From 58fe92c90f1060f2bc4cacc2d1a5fab14fde008b Mon Sep 17 00:00:00 2001
From: Eran Krakovsky <erank@ai21.com>
Date: Sun, 5 Jul 2026 11:21:58 +0300
Subject: [PATCH] fix: add RTL direction detection helpers

---
 apps/web/src/lib/rtl.test.ts | 49 ++++++++++++++++++++++++++++++++++++
 apps/web/src/lib/rtl.ts      | 40 +++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 apps/web/src/lib/rtl.test.ts
 create mode 100644 apps/web/src/lib/rtl.ts

diff --git a/apps/web/src/lib/rtl.test.ts b/apps/web/src/lib/rtl.test.ts
new file mode 100644
index 00000000000..f6c6db1f538
--- /dev/null
+++ b/apps/web/src/lib/rtl.test.ts
@@ -0,0 +1,49 @@
+import { describe, expect, it } from "vite-plus/test";
+import { containsRtl, dirFor, dirForMarkdown, isRtlText } from "./rtl";
+
+describe("isRtlText", () => {
+  const cases: Array<[string, boolean]> = [
+    ["שלום עולם", true],
+    ["مرحبا بالعالم", true],
+    ["سلام دنیا", true],
+    ["שלום hello עולם", true],
+    ["Note: بقية الجملة عربية طويلة جدا", true],
+    ["Hello world", false],
+    ["Hello world שלום", false],
+    ["", false],
+    ["12345 !@#$%", false],
+    ["ﭐﭑﭒ ﬠﬡ", true],
+  ];
+
+  for (const [input, expected] of cases) {
+    it(`${JSON.stringify(input)} -> ${expected}`, () => {
+      expect(isRtlText(input)).toBe(expected);
+    });
+  }
+});
+
+describe("containsRtl", () => {
+  it("detects RTL without classifying unrelated scripts as RTL", () => {
+    expect(containsRtl("hello שלום")).toBe(true);
+    expect(containsRtl("hello world")).toBe(false);
+    expect(containsRtl("")).toBe(false);
+    expect(containsRtl("中文")).toBe(false);
+    expect(containsRtl("あいう")).toBe(false);
+  });
+});
+
+describe("dirFor", () => {
+  it("returns rtl for RTL-majority text", () => {
+    expect(dirFor("שלום")).toBe("rtl");
+  });
+
+  it("returns ltr for LTR text", () => {
+    expect(dirFor("hi")).toBe("ltr");
+  });
+});
+
+describe("dirForMarkdown", () => {
+  it("ignores markdown code when deciding direction", () => {
+    expect(dirForMarkdown("- **`apps/web`**: אפליקציית React שמציגה צ'אט")).toBe("rtl");
+  });
+});
diff --git a/apps/web/src/lib/rtl.ts b/apps/web/src/lib/rtl.ts
new file mode 100644
index 00000000000..1cf444adc49
--- /dev/null
+++ b/apps/web/src/lib/rtl.ts
@@ -0,0 +1,40 @@
+function isRtlChar(c: number): boolean {
+  return (
+    (c >= 0x0590 && c <= 0x05ff) ||
+    (c >= 0x0600 && c <= 0x06ff) ||
+    (c >= 0x0750 && c <= 0x077f) ||
+    (c >= 0x08a0 && c <= 0x08ff) ||
+    (c >= 0xfb1d && c <= 0xfdff) ||
+    (c >= 0xfe70 && c <= 0xfeff)
+  );
+}
+
+function isLtrChar(c: number): boolean {
+  return (c >= 0x41 && c <= 0x5a) || (c >= 0x61 && c <= 0x7a) || (c >= 0xc0 && c <= 0x024f);
+}
+
+export function isRtlText(text: string): boolean {
+  let rtl = 0;
+  let ltr = 0;
+  for (let i = 0; i < text.length; i++) {
+    const c = text.charCodeAt(i);
+    if (isRtlChar(c)) rtl++;
+    else if (isLtrChar(c)) ltr++;
+  }
+  if (rtl === 0 && ltr === 0) return false;
+  return rtl > ltr;
+}
+
+export function containsRtl(text: string): boolean {
+  return Array.from(text).some((char) => isRtlChar(char.charCodeAt(0)));
+}
+
+export function dirFor(text: string): "rtl" | "ltr" {
+  return isRtlText(text) ? "rtl" : "ltr";
+}
+
+const MARKDOWN_DIRECTION_NEUTRAL_RE = /```[\s\S]*?(?:```|$)|`[^`\n]*`|https?:\/\/\S+/g;
+
+export function dirForMarkdown(text: string): "rtl" | "ltr" {
+  return dirFor(text.replace(MARKDOWN_DIRECTION_NEUTRAL_RE, ""));
+}