fix two bugs in grammar memoization for reject_candidates_for_stack (#2431)

TOKEN/TOKEN_NOT early return: operator[] pre-inserts an empty candidates
vector at line 1123, setting cache_target. The TOKEN/TOKEN_NOT branch
returns without writing to *cache_target, leaving an empty vector in the
map. Subsequent lookups return that empty vector, reporting "reject
nothing" for what should be a non-empty reject set. Fix: write to
cache_target before the early return.

Pointer aliasing in candidate key: candidate lists were hashed as raw
struct bytes, which includes the code_points pointer value. glibc can
reuse heap addresses across calls, so the same pointer value can point
to different content, causing false cache hits. Fix: hash actual content
(index, id, partial_utf8 fields, and the decoded code_points sequence)
rather than raw bytes including the pointer.

src/llama-grammar.cpp

@@ -1114,9 +1114,24 @@ llama_grammar_candidates llama_grammar_reject_candidates_for_stack(
     if (candidates_hash_size < hash_cutoff) {
         // Only check stash hash first - these are usually ~24b, and almost always under 64b
         if (auto cache_hit = memo_cache.find(stack_hash); cache_hit != memo_cache.end()) {
-            auto & candidates_memos      = cache_hit->second;
-            auto   candidates_hash_start = reinterpret_cast<const char *>(candidates.data());
-            auto   candidates_hash       = std::hash<bytes>{}({ candidates_hash_start, candidates_hash_size });
+            auto & candidates_memos = cache_hit->second;
+            // Hash candidate content (index, id, partial_utf8, decoded code_points sequence)
+            // rather than raw struct bytes, which include the code_points pointer value.
+            // Pointer values can be reused by the allocator across calls pointing to different
+            // content, causing false cache hits that return stale reject sets.
+            size_t candidates_hash = candidates.size();
+            auto combine = [&](size_t v) {
+                candidates_hash ^= v + 0x9e3779b9 + (candidates_hash << 6) + (candidates_hash >> 2);
+            };
+            for (const auto & c : candidates) {
+                combine(std::hash<size_t>{}(c.index));
+                combine(std::hash<llama_token>{}(c.id));
+                combine(std::hash<uint32_t>{}(c.partial_utf8.value));
+                combine(std::hash<int>{}(c.partial_utf8.n_remain));
+                for (const uint32_t * cp = c.code_points; *cp != 0; ++cp) {
+                    combine(std::hash<uint32_t>{}(*cp));
+                }
+            }
             if (auto cache_hit2 = candidates_memos.find(candidates_hash); cache_hit2 != candidates_memos.end()) {
                 return cache_hit2->second;
             } else {
@@ -1142,6 +1157,11 @@ llama_grammar_candidates llama_grammar_reject_candidates_for_stack(
                 rejects.push_back(tok);
             }
         }
+        // cache_target was set by operator[] which pre-inserted an empty vector; write the
+        // result here so subsequent lookups don't return an empty reject set.
+        if (cache_target) {
+            *cache_target = rejects;
+        }
         return rejects;
     }