fix: distribute rest files over shards

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Fixes
 
+- `[@jest/test-sequencer]` Make sure sharding does not produce empty groups ([#13476](https://github.com/facebook/jest/pull/13476))
+
 ### Chore & Maintenance
 
 ### Performance

packages/jest-test-sequencer/src/__tests__/test_sequencer.test.ts

@@ -368,6 +368,23 @@ test('returns expected 100/8 shards', async () => {
   );
 
   expect(shards.map(shard => shard.length)).toEqual([
-    13, 13, 13, 13, 13, 13, 13, 9,
+    13, 13, 13, 13, 12, 12, 12, 12,
+  ]);
+});
+
+test('returns expected 55/12 shards', async () => {
+  const allTests = toTests(new Array(55).fill(true).map((_, i) => `/${i}.js`));
+
+  const shards = await Promise.all(
+    new Array(12).fill(true).map((_, i) =>
+      sequencer.shard(allTests, {
+        shardCount: 12,
+        shardIndex: i + 1,
+      }),
+    ),
+  );
+
+  expect(shards.map(shard => shard.length)).toEqual([
+    5, 5, 5, 5, 5, 5, 5, 4, 4, 4, 4, 4,
   ]);
 });

packages/jest-test-sequencer/src/index.ts

@@ -24,6 +24,10 @@ export type ShardOptions = {
   shardCount: number;
 };
 
+type ShardPositionOptions = ShardOptions & {
+  suiteLength: number;
+};
+
 /**
  * The TestSequencer will ultimately decide which tests should run first.
  * It is responsible for storing and reading from a local cache
@@ -72,6 +76,19 @@ export default class TestSequencer {
     return cache;
   }
 
+  private _shardPosition(options: ShardPositionOptions): number {
+    const shardRest = options.suiteLength % options.shardCount;
+    const ratio = options.suiteLength / options.shardCount;
+
+    return new Array(options.shardIndex)
+      .fill(true)
+      .reduce<number>((acc, _, shardIndex) => {
+        const dangles = shardIndex < shardRest;
+        const shardSize = dangles ? Math.ceil(ratio) : Math.floor(ratio);
+        return acc + shardSize;
+      }, 0);
+  }
+
   /**
    * Select tests for shard requested via --shard=shardIndex/shardCount
    * Sharding is applied before sorting
@@ -97,9 +114,17 @@ export default class TestSequencer {
     tests: Array<Test>,
     options: ShardOptions,
   ): Array<Test> | Promise<Array<Test>> {
-    const shardSize = Math.ceil(tests.length / options.shardCount);
-    const shardStart = shardSize * (options.shardIndex - 1);
-    const shardEnd = shardSize * options.shardIndex;
+    const shardStart = this._shardPosition({
+      shardCount: options.shardCount,
+      shardIndex: options.shardIndex - 1,
+      suiteLength: tests.length,
+    });
+
+    const shardEnd = this._shardPosition({
+      shardCount: options.shardCount,
+      shardIndex: options.shardIndex,
+      suiteLength: tests.length,
+    });
 
     return tests
       .map(test => {