perf: optimize factor calculation

Maths/Factors.js

@@ -7,10 +7,30 @@
  *
  */
 
+/**
+ * Finds every positive factor of a number in ascending order.
+ *
+ * Time complexity: O(sqrt(number))
+ * Space complexity: O(k), where k is the number of factors
+ */
 const factorsOfANumber = (number = 0) => {
-  return Array.from(Array(number + 1).keys()).filter(
-    (num) => number % num === 0
-  )
+  if (!Number.isInteger(number) || number <= 0) return []
+
+  const lowerFactors = []
+  const upperFactors = []
+
+  for (let divisor = 1; divisor <= number / divisor; divisor++) {
+    if (number % divisor !== 0) continue
+
+    lowerFactors.push(divisor)
+
+    const pairedDivisor = number / divisor
+    if (pairedDivisor !== divisor) {
+      upperFactors.push(pairedDivisor)
+    }
+  }
+
+  return lowerFactors.concat(upperFactors.reverse())
 }
 
 export { factorsOfANumber }

Maths/test/Factors.test.js

@@ -1,10 +1,19 @@
 import { factorsOfANumber } from '../Factors'
 
 describe('Factors', () => {
-  factorsOfANumber(50).forEach((num) => {
-    it(`${num} is a factor of 50`, () => {
-      const isFactor = 50 % num === 0
-      expect(isFactor).toBeTruthy()
-    })
+  it.each([
+    [1, [1]],
+    [16, [1, 2, 4, 8, 16]],
+    [50, [1, 2, 5, 10, 25, 50]],
+    [97, [1, 97]]
+  ])('returns every factor of %i in ascending order', (number, expected) => {
+    expect(factorsOfANumber(number)).toEqual(expected)
   })
+
+  it.each([0, -1, 1.5])(
+    'returns an empty array for invalid input %s',
+    (number) => {
+      expect(factorsOfANumber(number)).toEqual([])
+    }
+  )
 })