feat: add interquartile range algorithm

Maths/Percentile.js

@@ -0,0 +1,32 @@
+/**
+ * @function percentile
+ * @description Calculates the nth percentile of an array of numbers.
+ *   The percentile indicates the value below which a given percentage
+ *   of observations fall.
+ * @param {number[]} arr - An array of numbers
+ * @param {number} p - The percentile to calculate (0-100)
+ * @returns {number} - The nth percentile value
+ * @see https://en.wikipedia.org/wiki/Percentile
+ * @example percentile([1, 2, 3, 4, 5], 50) => 3
+ * @example percentile([1, 2, 3, 4, 5], 75) => 4
+ */
+const percentile = (arr, p) => {
+  if (!Array.isArray(arr) || arr.length === 0) {
+    throw new TypeError('First argument must be a non-empty array')
+  }
+  if (!arr.every((n) => typeof n === 'number' && !isNaN(n))) {
+    throw new TypeError('All elements must be numbers')
+  }
+  if (typeof p !== 'number' || isNaN(p) || p < 0 || p > 100) {
+    throw new TypeError('Percentile must be a number between 0 and 100')
+  }
+  const sorted = [...arr].sort((a, b) => a - b)
+  if (p === 0) return sorted[0]
+  if (p === 100) return sorted[sorted.length - 1]
+  const index = (p / 100) * (sorted.length - 1)
+  const lower = Math.floor(index)
+  const upper = Math.ceil(index)
+  return sorted[lower] + (sorted[upper] - sorted[lower]) * (index - lower)
+}
+
+export { percentile }

Maths/test/Percentile.test.js

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+import { percentile } from '../Percentile.js'
+
+describe('percentile', () => {
+  test('percentile([1,2,3,4,5], 50) returns 3', () => {
+    expect(percentile([1, 2, 3, 4, 5], 50)).toBe(3)
+  })
+
+  test('percentile([1,2,3,4,5], 0) returns 1', () => {
+    expect(percentile([1, 2, 3, 4, 5], 0)).toBe(1)
+  })
+
+  test('percentile([1,2,3,4,5], 100) returns 5', () => {
+    expect(percentile([1, 2, 3, 4, 5], 100)).toBe(5)
+  })
+
+  test('percentile([1,2,3,4,5], 75) returns 4', () => {
+    expect(percentile([1, 2, 3, 4, 5], 75)).toBe(4)
+  })
+
+  test('percentile([1,2,3,4,5], 25) returns 2', () => {
+    expect(percentile([1, 2, 3, 4, 5], 25)).toBe(2)
+  })
+
+  test('percentile works with unsorted array', () => {
+    expect(percentile([5, 3, 1, 4, 2], 50)).toBe(3)
+  })
+
+  test('percentile single element returns that element', () => {
+    expect(percentile([5], 50)).toBe(5)
+  })
+
+  test('percentile does not mutate original array', () => {
+    const arr = [5, 3, 1, 4, 2]
+    percentile(arr, 50)
+    expect(arr).toEqual([5, 3, 1, 4, 2])
+  })
+
+  test('throws TypeError for empty array', () => {
+    expect(() => percentile([], 50)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for invalid percentile > 100', () => {
+    expect(() => percentile([1, 2, 3], 101)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for invalid percentile < 0', () => {
+    expect(() => percentile([1, 2, 3], -1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for non-array input', () => {
+    expect(() => percentile('123', 50)).toThrow(TypeError)
+  })
+})