1 files changed, 105 insertions, 0 deletions
diff --git a/src/background/completion/impl/PrefetchAndCache.ts b/src/background/completion/impl/PrefetchAndCache.ts
new file mode 100644
index 0000000..3c074c2
--- /dev/null
+++ b/src/background/completion/impl/PrefetchAndCache.ts
@@ -0,0 +1,105 @@
+type Getter<T> = (query: string) => Promise<T[]>;
+type Filter<T> = (src: T[], query: string) => T[];
+
+const WHITESPACE = /\s/;
+
+// `shortKey` returns a shorten key to pre-fetch completions and store in the
+// cache.  The shorten key is generated by the following rules:
+// 
+// 1. If the query contains a space in the middle: i.e. the query consists of
+//    multiple words, the method removes the last word from the query, and
+//    returns joined remaining words with space.
+// 
+// 2. If the query is a single word and it's an URL, the method returns a new
+//    URL excluding search query with the upper path of the original URL.
+// 
+// 3. If the query is a single word and it's not an URL, the method returns a
+//    word with the half-length of the original query.
+//
+//  Examples:
+//
+//    shortKey("hello world good bye")
+//    => "hello world good"
+//
+//    shortKey("https://example.com/path/to/resource?q=hello")
+//    => "https://example.com/path/to/"
+//
+//    shortKey("the-query-with-super-long-word")
+//    => "the-query-with-"
+//
+export const shortKey = (query: string): string => {
+  if (WHITESPACE.test(query)) {
+      return query.split(WHITESPACE).filter(word => word.length > 0).slice(0, -1).join(' ');
+  }
+  let url;
+  try {
+    url = new URL(query)
+  } catch (e) {
+    return query.slice(0, query.length / 2);
+  }
+
+  if (url.origin === query) {
+    // may be on typing or removing URLs such as "such as https://goog"
+    return query.slice(0, query.length / 2);
+  }
+  if (url.pathname.endsWith('/')) {
+    // remove parameters and move to upper path
+    return new URL('..', url).href;
+  }
+  // remove parameters
+  return new URL('.', url).href;
+};
+
+export default class PrefetchAndCache<T> {
+  private shortKey: string | undefined;
+
+  private shortKeyCache: T[] = [];
+
+  constructor(
+    private getter: Getter<T>,
+    private filter: Filter<T>,
+    private prefetchThrethold: number = 1,
+  ) {
+  }
+
+  async get(query: string): Promise<T[]> {
+    query = query.trim();
+    if (query.length < this.prefetchThrethold) {
+      this.shortKey = undefined;
+      return this.getter(query);
+    }
+
+    if (this.needToRefresh(query)) {
+      this.shortKey = shortKey(query);
+      this.shortKeyCache = await this.getter(this.shortKey);
+    }
+    return this.filter(this.shortKeyCache, query);
+  }
+
+  private needToRefresh(query: string): boolean {
+    if (!this.shortKey) {
+      // no cache
+      return true
+    }
+
+    if (query.length < this.shortKey.length) {
+      // query: "hello"
+      // cache: "hello_world"
+      return true;
+    }
+
+    if (!query.startsWith(this.shortKey)) {
+        // queyr:   "hello_w"
+        // shorten: "hello_morning"
+        return true
+    }
+
+    if (query.slice(this.shortKey.length).includes(' ')) {
+      // queyr:   "hello x"
+      // shorten: "hello"
+      return true;
+    }
+
+    return false
+  }
+}