Experiment with a compact mode for simple preloaded entries.

net/tools/transport_security_state_generator/trie/trie_writer.cc

Index: net/tools/transport_security_state_generator/trie/trie_writer.cc
diff --git a/net/tools/transport_security_state_generator/trie/trie_writer.cc b/net/tools/transport_security_state_generator/trie/trie_writer.cc
index 3dd8163bb7de20ce043c944365ed5c711dbbcafc..0f92d5be611c2b832d319dc0e296c5653aaf0574 100644
--- a/net/tools/transport_security_state_generator/trie/trie_writer.cc
+++ b/net/tools/transport_security_state_generator/trie/trie_writer.cc
@@ -20,6 +20,14 @@ bool CompareReversedEntries(const std::unique_ptr<ReversedEntry>& lhs,
   return lhs->reversed_name < rhs->reversed_name;
 }
 
+// Returns true if the entry only configures HSTS with includeSubdomains.
+// Such entries, when written, can be represented more compactly, and thus
+// reduce the overall size of the trie.
+bool IsSimpleEntry(const TransportSecurityStateEntry* entry) {
+  return entry->force_https && entry->include_subdomains &&
+         entry->pinset.empty() && !entry->expect_ct && !entry->expect_staple;
+}
+
 }  // namespace
 
 ReversedEntry::ReversedEntry(std::vector<uint8_t> reversed_name,
@@ -124,6 +132,13 @@ bool TrieWriter::WriteDispatchTables(ReversedEntries::iterator start,
 
 bool TrieWriter::WriteEntry(const TransportSecurityStateEntry* entry,
                             TrieBitBuffer* writer) {
+  if (IsSimpleEntry(entry)) {
+    writer->WriteBit(1);
+    return true;
+  } else {
+    writer->WriteBit(0);
+  }
+
   uint8_t include_subdomains = 0;
   if (entry->include_subdomains) {
     include_subdomains = 1;