Improve performance of String.prototype.includes

Summary:
Our current implmentation extracts every substring (of the source string) that
matches the length of the search string and check to see if any of them match.

This change just does one `std::search` of the search string in the source
string.

Reviewed By: avp

Differential Revision: D18359912

fbshipit-source-id: 24ce96f777aab2e0e6ab789f59c1be5030002dbc

lib/VM/JSLib/String.cpp

@@ -2080,12 +2080,17 @@ CallResult<HermesValue> stringPrototypeIncludesOrStartsWith(
   // than searchLen, the code unit at index k+j of S is the same as the code
   // unit at index j of searchStr, return true; but if there is no such integer
   // k, return false.
-  for (double k = start; k + searchLength <= len; ++k) {
-    if (S->sliceEquals(k, searchLength, *searchStr)) {
-      return HermesValue::encodeBoolValue(true);
-    }
-  }
-  return HermesValue::encodeBoolValue(false);
+  auto SView = StringPrimitive::createStringView(runtime, S);
+  auto searchStrView = StringPrimitive::createStringView(runtime, searchStr);
+  auto foundIter = std::search(
+      SView.begin() + start,
+      SView.end(),
+      searchStrView.begin(),
+      searchStrView.end());
+  // Note: searchStrView.empty check is needed in the special case that S is
+  // empty, searchStr is empty, and start = 0
+  return HermesValue::encodeBoolValue(
+      foundIter != SView.end() || searchStrView.empty());
 }
 
 /// Shared implementation of string.indexOf and string.lastIndexOf