[Feature] Cover JSON schema string format

cpp/grammar_builder.h

@@ -28,6 +28,15 @@ class GrammarBuilder {
   /*! \brief Default constructor. Creates a new grammar object. */
   GrammarBuilder() : grammar_(std::make_shared<Grammar::Impl>()) {}
 
+  /*! \brief Constructor. Creates a new grammar object from an existing grammar. */
+  GrammarBuilder(const Grammar& grammar)
+      : grammar_(std::make_shared<Grammar::Impl>(*grammar.operator->())) {
+    for (int i = 0; i < static_cast<int>(grammar->NumRules()); ++i) {
+      auto rule = grammar->GetRule(i);
+      rule_name_to_id_[rule.name] = i;
+    }
+  }
+
   /*!
    * \brief Get the result grammar. This function will also set the root rule to the rule with the
    * specified name. The rule should be already added to the grammar.

cpp/grammar_compiler.cc

@@ -249,7 +249,7 @@ bool GrammarMatcherForTokenMaskCache::IsTokenPassLookaheadAssertion(
 
   // Find all positions that can come to and end. Then check if the suffix from that position
   // can be accepted by the lookahead assertion.
-  for (int i = static_cast<int>(can_reach_end_stack.size()); i >= 0; --i) {
+  for (int i = static_cast<int>(can_reach_end_stack.size()) - 1; i >= 0; --i) {
     if (!can_reach_end_stack[i]) {
       continue;
     }

cpp/grammar_functor.cc

@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <queue>
+#include <set>
 #include <unordered_set>
 #include <vector>
 
@@ -284,7 +285,17 @@ class NestedRuleUnwrapper : public GrammarMutator {
   }
 };
 
-class ByteStringFuser : public GrammarMutator {
+class StructureNormalizerImpl : public GrammarMutator {
+ public:
+  using GrammarMutator::Apply;
+  using GrammarMutator::GrammarMutator;
+
+  Grammar Apply(const Grammar& grammar) final {
+    return NestedRuleUnwrapper().Apply(SingleElementExprEliminator().Apply(grammar));
+  }
+};
+
+class ByteStringFuserImpl : public GrammarMutator {
  public:
   using GrammarMutator::Apply;
   using GrammarMutator::GrammarMutator;
@@ -316,6 +327,249 @@ class ByteStringFuser : public GrammarMutator {
     return builder_.AddSequence(new_sequence_ids);
   }
 };
+
+class RuleInlinerImpl : public GrammarMutator {
+ public:
+  using GrammarMutator::Apply;
+  using GrammarMutator::GrammarMutator;
+
+ private:
+  int32_t VisitChoices(const RuleExpr& rule_expr) final {
+    std::vector<int32_t> new_choice_ids;
+    for (int i : rule_expr) {
+      auto choice_expr = base_grammar_->GetRuleExpr(i);
+      if (choice_expr.type == RuleExprType::kEmptyStr) {
+        new_choice_ids.push_back(VisitExpr(i));
+        continue;
+      }
+      XGRAMMAR_ICHECK(choice_expr.type == RuleExprType::kSequence);
+      auto first_element = base_grammar_->GetRuleExpr(choice_expr[0]);
+      if (first_element.type != RuleExprType::kRuleRef) {
+        new_choice_ids.push_back(VisitExpr(choice_expr));
+        continue;
+      }
+      auto rule_ref_id = first_element[0];
+      if (can_rule_be_inlined_.count(rule_ref_id) == 0) {
+        can_rule_be_inlined_[rule_ref_id] = CheckIfRuleCanBeInlined(rule_ref_id);
+      }
+      if (!can_rule_be_inlined_[rule_ref_id]) {
+        new_choice_ids.push_back(VisitExpr(choice_expr));
+        continue;
+      }
+
+      // Do inlining
+      std::vector<int32_t> other_elements;
+      for (int i = 1; i < choice_expr.size(); ++i) {
+        other_elements.push_back(VisitExpr(choice_expr[i]));
+      }
+
+      auto ref_rule = base_grammar_->GetRule(rule_ref_id);
+      auto ref_rule_expr = base_grammar_->GetRuleExpr(ref_rule.body_expr_id);
+
+      for (auto ref_choice_id : ref_rule_expr) {
+        auto ref_choice_expr = base_grammar_->GetRuleExpr(ref_choice_id);
+        XGRAMMAR_ICHECK(ref_choice_expr.type == RuleExprType::kSequence);
+        std::vector<int32_t> choice_to_add;
+        for (auto ref_element_id : ref_choice_expr) {
+          choice_to_add.push_back(VisitExpr(ref_element_id));
+        }
+        choice_to_add.insert(choice_to_add.end(), other_elements.begin(), other_elements.end());
+        new_choice_ids.push_back(builder_.AddSequence(choice_to_add));
+      }
+    }
+    return builder_.AddChoices(new_choice_ids);
+  }
+
+  /**
+   * The rule should be: a sequence of choices, cannot be empty, cannot refer to other rules
+   */
+  bool CheckIfRuleCanBeInlined(int32_t rule_id) {
+    auto rule = base_grammar_->GetRule(rule_id);
+    auto rule_expr = base_grammar_->GetRuleExpr(rule.body_expr_id);
+    if (rule_expr.type != RuleExprType::kChoices) {
+      return false;
+    }
+    if (rule_expr.size() == 0) {
+      return false;
+    }
+    for (auto choice_id : rule_expr) {
+      auto choice_expr = base_grammar_->GetRuleExpr(choice_id);
+      if (choice_expr.type == RuleExprType::kEmptyStr) {
+        return false;
+      }
+      XGRAMMAR_ICHECK(choice_expr.type == RuleExprType::kSequence);
+      for (auto element_id : choice_expr) {
+        auto element_expr = base_grammar_->GetRuleExpr(element_id);
+        if (element_expr.type == RuleExprType::kRuleRef) {
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
+  std::unordered_map<int32_t, bool> can_rule_be_inlined_;
+};
+
+/*!
+ * \brief Analyze all referenced rules or the main rule. Return a list of all referenced rule ids.
+ * This is useful for dead code elimination.
+ */
+class UsedRulesAnalyzer : public GrammarVisitor<std::vector<int32_t>> {
+ public:
+  UsedRulesAnalyzer() = default;
+
+  std::vector<int32_t> Apply(const Grammar& grammar) final {
+    base_grammar_ = grammar;
+
+    std::set<int32_t> visited;
+
+    std::queue<int32_t>().swap(visit_queue_);
+
+    visit_queue_.push(base_grammar_->GetRootRuleId());
+    while (!visit_queue_.empty()) {
+      auto rule_id = visit_queue_.front();
+      visit_queue_.pop();
+      if (visited.count(rule_id)) {
+        continue;
+      }
+      visited.insert(rule_id);
+      auto rule = base_grammar_->GetRule(rule_id);
+      VisitExpr(rule.body_expr_id);
+    }
+
+    return std::vector<int32_t>(visited.begin(), visited.end());
+  }
+
+  void VisitTagDispatch(const RuleExpr& rule_expr) {
+    for (int i = 0; i < rule_expr.size(); i += 2) {
+      visit_queue_.push(rule_expr[i + 1]);
+    }
+  }
+
+  void VisitRuleRef(const RuleExpr& rule_expr) { visit_queue_.push(rule_expr[0]); }
+
+ private:
+  std::queue<int32_t> visit_queue_;
+};
+
+class DeadCodeEliminatorImpl : public GrammarMutator {
+ public:
+  using GrammarMutator::Apply;
+  using GrammarMutator::GrammarMutator;
+
+  Grammar Apply(const Grammar& grammar) final {
+    Init(grammar);
+    auto used_rules = UsedRulesAnalyzer().Apply(grammar);
+    rule_id_map_.clear();
+    for (auto rule_id : used_rules) {
+      rule_id_map_[rule_id] = builder_.AddEmptyRule(grammar->GetRule(rule_id).name);
+    }
+    for (auto rule_id : used_rules) {
+      auto rule = grammar->GetRule(rule_id);
+      auto new_body_expr_id = VisitExpr(rule.body_expr_id);
+      builder_.UpdateRuleBody(rule_id_map_[rule_id], new_body_expr_id);
+      builder_.AddLookaheadAssertion(
+          rule_id_map_[rule_id], VisitLookaheadAssertion(rule.lookahead_assertion_id)
+      );
+    }
+    XGRAMMAR_CHECK(rule_id_map_.count(grammar->GetRootRuleId()) > 0);
+    return builder_.Get(rule_id_map_[grammar->GetRootRuleId()]);
+  }
+
+  int32_t VisitTagDispatch(const RuleExpr& rule_expr) final {
+    std::vector<std::pair<int32_t, int32_t>> tag_dispatch_list;
+    for (int i = 0; i < rule_expr.size(); i += 2) {
+      XGRAMMAR_DCHECK(rule_id_map_.count(rule_expr[i + 1]) > 0);
+      auto new_rule_id = rule_id_map_[rule_expr[i + 1]];
+      tag_dispatch_list.push_back({VisitExpr(rule_expr[i]), new_rule_id});
+    }
+    return builder_.AddTagDispatch(tag_dispatch_list);
+  }
+
+  int32_t VisitRuleRef(const RuleExpr& rule_expr) final {
+    XGRAMMAR_DCHECK(rule_id_map_.count(rule_expr[0]) > 0);
+    auto new_rule_id = rule_id_map_[rule_expr[0]];
+    return builder_.AddRuleRef(new_rule_id);
+  }
+
+ private:
+  std::unordered_map<int32_t, int32_t> rule_id_map_;
+};
+
+class LookaheadAssertionAnalyzerImpl : public GrammarMutator {
+ public:
+  using GrammarMutator::GrammarMutator;
+
+  Grammar Apply(const Grammar& grammar) final {
+    InitWithCopy(grammar);
+    auto root_rule = grammar->GetRootRule();
+    auto root_rule_expr = base_grammar_->GetRuleExpr(root_rule.body_expr_id);
+    if (root_rule_expr.type == RuleExprType::kTagDispatch) {
+      return grammar;
+    }
+    for (int i = 0; i < static_cast<int>(grammar->NumRules()); ++i) {
+      auto rule = grammar->GetRule(i);
+      if (i == grammar->GetRootRuleId() || rule.lookahead_assertion_id != -1) {
+        continue;
+      }
+      auto look_head_assertion_id = DetectLookaheadAssertion(i);
+      if (look_head_assertion_id != -1) {
+        builder_.AddLookaheadAssertion(i, look_head_assertion_id);
+      }
+    }
+    return builder_.Get(grammar->GetRootRuleId());
+  }
+
+  int32_t DetectLookaheadAssertion(int32_t rule_id) {
+    std::vector<int32_t> found_sequence;  // Element ids
+    bool found = false;
+    for (int i = 0; i < static_cast<int>(base_grammar_->NumRules()); ++i) {
+      auto rule = base_grammar_->GetRule(i);
+      auto rule_expr = base_grammar_->GetRuleExpr(rule.body_expr_id);
+      if (rule_expr.type == RuleExprType::kTagDispatch) {
+        for (int j = 1; j < rule_expr.size(); j += 2) {
+          if (rule_expr[j] == rule_id) {
+            return -1;
+          }
+        }
+        continue;
+      }
+      XGRAMMAR_DCHECK(rule_expr.type == RuleExprType::kChoices);
+      for (auto sequence_id : rule_expr) {
+        auto sequence_expr = base_grammar_->GetRuleExpr(sequence_id);
+        if (sequence_expr.type != RuleExprType::kSequence) {
+          continue;
+        }
+        auto last_element = base_grammar_->GetRuleExpr(sequence_expr.end()[-1]);
+        if (last_element.type == RuleExprType::kRuleRef && last_element[0] == rule_id &&
+            i != rule_id) {
+          return -1;
+        }
+
+        for (int j = 0; j < sequence_expr.size() - 1; ++j) {
+          auto element_expr = base_grammar_->GetRuleExpr(sequence_expr[j]);
+          if (element_expr.type != RuleExprType::kRuleRef || element_expr[0] != rule_id) {
+            continue;
+          }
+          if (found) {
+            return -1;
+          }
+          found = true;
+          for (int k = j + 1; k < sequence_expr.size(); ++k) {
+            found_sequence.push_back(sequence_expr[k]);
+          }
+        }
+      }
+    }
+
+    if (!found) {
+      return -1;
+    }
+    return builder_.AddSequence(found_sequence);
+  }
+};
+
 /*!
  * \brief A class that normalizes a grammar by applying a series of transformations.
  *
@@ -341,9 +595,11 @@ class GrammarNormalizerImpl : public GrammarMutator {
   // Return the list of all normalizers in the class. The normalizers are applied one by one.
   std::vector<std::unique_ptr<GrammarMutator>> GetNormalizerList() {
     std::vector<std::unique_ptr<GrammarMutator>> normalizer_mutators;
-    normalizer_mutators.emplace_back(std::make_unique<SingleElementExprEliminator>());
-    normalizer_mutators.emplace_back(std::make_unique<NestedRuleUnwrapper>());
-    normalizer_mutators.emplace_back(std::make_unique<ByteStringFuser>());
+    normalizer_mutators.emplace_back(std::make_unique<StructureNormalizerImpl>());
+    normalizer_mutators.emplace_back(std::make_unique<ByteStringFuserImpl>());
+    normalizer_mutators.emplace_back(std::make_unique<RuleInlinerImpl>());
+    normalizer_mutators.emplace_back(std::make_unique<DeadCodeEliminatorImpl>());
+    normalizer_mutators.emplace_back(std::make_unique<LookaheadAssertionAnalyzerImpl>());
     return normalizer_mutators;
   }
 };
@@ -515,8 +771,8 @@ class AllowEmptyRuleAnalyzerImpl : public GrammarVisitor<std::vector<int32_t>> {
     std::unordered_set<int32_t> empty_rule_id_set;
     FindExplicitEmptyRules(&empty_rule_id_set);
 
-    // Step 2: Find rules that indirectly allow empty string. Using the Bellman-Ford algorithm on
-    // the rule reference graph.
+    // Step 2: Find rules that indirectly allow empty string. Using the Bellman-Ford algorithm
+    // on the rule reference graph.
     std::vector<std::vector<int32_t>> rule_ref_graph = RuleRefGraphFinder().Apply(grammar);
     FindIndirectEmptyRules(&empty_rule_id_set, rule_ref_graph);
 
@@ -703,4 +959,22 @@ Grammar StructuralTagGrammarCreator::Apply(
   return StructuralTagGrammarCreatorImpl().Apply(triggers, tag_groups);
 }
 
+Grammar RuleInliner::Apply(const Grammar& grammar) { return RuleInlinerImpl().Apply(grammar); }
+
+Grammar ByteStringFuser::Apply(const Grammar& grammar) {
+  return ByteStringFuserImpl().Apply(grammar);
+}
+
+Grammar DeadCodeEliminator::Apply(const Grammar& grammar) {
+  return DeadCodeEliminatorImpl().Apply(grammar);
+}
+
+Grammar StructureNormalizer::Apply(const Grammar& grammar) {
+  return StructureNormalizerImpl().Apply(grammar);
+}
+
+Grammar LookaheadAssertionAnalyzer::Apply(const Grammar& grammar) {
+  return LookaheadAssertionAnalyzerImpl().Apply(grammar);
+}
+
 }  // namespace xgrammar