From 4c74710aa0c9827678a3f625cf77538f1021bd35 Mon Sep 17 00:00:00 2001
From: Devin Ivy <devinivy@gmail.com>
Date: Fri, 24 Apr 2026 17:18:36 -0400
Subject: [PATCH] feat(serve): leastConns and roundRobin balance strategies

---
 src/serve/index.ts            |  1 +
 src/serve/strategies.ts       | 30 ++++++++++++++++++++++++
 test/serve/strategies.test.ts | 43 +++++++++++++++++++++++++++++++++++
 3 files changed, 74 insertions(+)
 create mode 100644 src/serve/strategies.ts
 create mode 100644 test/serve/strategies.test.ts

diff --git a/src/serve/index.ts b/src/serve/index.ts
index 8489f72..fc44d2a 100644
--- a/src/serve/index.ts
+++ b/src/serve/index.ts
@@ -1 +1,2 @@
 export type { ListenArgs, ListenOptions, Balance, ServerThreads, ServerThreadsOptions } from './types.ts';
+export { leastConns, roundRobin } from './strategies.ts';
diff --git a/src/serve/strategies.ts b/src/serve/strategies.ts
new file mode 100644
index 0000000..db1ef5e
--- /dev/null
+++ b/src/serve/strategies.ts
@@ -0,0 +1,30 @@
+import type { Balance } from './types.ts';
+
+/** Pick the worker with the fewest active connections; ties go to the lowest index. */
+export function leastConns(): Balance {
+  return {
+    pick(counters) {
+      let bestIdx = 0;
+      let bestCount = counters[0];
+      for (let i = 1; i < counters.length; i++) {
+        if (counters[i] < bestCount) {
+          bestIdx = i;
+          bestCount = counters[i];
+        }
+      }
+      return bestIdx;
+    },
+  };
+}
+
+/** Cycle through worker indices in order, ignoring counts. */
+export function roundRobin(): Balance {
+  let cursor = 0;
+  return {
+    pick(counters) {
+      const idx = cursor % counters.length;
+      cursor = (cursor + 1) % counters.length;
+      return idx;
+    },
+  };
+}
diff --git a/test/serve/strategies.test.ts b/test/serve/strategies.test.ts
new file mode 100644
index 0000000..e6dd607
--- /dev/null
+++ b/test/serve/strategies.test.ts
@@ -0,0 +1,43 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { leastConns, roundRobin } from 'moroutine/serve';
+
+describe('leastConns', () => {
+  it('picks the index with the lowest count', () => {
+    const b = leastConns();
+    assert.equal(b.pick([5, 2, 7]), 1);
+    assert.equal(b.pick([0, 0, 0]), 0); // ties → first min
+    assert.equal(b.pick([3, 3, 1]), 2);
+  });
+
+  it('is stateless across calls', () => {
+    const b = leastConns();
+    assert.equal(b.pick([1, 0, 0]), 1);
+    assert.equal(b.pick([1, 0, 0]), 1); // same input → same result
+  });
+});
+
+describe('roundRobin', () => {
+  it('cycles through indices in order', () => {
+    const b = roundRobin();
+    const counters = [0, 0, 0, 0];
+    assert.equal(b.pick(counters), 0);
+    assert.equal(b.pick(counters), 1);
+    assert.equal(b.pick(counters), 2);
+    assert.equal(b.pick(counters), 3);
+    assert.equal(b.pick(counters), 0);
+  });
+
+  it('ignores counter values', () => {
+    const b = roundRobin();
+    assert.equal(b.pick([100, 0, 0]), 0); // does not pick the idle one
+    assert.equal(b.pick([0, 100, 0]), 1);
+  });
+
+  it('each instance has independent state', () => {
+    const b1 = roundRobin();
+    const b2 = roundRobin();
+    b1.pick([0, 0]); // b1 cursor: 1
+    assert.equal(b2.pick([0, 0]), 0); // b2 starts fresh
+  });
+});
-- 
2.51.2