refactor: cache lowercasing all the locales

packages/next/src/shared/lib/i18n/normalize-locale-path.ts

@@ -3,6 +3,13 @@ export interface PathLocale {
   pathname: string
 }
 
+/**
+ * A cache of lowercased locales for each list of locales. This is stored as a
+ * WeakMap so if the locales are garbage collected, the cache entry will be
+ * removed as well.
+ */
+const cache = new WeakMap<readonly string[], readonly string[]>()
+
 /**
  * For a pathname that may include a locale from a list of locales, it
  * removes the locale from the pathname returning it alongside with the
@@ -16,25 +23,39 @@ export function normalizeLocalePath(
   pathname: string,
   locales?: readonly string[]
 ): PathLocale {
-  let detectedLocale: string | undefined
-  // first item will be empty string from splitting at first char
-  const pathnameParts = pathname.split('/')
-
-  ;(locales || []).some((locale) => {
-    if (
-      pathnameParts[1] &&
-      pathnameParts[1].toLowerCase() === locale.toLowerCase()
-    ) {
-      detectedLocale = locale
-      pathnameParts.splice(1, 1)
-      pathname = pathnameParts.join('/') || '/'
-      return true
-    }
-    return false
-  })
-
-  return {
-    pathname,
-    detectedLocale,
+  // If locales is undefined, return the pathname as is.
+  if (!locales) return { pathname }
+
+  // Get the cached lowercased locales or create a new cache entry.
+  let lowercasedLocales = cache.get(locales)
+  if (!lowercasedLocales) {
+    lowercasedLocales = locales.map((locale) => locale.toLowerCase())
+    cache.set(locales, lowercasedLocales)
   }
+
+  let detectedLocale: string | undefined
+
+  // The first segment will be empty, because it has a leading `/`. If
+  // there is no further segment, there is no locale (or it's the default).
+  const segments = pathname.split('/', 2)
+
+  // If there's no second segment (ie, the pathname is just `/`), there's no
+  // locale.
+  if (!segments[1]) return { pathname }
+
+  // The second segment will contain the locale part if any.
+  const segment = segments[1].toLowerCase()
+
+  // See if the segment matches one of the locales. If it doesn't, there is
+  // no locale (or it's the default).
+  const index = lowercasedLocales.indexOf(segment)
+  if (index < 0) return { pathname }
+
+  // Return the case-sensitive locale.
+  detectedLocale = locales[index]
+
+  // Remove the `/${locale}` part of the pathname.
+  pathname = pathname.slice(detectedLocale.length + 1) || '/'
+
+  return { pathname, detectedLocale }
 }