feat(algorithms): add counting sort for array of numbers (#77)

* feat: add counting sort for array of numbers

* refactor(counting-sort.ts): change for loop syntax for readability

src/algorithms/sort/counting-sort.ts

@@ -0,0 +1,55 @@
+/**
+ * Sort an array of integers in-place and in ascending order using counting sort.
+ *
+ * Time complexity: O(n + k), where n is the number of elements in arr
+ * and k is the range of numbers in arr.
+ *
+ * Space complexity: O(n + k)
+ *
+ * @param arr - array of integers to sort
+ */
+const countingSort = (arr: number[]): void => {
+  if (arr.length == 0) {
+    return
+  }
+
+  // check that arr elements are integers
+  for (let idx = 0; idx < arr.length; idx++) {
+    if (!Number.isInteger(arr[idx])) {
+      console.error(`arr[${idx}] is not an integer`)
+      return
+    }
+  }
+
+  // implementation of counting sort below
+  const max = Math.max(...arr) // find largest integer in arr
+  const min = Math.min(...arr) // find smallest integer in arr
+  const range = max - min + 1 // calculate range of possible numbers
+  const counts: number[] = new Array(range).fill(0) // contains counts for each number in arr
+  const sortedArr: number[] = new Array(arr.length) // final sorted array
+
+  // get counts of each number in arr
+  for (const num of arr) {
+    counts[num - min]++
+  }
+
+  // modify counts array such that each element is the sum of previous counts
+  for (let i = 1; i < counts.length; i++) {
+    counts[i] += counts[i - 1]
+  }
+
+  // fill in sortedArr using the indices stored in the counts array
+  // traversing arr in reverse keeps the sort stable
+  for (let i = arr.length - 1; i >= 0; i--) {
+    const sortedIndex = counts[arr[i] - min] - 1
+    counts[arr[i] - min]--
+    sortedArr[sortedIndex] = arr[i]
+  }
+
+  // copy elements of sortedArr to input array
+  for (let i = 0; i < arr.length; i++) {
+    arr[i] = sortedArr[i]
+  }
+}
+
+export default countingSort

test/algorithms/sort/counting-sort.test.ts

@@ -0,0 +1,27 @@
+import countingSort from '../../../src/algorithms/sort/counting-sort'
+import testCases from './test-cases'
+
+describe('countingSort', () => {
+  console.error = jest.fn()
+
+  beforeEach(() => {
+    jest.clearAllMocks()
+  })
+
+  for (const [description, inputArray] of Object.entries(testCases)) {
+    if (description === 'array of floats') {
+      it(`raises error for ${description}`, () => {
+        countingSort(inputArray)
+        expect(console.error).toHaveBeenCalledTimes(1)
+      })
+      continue
+    }
+
+    it(`correctly sorts ${description}`, () => {
+      const soln = [...inputArray]
+      soln.sort((a, b) => a - b) // sort the copy in-place using native JS function
+      countingSort(inputArray)
+      expect(inputArray).toEqual(soln)
+    })
+  }
+})