Add left recursion check: quit early instead of going into an infinite loop

llama.cpp

@@ -13178,6 +13178,25 @@ static std::vector<llama_grammar_candidate> llama_grammar_reject_candidates(
 // grammar - external
 //
 
+enum detect_left_recursion_status {
+    // haven't searched this nonterminal
+    LLAMA_LEFT_REC_NOT_SEARCHED    = 0,
+
+    // searching this nonterminal in progress
+    LLAMA_LEFT_REC_IN_PROGRESS     = 1,
+
+    // finished searching this nonterminal
+    LLAMA_LEFT_REC_FINISHED_SEARCH = 2,
+
+    // detected a cycle
+    LLAMA_LEFT_REC_FOUND_CYCLE     = 3,
+};
+
+static void detect_left_recursion(
+        const std::vector<std::vector<llama_grammar_element>> & rules,
+        size_t                                                  rule_index,
+        std::vector<enum detect_left_recursion_status>        * rules_visited);
+
 struct llama_grammar * llama_grammar_init(
             const llama_grammar_element ** rules,
                                  size_t    n_rules,
@@ -13193,6 +13212,17 @@ struct llama_grammar * llama_grammar_init(
         vec_rules[i].push_back({LLAMA_GRETYPE_END, 0});
     }
 
+    // Check for left recursion
+    std::vector<enum detect_left_recursion_status> rules_visited(n_rules);
+    for (size_t i = 0; i < n_rules; i++) {
+        detect_left_recursion(vec_rules, i, &rules_visited);
+    }
+
+    auto iter = std::find(rules_visited.begin(), rules_visited.end(), LLAMA_LEFT_REC_FOUND_CYCLE);
+    if (iter != rules_visited.end()) {
+        throw std::runtime_error(format("unsupported grammar, left recursion detected for nonterminal at index %d", (int)(iter - rules_visited.begin())));
+    }
+
     // loop over alternates of start rule to build initial stacks
     std::vector<std::vector<const llama_grammar_element *>> stacks;
     pos = vec_rules[start_rule_index].data();
@@ -13215,9 +13245,51 @@ struct llama_grammar * llama_grammar_init(
         }
     } while (true);
 
+    // Important: vec_rules has to be moved here, not copied, because stacks contains
+    // pointers to elements of vec_rules. If vec_rules were copied into llama_grammar
+    // then the pointers would be invalidated when the local vec_rules goes out of scope.
     return new llama_grammar{ std::move(vec_rules), std::move(stacks), {} };
 }
 
+static void detect_left_recursion(
+        const std::vector<std::vector<llama_grammar_element>> & rules,
+        size_t                                                  rule_index,
+        std::vector<enum detect_left_recursion_status>        * rules_visited) {
+
+    int visit_status = (*rules_visited)[rule_index];
+    if (visit_status == LLAMA_LEFT_REC_IN_PROGRESS) {
+        // in progress -- we're in a cycle
+        (*rules_visited)[rule_index] = LLAMA_LEFT_REC_FOUND_CYCLE;
+        return;
+    } else if (visit_status == LLAMA_LEFT_REC_NOT_SEARCHED) {
+        // haven't visited yet. mark in progress, recurse, then mark complete.
+        // mark in progress
+        (*rules_visited)[rule_index] = LLAMA_LEFT_REC_IN_PROGRESS;
+
+        // recurse
+        const std::vector<llama_grammar_element> & rule = rules[rule_index];
+        size_t i = 0;
+        do {
+            if (rule[i].type == LLAMA_GRETYPE_RULE_REF) {
+                detect_left_recursion(rules, (size_t)rule[i].value, rules_visited);
+            }
+            while (!llama_grammar_is_end_of_sequence(&rule[i])) {
+                i++;
+            }
+            i++;
+        } while (i < rule.size());
+
+        // mark complete, but only if the recursive call didn't mark a cycle.
+        // that doesn't mean there's definitely no cycle *for this rule* -- the recursive call
+        // might have found a different cycle and stopped early.
+        if ((*rules_visited)[rule_index] == LLAMA_LEFT_REC_IN_PROGRESS) {
+            (*rules_visited)[rule_index] = LLAMA_LEFT_REC_FINISHED_SEARCH;
+        }
+    }
+
+    return;
+}
+
 void llama_grammar_free(struct llama_grammar * grammar) {
     delete grammar;
 }

tests/test-grammar-integration.cpp

@@ -28,6 +28,18 @@ static llama_grammar* build_grammar(const std::string & grammar_str) {
     return grammar;
 }
 
+static bool test_build_grammar_fails(const std::string & grammar_str) {
+    bool grammar_fails = false;
+    try {
+        build_grammar(grammar_str);
+        fprintf(stderr, "❌ Expected build failure, but succeeded: %s\n", grammar_str.c_str());
+    } catch (const std::exception & err) {
+        grammar_fails = true;
+        fprintf(stdout, "✅︎\n");
+    }
+    return grammar_fails;
+}
+
 static bool match_string(const std::string & input, llama_grammar* grammar) {
     auto decoded = decode_utf8(input, {});
 
@@ -320,6 +332,30 @@ number ::= [0-9]+)""";
     fprintf(stderr, "  ✅︎ Passed\n");
 }
 
+static void test_failure_left_recursion() {
+    fprintf(stderr, "⚫ Testing left recursion detection:\n");
+
+    // Test simple left recursion detection
+    const std::string simple_str = R"""(root ::= "a" | root "a")""";
+    assert(test_build_grammar_fails(simple_str));
+
+    // Test more complicated left recursion detection
+    const std::string medium_str = R"""(
+root ::= asdf
+asdf ::= "a" | asdf "a"
+)""";
+    assert(test_build_grammar_fails(medium_str));
+
+    // Test even more complicated left recursion detection
+    const std::string hard_str = R"""(
+root ::= asdf
+asdf ::= "a" | foo "b"
+foo ::= "c" | asdf "d" | "e")""";
+    assert(test_build_grammar_fails(hard_str));
+
+    fprintf(stderr, "  ✅︎ Passed\n");
+}
+
 int main() {
     fprintf(stdout, "Running grammar integration tests...\n");
     test_simple_grammar();
@@ -327,6 +363,7 @@ int main() {
     test_quantifiers();
     test_failure_missing_root();
     test_failure_missing_reference();
+    test_failure_left_recursion();
     fprintf(stdout, "All tests passed.\n");
     return 0;
 }