
+  })
+
+  test('wiggle sort with duplicates, even array', () => {
+    const src = [2, 2, 1, 3]
+    expect(simplifiedWiggleSort(src)).toEqual([1, 3, 2, 2])
+  })
+
+  test('wiggle sort with duplicates, odd array', () => {
+    const src = [1, 1, 1, 2, 4]
+    expect(simplifiedWiggleSort(src)).toEqual([1, 4, 1, 2, 1])
+  })
+
+  test('simplified wiggle sort which leads to equal values next to ' +
+    'each other', () => {
+    const src = [3, 3, 5, 1]
+    expect(simplifiedWiggleSort(src)).toEqual([1, 5, 3, 3])
+  })
+})