common/peg : refactor until gbnf grammar generation (#24839)

* common/peg : refactor until gbnf grammar into an ac automaton

* cont : add a test with multiple strings

* cont : pad state with 0s so rules line up

* cont : clean up comments

* cont : use set everywhere

* cont : inline state num string padding

* cont : add a ref to PR

* cont : fix regression in server-tools.cpp

common/peg-parser.cpp

@@ -6,13 +6,14 @@
 #include "unicode.h"
 
 #include <algorithm>
+#include <deque>
 #include <initializer_list>
 #include <map>
 #include <memory>
 #include <nlohmann/json.hpp>
 #include <regex>
+#include <set>
 #include <stdexcept>
-#include <unordered_set>
 
 // Trick to catch missing branches
 template <typename T>
@@ -88,40 +89,7 @@ struct trie {
         return match_result{match_result::NO_MATCH};
     }
 
-    struct prefix_and_next {
-        std::vector<uint32_t> prefix;
-        std::vector<uint32_t> next_chars;
-    };
-
-    std::vector<prefix_and_next> collect_prefix_and_next() {
-        std::vector<uint32_t>        prefix;
-        std::vector<prefix_and_next> result;
-        collect_prefix_and_next(0, prefix, result);
-        return result;
-    }
-
   private:
-    void collect_prefix_and_next(size_t index, std::vector<uint32_t> & prefix, std::vector<prefix_and_next> & out) {
-        if (!nodes[index].is_word) {
-            if (!nodes[index].children.empty()) {
-                std::vector<uint32_t> chars;
-                chars.reserve(nodes[index].children.size());
-                for (const auto & p : nodes[index].children) {
-                    chars.push_back(p.first);
-                }
-                out.emplace_back(prefix_and_next{prefix, chars});
-            }
-        }
-
-        for (const auto & p : nodes[index].children) {
-            uint32_t ch = p.first;
-            auto child = p.second;
-            prefix.push_back(ch);
-            collect_prefix_and_next(child, prefix, out);
-            prefix.pop_back();
-        }
-    }
-
     size_t create_node() {
         size_t index = nodes.size();
         nodes.emplace_back();
@@ -153,6 +121,65 @@ struct trie {
     }
 };
 
+// Aho-Corasick automaton
+struct aho_corasick {
+    trie                t;
+    std::vector<size_t> fail;      // failure links
+    std::vector<size_t> order;     // states in BFS order
+    std::vector<bool>   terminal;  // match states (directly or via a suffix link)
+    std::set<uint32_t>  alphabet;  // every character with a transition
+
+    aho_corasick(const std::vector<std::string> & strings) : t(strings) {
+        const auto & nodes = t.nodes;
+        const size_t n = nodes.size();
+
+        fail.assign(n, 0);
+        order.reserve(n);
+
+        std::deque<size_t> queue{ 0 };
+        while (!queue.empty()) {
+            size_t u = queue.front();
+            queue.pop_front();
+            order.push_back(u);
+            for (const auto & [ch, v] : nodes[u].children) {
+                if (u != 0) {
+                    size_t f = fail[u];
+                    while (f && nodes[f].children.find(ch) == nodes[f].children.end()) {
+                        f = fail[f];
+                    }
+                    auto it = nodes[f].children.find(ch);
+                    fail[v] = (it != nodes[f].children.end() && it->second != v) ? it->second : 0;
+                }
+                queue.push_back(v);
+            }
+        }
+
+        terminal.assign(n, false);
+        for (size_t u : order) {
+            terminal[u] = nodes[u].is_word || (u != 0 && terminal[fail[u]]);
+        }
+
+        for (const auto & node : nodes) {
+            for (const auto & [ch, v] : node.children) {
+                alphabet.insert(ch);
+            }
+        }
+    }
+
+    size_t num_states()          const { return t.nodes.size(); }
+    bool   is_terminal(size_t s) const { return terminal[s]; }
+
+    // follow failure links until a transition on `ch` exists.
+    size_t next(size_t state, uint32_t ch) const {
+        const auto & nodes = t.nodes;
+        while (state && nodes[state].children.find(ch) == nodes[state].children.end()) {
+            state = fail[state];
+        }
+        auto it = nodes[state].children.find(ch);
+        return it != nodes[state].children.end() ? it->second : 0;
+    }
+};
+
 static std::pair<uint32_t, size_t> parse_hex_escape(const std::string & str, size_t pos, int hex_count) {
     if (pos + hex_count > str.length()) {
         return {0, 0};
@@ -992,12 +1019,12 @@ void common_peg_arena::resolve_refs() {
 }
 
 std::string common_peg_arena::dump(common_peg_parser_id id) const {
-    std::unordered_set<common_peg_parser_id> visited;
+    std::set<common_peg_parser_id> visited;
     return dump_impl(id, visited);
 }
 
 std::string common_peg_arena::dump_impl(common_peg_parser_id                       id,
-                                        std::unordered_set<common_peg_parser_id> & visited) const {
+                                        std::set<common_peg_parser_id> & visited) const {
     // Check for cycles
     if (visited.count(id)) {
         return "[cycle]";
@@ -1502,61 +1529,74 @@ static std::string gbnf_escape_char_class(uint32_t c) {
     return std::string(buf);
 }
 
-static std::string gbnf_excluding_pattern(const std::vector<std::string> & strings) {
-    trie matcher(strings);
-    auto pieces = matcher.collect_prefix_and_next();
+// GBNF grammar matching strings that contain no string in `strings` as a
+// substring. Emits the complement of an Aho-Corasick automaton DFA and returns
+// the start state rule name.
+//
+// ref: https://github.com/ggml-org/llama.cpp/pull/24839
+static std::string gbnf_excluding_grammar(const common_grammar_builder & builder,
+                                          const std::string &            prefix,
+                                          const std::vector<std::string> & strings) {
+    aho_corasick ac(strings);
 
-    std::string pattern;
-    std::string trailing;  // optional proper-prefix of a delimiter, allowed only at the very end
-    for (size_t i = 0; i < pieces.size(); ++i) {
-        if (i > 0) {
-            pattern += " | ";
+    auto state_name = [&](size_t s) -> std::string {
+        if (s == 0) {
+            return prefix;
         }
+        std::string num = std::to_string(s);
+        num = num.size() == 1 ? ("0" + num) : num;
+        return prefix + "-" + num;
+    };
 
-        const auto & pre = pieces[i].prefix;
-        const auto & chars = pieces[i].next_chars;
-
-        std::string cls;
-        cls.reserve(chars.size());
+    auto char_class = [](const std::vector<uint32_t> & chars, bool negate) {
+        std::string s = negate ? "[^" : "[";
         for (uint32_t ch : chars) {
-            cls += gbnf_escape_char_class(ch);
+            s += gbnf_escape_char_class(ch);
+        }
+        return s + "]";
+    };
+
+    for (size_t q = 0; q < ac.num_states(); q++) {
+        if (ac.is_terminal(q)) {
+            continue; // match states are dropped
         }
 
-        if (!pre.empty()) {
-            std::string pre_literal = gbnf_format_literal(common_unicode_cpts_to_utf8(pre));
-            pattern += pre_literal + " [^" + cls + "]";
-            // Each interior alternative consumes a delimiter-prefix plus a disambiguating
-            // char, so the repetition alone cannot match a value that *ends* on a proper
-            // prefix of a delimiter (e.g. a trailing "\n" when the delimiter is
-            // "\n</parameter>\n"). The runtime until() (greedy first-match) accepts such
-            // values, so without this the grammar would reject input the parser accepts.
-            // Allow the value to terminate on any proper prefix as an optional tail.
-            // This makes the grammar a slight superset of the runtime language (a value
-            // may end on the longest prefix, which greedy first-match would not itself
-            // produce); harmless for constrained generation, which only needs to admit
-            // every runtime-valid string.
-            if (!trailing.empty()) {
-                trailing += " | ";
+        std::map<size_t, std::vector<uint32_t>> buckets;
+        std::vector<uint32_t> excluded;
+        for (uint32_t c : ac.alphabet) {
+            size_t d = ac.next(q, c);
+            if (ac.is_terminal(d)) {
+                excluded.push_back(c); // completes a forbidden string -> omit
+            } else if (d != 0) {
+                buckets[d].push_back(c); // specific non-root destination
+                excluded.push_back(c);
             }
-            trailing += pre_literal;
-        } else {
-            pattern += "[^" + cls + "]";
         }
+
+        std::string rhs = "|"; // every state is accepting
+        for (const auto & [d, chars] : buckets) {
+            rhs += " " + char_class(chars, false) + " " + state_name(d) + " |";
+        }
+        rhs += " " + char_class(excluded, true) + " " + state_name(0);
+
+        builder.add_rule(state_name(q), rhs);
     }
 
-    std::string result = "(" + pattern + ")*";
-    if (!trailing.empty()) {
-        result += " (" + trailing + ")?";
+    // An empty delimiter makes the start state terminal. Emit an entry rule
+    // that matches nothing so the returned reference stays valid.
+    if (ac.is_terminal(0)) {
+        builder.add_rule(prefix, "|");
     }
-    return result;
+
+    return state_name(0);
 }
 
-static std::unordered_set<std::string> collect_reachable_rules(
+static std::set<std::string> collect_reachable_rules(
     const common_peg_arena & arena,
     const common_peg_parser_id & rule
 ) {
-    std::unordered_set<std::string> reachable;
-    std::unordered_set<std::string> visited;
+    std::set<std::string> reachable;
+    std::set<std::string> visited;
 
     std::function<void(common_peg_parser_id)> visit = [&](common_peg_parser_id id) {
         const auto & parser = arena.get(id);
@@ -1765,7 +1805,7 @@ void common_peg_arena::build_grammar(const common_grammar_builder & builder, boo
                 if (p.delimiters.empty()) {
                     return ".*";
                 }
-                return gbnf_excluding_pattern(p.delimiters);
+                return gbnf_excluding_grammar(builder, "until-" + std::to_string(id), p.delimiters);
             } else if constexpr (std::is_same_v<T, common_peg_schema_parser>) {
                 if (schema_delegates(p)) {
                     return to_gbnf(p.child);
@@ -1789,7 +1829,7 @@ void common_peg_arena::build_grammar(const common_grammar_builder & builder, boo
     };
 
     // Collect reachable rules
-    std::unordered_set<std::string> reachable_rules;
+    std::set<std::string> reachable_rules;
 
     if (lazy) {
         // Collect rules reachable from trigger rules

common/peg-parser.h

@@ -3,8 +3,8 @@
 #include <nlohmann/json_fwd.hpp>
 
 #include <memory>
+#include <set>
 #include <unordered_map>
-#include <unordered_set>
 #include <string>
 #include <string_view>
 #include <functional>
@@ -335,7 +335,7 @@ class common_peg_arena {
     friend class common_peg_parser_builder;
 
   private:
-    std::string dump_impl(common_peg_parser_id id, std::unordered_set<common_peg_parser_id> & visited) const;
+    std::string dump_impl(common_peg_parser_id id, std::set<common_peg_parser_id> & visited) const;
 
     common_peg_parser_id add_parser(common_peg_parser_variant parser);
     void add_rule(const std::string & name, common_peg_parser_id id);

tests/peg-parser/test-gbnf-generation.cpp

@@ -129,8 +129,86 @@ void test_gbnf_generation(testing &t) {
         });
 
         assert_gbnf_equal(t, R"""(
-            root ::= ([^<] | "<" [^/] | "</" [^t] | "</t" [^a] | "</ta" [^g] | "</tag" [^>])* ("<" | "</" | "</t" | "</ta" | "</tag")?
+            root ::= until-0
             space ::= | " " | "\n"{1,2} [ \t]{0,20}
+            until-0 ::= | [<] until-0-01 | [^<] until-0
+            until-0-01 ::= | [<] until-0-01 | [/] until-0-02 | [^/<] until-0
+            until-0-02 ::= | [<] until-0-01 | [t] until-0-03 | [^<t] until-0
+            until-0-03 ::= | [<] until-0-01 | [a] until-0-04 | [^<a] until-0
+            until-0-04 ::= | [<] until-0-01 | [g] until-0-05 | [^<g] until-0
+            until-0-05 ::= | [<] until-0-01 | [^<>] until-0
+        )""", gbnf);
+    });
+
+    t.test("until grammar overlapping delimiter", [](testing &t) {
+        auto parser = build_peg_parser([](common_peg_parser_builder & p)  {
+            return p.until("\n</parameter>\n");
+        });
+
+        auto gbnf = build_grammar([&](const common_grammar_builder & builder) {
+            parser.build_grammar(builder);
+        });
+
+        assert_gbnf_equal(t, R"""(
+            root ::= until-0
+            space ::= | " " | "\n"{1,2} [ \t]{0,20}
+            until-0 ::= | [\n] until-0-01 | [^\n] until-0
+            until-0-01 ::= | [\n] until-0-01 | [<] until-0-02 | [^\n<] until-0
+            until-0-02 ::= | [\n] until-0-01 | [/] until-0-03 | [^\n/] until-0
+            until-0-03 ::= | [\n] until-0-01 | [p] until-0-04 | [^\np] until-0
+            until-0-04 ::= | [\n] until-0-01 | [a] until-0-05 | [^\na] until-0
+            until-0-05 ::= | [\n] until-0-01 | [r] until-0-06 | [^\nr] until-0
+            until-0-06 ::= | [\n] until-0-01 | [a] until-0-07 | [^\na] until-0
+            until-0-07 ::= | [\n] until-0-01 | [m] until-0-08 | [^\nm] until-0
+            until-0-08 ::= | [\n] until-0-01 | [e] until-0-09 | [^\ne] until-0
+            until-0-09 ::= | [\n] until-0-01 | [t] until-0-10 | [^\nt] until-0
+            until-0-10 ::= | [\n] until-0-01 | [e] until-0-11 | [^\ne] until-0
+            until-0-11 ::= | [\n] until-0-01 | [r] until-0-12 | [^\nr] until-0
+            until-0-12 ::= | [\n] until-0-01 | [>] until-0-13 | [^\n>] until-0
+            until-0-13 ::= | [^\n] until-0
+        )""", gbnf);
+    });
+
+    // DeepSeek-V3.2 tag prefix. The DSML token (｜DSML｜) embeds U+FF5C,
+    // so the delimiter mixes ASCII and multi-byte codepoints.
+    t.test("until grammar unicode delimiter", [](testing &t) {
+        auto parser = build_peg_parser([](common_peg_parser_builder & p)  {
+            return p.until("<｜DSML｜");
+        });
+
+        auto gbnf = build_grammar([&](const common_grammar_builder & builder) {
+            parser.build_grammar(builder);
+        });
+
+        assert_gbnf_equal(t, R"""(
+            root ::= until-0
+            space ::= | " " | "\n"{1,2} [ \t]{0,20}
+            until-0 ::= | [<] until-0-01 | [^<] until-0
+            until-0-01 ::= | [<] until-0-01 | [\uFF5C] until-0-02 | [^<\uFF5C] until-0
+            until-0-02 ::= | [<] until-0-01 | [D] until-0-03 | [^<D] until-0
+            until-0-03 ::= | [<] until-0-01 | [S] until-0-04 | [^<S] until-0
+            until-0-04 ::= | [<] until-0-01 | [M] until-0-05 | [^<M] until-0
+            until-0-05 ::= | [<] until-0-01 | [L] until-0-06 | [^<L] until-0
+            until-0-06 ::= | [<] until-0-01 | [^<\uFF5C] until-0
+        )""", gbnf);
+    });
+
+    t.test("until grammar multiple delimiters", [](testing &t) {
+        auto parser = build_peg_parser([](common_peg_parser_builder & p)  {
+            return p.until_one_of({"ab", "cd", "ef"});
+        });
+
+        auto gbnf = build_grammar([&](const common_grammar_builder & builder) {
+            parser.build_grammar(builder);
+        });
+
+        assert_gbnf_equal(t, R"""(
+            root ::= until-0
+            space ::= | " " | "\n"{1,2} [ \t]{0,20}
+            until-0 ::= | [a] until-0-01 | [c] until-0-03 | [e] until-0-05 | [^ace] until-0
+            until-0-01 ::= | [a] until-0-01 | [c] until-0-03 | [e] until-0-05 | [^abce] until-0
+            until-0-03 ::= | [a] until-0-01 | [c] until-0-03 | [e] until-0-05 | [^acde] until-0
+            until-0-05 ::= | [a] until-0-01 | [c] until-0-03 | [e] until-0-05 | [^acef] until-0
         )""", gbnf);
     });
 

tools/server/server-tools.cpp

@@ -11,6 +11,7 @@
 #include <cstring>
 #include <climits>
 #include <algorithm>
+#include <unordered_set>
 
 namespace fs = std::filesystem;