Updated quantile to use sorted values for min and max

Fixes mathigon#192

src/statistics.ts

@@ -19,8 +19,8 @@ export function quantile(values: number[], p: number, method: number = 1): numbe
   if (!n) return 0;
 
   const sorted = values.slice(0).sort((a, b) => (a - b));
-  if (p === 0) return values[0];
-  if (p === 1) return values[n - 1];
+  if (p === 0) return sorted[0];
+  if (p === 1) return sorted[n - 1];
 
   // See https://en.wikipedia.org/wiki/Quantile#Estimating_quantiles_from_a_sample
   if (![1, 2, 3].includes(method)) throw new RangeError('Invalid quantile method.');

test/statistics-test.ts

@@ -3,12 +3,10 @@
 // (c) Mathigon
 // =============================================================================
 
+import tape from "tape";
+import { mode, quantile } from "../src";
 
-import tape from 'tape';
-import {mode, quantile} from '../src';
-
-
-tape('mode', (test) => {
+tape("mode", (test) => {
   test.equal(mode([]), undefined);
   test.equal(mode([2]), 2);
   test.equal(mode([2, 3]), undefined);
@@ -19,7 +17,7 @@ tape('mode', (test) => {
   test.end();
 });
 
-tape('quantile', (test) => {
+tape("quantile", (test) => {
   test.equal(quantile([], 0.5), 0);
 
   const evenPopulation = [1, 2, 3, 4, 5];
@@ -35,5 +33,12 @@ tape('quantile', (test) => {
   test.equal(quantile(oddPopulation, 0.5), 3.5);
   test.equal(quantile(oddPopulation, 0.75), 5);
   test.equal(quantile(oddPopulation, 1), 6);
+
+  const randomPopulation = [4, 1, 2, 6, 3, 5];
+  test.equal(quantile(randomPopulation, 0), 1);
+  test.equal(quantile(randomPopulation, 0.25), 2);
+  test.equal(quantile(randomPopulation, 0.5), 3.5);
+  test.equal(quantile(randomPopulation, 0.75), 5);
+  test.equal(quantile(randomPopulation, 1), 6);
   test.end();
 });