chore: Merge pull request #800 from arthurvergacas/pancake-sort

Add Pancake Sort to the sorting algorithms

Sorts/PancakeSort.js

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+/*
+ * Unlike a traditional sorting algorithm, which attempts to sort with the fewest
+ * comparisons possible, the goal of pancake sort is to sort the sequence in as few reversals as
+ * possible. The idea is to do something similar to Selection Sort. We one by one place
+ * maximum element at the end and reduce the size of current array by one.
+ *
+ * Source: https://www.geeksforgeeks.org/pancake-sorting/
+ *
+ * This sorting algorithm is inspired by the pancake problem (hence the name),
+ * where a spatula can be placed anywhere between two pancakes and flip all pancakes
+ * above.
+ *
+ * The interesting about this algorithm (besides its name) is that instead of comparisons,
+ * the algorithm relies on flipping an array.
+ *
+ * Source: https://en.wikipedia.org/wiki/Pancake_sorting#The_original_pancake_problem
+ *
+ */
+
+/**
+ * Unlike Array.prototype.reverse, flipArray reverses only a subarray of the given
+ * array, determined by the parameters startIndex and endIndex
+ *
+ * @param {number[]} array The array to flip
+ * @param {number} startIndex The start of the subarray
+ * @param {number} endIndex The end of the subarray
+ * @returns The flipped array
+ */
+export function flipArray(array, startIndex, endIndex) {
+  while (startIndex < endIndex) {
+    // swap front and back of the subarray
+    const temp = array[startIndex]
+    array[startIndex] = array[endIndex]
+    array[endIndex] = temp
+
+    // essentialy reducing the problem to a smaller subarray
+    startIndex++
+    endIndex--
+  }
+
+  return array
+}
+
+/**
+ * Returns the index of the maximum number of a subarray in a given array
+ *
+ * @param {number[]} array The array to found the maximum number's index
+ * @param {*} startIndex The start of the subarray
+ * @param {*} endIndex The end of the subarray
+ * @returns The index of the maximum number
+ */
+export function findMax(array, startIndex, endIndex) {
+  let maxIndex = 0
+  for (let i = startIndex; i <= endIndex; i++) {
+    if (array[i] > array[maxIndex]) maxIndex = i
+  }
+
+  return maxIndex
+}
+
+/**
+ * The Pancake Sort algorithm.
+ *
+ * Note that even though it's a completely different concept of sorting an
+ * array, it's rather simple!
+ *
+ * @param {number[]} array The array to sort
+ * @returns The sorted array
+ */
+export function pancakeSort(array) {
+  for (let subarraySize = array.length; subarraySize > 1; subarraySize--) {
+    const maximumIndex = findMax(array, 0, subarraySize - 1)
+
+    if (maximumIndex !== subarraySize - 1) {
+      flipArray(array, 0, maximumIndex)
+      flipArray(array, 0, subarraySize - 1)
+    }
+  }
+
+  return array
+}

Sorts/test/PancakeSort.test.js

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+import { flipArray, findMax, pancakeSort } from '../PancakeSort'
+
+describe('flipArray', () => {
+  it('should flip any subarray of any array', () => {
+    expect(flipArray([1, 2, 3, 4], 0, 3)).toEqual([4, 3, 2, 1])
+    expect(flipArray([1, 2, 3, 4, 5], 2, 4)).toEqual([1, 2, 5, 4, 3])
+    expect(flipArray([], 0, 0)).toEqual([])
+  })
+})
+
+describe('findMax', () => {
+  it('should find the index of the maximum value in any subarray of any array', () => {
+    expect(findMax([1, 3, 2, 5], 0, 3)).toEqual(3)
+    expect(findMax([1, 3, 2, 5], 0, 2)).toEqual(1)
+  })
+})
+
+describe('pancakeSort', () => {
+  it('should sort any array', () => {
+    expect(pancakeSort([4, 3, 2, 1])).toEqual([1, 2, 3, 4])
+    expect(pancakeSort([3, 1, 4, 2])).toEqual([1, 2, 3, 4])
+    expect(pancakeSort([100, 1000, 10, 1])).toEqual([1, 10, 100, 1000])
+  })
+})