refactor: cache lowercasing all the locales (vercel#74038)

This improves the locale normalization method used in several looping
functions. This caches the lowercasing of locales so the operation is
only performed once.

It also duplicates the logic from the `I18nProvider` for the more
efficient locale handling.

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---------

Co-authored-by: Hendrik Liebau <[email protected]>

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 }
 }