Add locale resolver with Language type and URL prefix parsing

src/i18n/resolver.test.ts

@@ -0,0 +1,74 @@
+import { describe, expect, it } from "vitest";
+import {
+  LANGUAGES,
+  isLanguage,
+  resolveLocaleFromPath,
+  stripLocaleFromPath,
+} from "./resolver.js";
+
+describe("LANGUAGES", () => {
+  it("contains exactly 9 supported languages", () => {
+    expect(LANGUAGES).toHaveLength(9);
+    expect(LANGUAGES).toContain("ru");
+    expect(LANGUAGES).toContain("en");
+    expect(LANGUAGES).toContain("de");
+  });
+});
+
+describe("isLanguage", () => {
+  it("returns true for valid languages", () => {
+    expect(isLanguage("ru")).toBe(true);
+    expect(isLanguage("en")).toBe(true);
+    expect(isLanguage("zh")).toBe(true);
+  });
+
+  it("returns false for invalid strings", () => {
+    expect(isLanguage("xx")).toBe(false);
+    expect(isLanguage("")).toBe(false);
+    expect(isLanguage("RU")).toBe(false);
+    expect(isLanguage("russian")).toBe(false);
+  });
+});
+
+describe("resolveLocaleFromPath", () => {
+  it("extracts locale from the first path segment", () => {
+    expect(resolveLocaleFromPath("/ru/onlineboard")).toBe("ru");
+    expect(resolveLocaleFromPath("/en/onlineboard/flight/SU100")).toBe("en");
+    expect(resolveLocaleFromPath("/de/schedule")).toBe("de");
+  });
+
+  it("returns null for paths without a valid locale prefix", () => {
+    expect(resolveLocaleFromPath("/onlineboard")).toBeNull();
+    expect(resolveLocaleFromPath("/xx/onlineboard")).toBeNull();
+    expect(resolveLocaleFromPath("/")).toBeNull();
+    expect(resolveLocaleFromPath("")).toBeNull();
+  });
+
+  it("handles bare locale path (e.g., /ru)", () => {
+    expect(resolveLocaleFromPath("/ru")).toBe("ru");
+    expect(resolveLocaleFromPath("/ru/")).toBe("ru");
+  });
+});
+
+describe("stripLocaleFromPath", () => {
+  it("strips locale and returns the rest", () => {
+    expect(stripLocaleFromPath("/ru/onlineboard")).toEqual({
+      locale: "ru",
+      rest: "/onlineboard",
+    });
+    expect(stripLocaleFromPath("/en/onlineboard/flight/SU100")).toEqual({
+      locale: "en",
+      rest: "/onlineboard/flight/SU100",
+    });
+  });
+
+  it("returns / as rest for bare locale path", () => {
+    expect(stripLocaleFromPath("/ru")).toEqual({ locale: "ru", rest: "/" });
+    expect(stripLocaleFromPath("/ru/")).toEqual({ locale: "ru", rest: "/" });
+  });
+
+  it("returns null for paths without a valid locale prefix", () => {
+    expect(stripLocaleFromPath("/onlineboard")).toBeNull();
+    expect(stripLocaleFromPath("/xx/schedule")).toBeNull();
+  });
+});

src/i18n/resolver.ts

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+export type Language = "ru" | "en" | "es" | "fr" | "it" | "ja" | "ko" | "zh" | "de";
+
+export const LANGUAGES: readonly Language[] = [
+  "ru", "en", "es", "fr", "it", "ja", "ko", "zh", "de",
+] as const;
+
+const languageSet: ReadonlySet<string> = new Set(LANGUAGES);
+
+export function isLanguage(x: string): x is Language {
+  return languageSet.has(x);
+}
+
+export function resolveLocaleFromPath(pathname: string): Language | null {
+  const segments = pathname.split("/").filter(Boolean);
+  const first = segments[0];
+  if (first !== undefined && isLanguage(first)) {
+    return first;
+  }
+  return null;
+}
+
+export function stripLocaleFromPath(
+  pathname: string,
+): { locale: Language; rest: string } | null {
+  const locale = resolveLocaleFromPath(pathname);
+  if (locale === null) return null;
+
+  const rest = pathname.slice(`/${locale}`.length);
+  return {
+    locale,
+    rest: rest === "" || rest === "/" ? "/" : rest,
+  };
+}