fix(helpers): uniform distribution in helpers.arrayElements (#1770)

Author: ST-DDT

src/modules/helpers/index.ts

@@ -594,8 +594,9 @@ export class HelpersModule {
     let temp: T;
     let index: number;
 
+    // Shuffle the last `count` elements of the array
     while (i-- > min) {
-      index = Math.floor((i + 1) * this.faker.number.float({ max: 0.99 }));
+      index = this.faker.number.int(i);
       temp = arrayCopy[index];
       arrayCopy[index] = arrayCopy[i];
       arrayCopy[i] = temp;

test/__snapshots__/helpers.spec.ts.snap

@@ -195,15 +195,15 @@ exports[`helpers > 1211 > arrayElements > noArgs 1`] = `
 
 exports[`helpers > 1211 > arrayElements > with array 1`] = `
 [
-  "r",
+  "d",
   "o",
-  "l",
+  "r",
   "!",
   "l",
-  "d",
-  "W",
   "H",
+  "W",
   "e",
+  "l",
   "o",
   "l",
   " ",
@@ -213,7 +213,7 @@ exports[`helpers > 1211 > arrayElements > with array 1`] = `
 exports[`helpers > 1211 > arrayElements > with array and count 1`] = `
 [
   "r",
-  "o",
+  " ",
   "!",
 ]
 `;

test/helpers.spec.ts

@@ -292,6 +292,45 @@ describe('helpers', () => {
 
           expect(result).toHaveLength(0);
         });
+
+        it('should return the only element in the array when there is only 1', () => {
+          const testArray = ['hello'];
+          const actual = faker.helpers.arrayElements(testArray);
+          expect(actual).toEqual(testArray);
+        });
+
+        it('should return each element with a somewhat equal distribution with 2 elements', () => {
+          const input = Array.from({ length: 2 }, (_, i) => i);
+          const occurrences = Array.from({ length: 2 }, () => 0);
+
+          for (let i = 0; i < 1000; i++) {
+            const [result] = faker.helpers.arrayElements(input, 1);
+            occurrences[result]++;
+          }
+
+          for (const occurrence of occurrences) {
+            expect(occurrence).toBeGreaterThanOrEqual(400);
+            expect(occurrence).toBeLessThanOrEqual(600);
+          }
+        });
+
+        it.each([10, 100, 1000])(
+          'should return each element with a somewhat equal distribution with %s elements',
+          (length) => {
+            const input = Array.from({ length }, (_, i) => i % 10);
+            const occurrences = Array.from({ length: 10 }, () => 0);
+
+            for (let i = 0; i < 1000; i++) {
+              const [result] = faker.helpers.arrayElements(input, 1);
+              occurrences[result]++;
+            }
+
+            for (const occurrence of occurrences) {
+              expect(occurrence).toBeGreaterThanOrEqual(70);
+              expect(occurrence).toBeLessThanOrEqual(130);
+            }
+          }
+        );
       });
 
       describe('slugify()', () => {