Use a fixed flat set to store stopwords.

This is more memory efficient than std::unordered_set and requires no
heap allocations. However, since the list of stop words is quite long,
sorting it dynamically at compile time exceeds the maximum number of
steps allowed in constexpr evaluation. To work around this, add
MakeFixedFlatSetSorted() which requires already sorted input as an
analogue to the already-existing MakeFixedFlatMapSorted().

Change-Id: I93e3cc685950ade5a1f054debb4638e5e1fc5d95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4660000
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiangdong Kong <xiangdongkong@google.com>
Cr-Commit-Position: refs/heads/main@{#1164820}

base/containers/fixed_flat_set.h

@@ -74,6 +74,25 @@ namespace base {
 template <class Key, size_t N, class Compare = std::less<>>
 using fixed_flat_set = base::flat_set<Key, Compare, std::array<const Key, N>>;
 
+// Utility function to simplify constructing a fixed_flat_set from a fixed list
+// of keys and values. Requires that the passed in `data` contains unique keys
+// and be sorted by key. See `MakeFixedFlatSet` for a variant that sorts the
+// input automatically.
+//
+// Example usage:
+//   constexpr auto kSet = base::MakeFixedFlatSetSorted<base::StringPiece>(
+//       {"bar", "baz", "foo", "qux"});
+template <class Key, size_t N, class Compare = std::less<>>
+constexpr fixed_flat_set<Key, N, Compare> MakeFixedFlatSetSorted(
+    std::common_type_t<Key> (&&data)[N],
+    const Compare& comp = Compare()) {
+  CHECK(internal::is_sorted_and_unique(data, comp));
+  // Specify the value_type explicitly to ensure that the returned array has
+  // immutable keys.
+  return fixed_flat_set<Key, N, Compare>(
+      sorted_unique, internal::ToArray<const Key>(data), comp);
+}
+
 // Utility function to simplify constructing a fixed_flat_set from a fixed list
 // of keys. Requires that the passed in `data` contains unique keys.
 //