LibRegex: Don't repeat the same fork again

If some state has already been tried, skip over it as it would never
lead to a match regardless.
This fixes performance/memory issues in cases like
/(a+)+b/.exec("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
or
/(a|a?)+b/...

Fixes #2622.

Libraries/LibRegex/RegexMatch.h

@@ -561,6 +561,33 @@ struct MatchState {
     COWVector<Vector<Match>> capture_group_matches;
     COWVector<u64> repetition_marks;
     Vector<u64, 64> checkpoints;
+
+    // For size_t in {0..100}, ips in {0..500} and repetitions in {0..30}, there are zero collisions.
+    // For the full range, zero collisions were found in 8 million random samples.
+    u64 u64_hash() const
+    {
+        u64 hash = 0xcbf29ce484222325;
+        auto combine = [&hash](auto value) {
+            hash ^= value + 0x9e3779b97f4a7c15 + (hash << 6) + (hash >> 2);
+        };
+        auto combine_vector = [&hash](auto const& vector) {
+            for (auto& value : vector) {
+                hash ^= value;
+                hash *= 0x100000001b3;
+            }
+        };
+
+        combine(string_position_before_match);
+        combine(string_position);
+        combine(string_position_in_code_units);
+        combine(instruction_position);
+        combine(fork_at_position);
+        combine(initiating_fork.value_or(0) + initiating_fork.has_value());
+        combine_vector(repetition_marks);
+        combine_vector(checkpoints);
+
+        return hash;
+    }
 };
 
 }

Libraries/LibRegex/RegexMatcher.cpp

@@ -468,6 +468,7 @@ bool Matcher<Parser>::execute(MatchInput const& input, MatchState& state, size_t
     }
 
     BumpAllocatedLinkedList<MatchState> states_to_try_next;
+    HashTable<u64> seen_state_hashes;
 #if REGEX_DEBUG
     size_t recursion_level = 0;
 #endif
@@ -545,17 +546,34 @@ bool Matcher<Parser>::execute(MatchInput const& input, MatchState& state, size_t
             continue;
         case ExecutionResult::Succeeded:
             return true;
-        case ExecutionResult::Failed:
+        case ExecutionResult::Failed: {
-            if (!states_to_try_next.is_empty()) {
+            bool found = false;
+            while (!states_to_try_next.is_empty()) {
                 state = states_to_try_next.take_last();
+                if (auto hash = state.u64_hash(); seen_state_hashes.set(hash) != HashSetResult::InsertedNewEntry) {
+                    dbgln_if(REGEX_DEBUG, "Already seen state, skipping: {}", hash);
+                    continue;
+                }
+                found = true;
+                break;
+            }
+            if (found)
                 continue;
-            }
             return false;
+        }
         case ExecutionResult::Failed_ExecuteLowPrioForks: {
-            if (states_to_try_next.is_empty()) {
+            bool found = false;
-                return false;
+            while (!states_to_try_next.is_empty()) {
+                state = states_to_try_next.take_last();
+                if (auto hash = state.u64_hash(); seen_state_hashes.set(hash) != HashSetResult::InsertedNewEntry) {
+                    dbgln_if(REGEX_DEBUG, "Already seen state, skipping: {}", hash);
+                    continue;
+                }
+                found = true;
+                break;
             }
-            state = states_to_try_next.take_last();
+            if (!found)
+                return false;
 #if REGEX_DEBUG
             ++recursion_level;
 #endif

Tests/LibRegex/Regex.cpp

@@ -1004,9 +1004,21 @@ static auto g_lots_of_a_s = ByteString::repeated('a', 10'000'000);
 
 BENCHMARK_CASE(fork_performance)
 {
-    Regex<ECMA262> re("(?:aa)*");
+    {
-    auto result = re.match(g_lots_of_a_s);
+        Regex<ECMA262> re("(?:aa)*");
-    EXPECT_EQ(result.success, true);
+        auto result = re.match(g_lots_of_a_s);
+        EXPECT_EQ(result.success, true);
+    }
+    {
+        Regex<ECMA262> re("(a+)+b");
+        auto result = re.match(g_lots_of_a_s.substring_view(0, 100));
+        EXPECT_EQ(result.success, false);
+    }
+    {
+        Regex<ECMA262> re("^(a|a?)+$");
+        auto result = re.match(ByteString::formatted("{}b", g_lots_of_a_s.substring_view(0, 100)));
+        EXPECT_EQ(result.success, false);
+    }
 }
 
 BENCHMARK_CASE(anchor_performance)