Fix infinite recursion in RagTokenizer when processing repetitive characters (#6109)

### What problem does this PR solve?
fix #6085 
RagTokenizer's dfs_() function falls into infinite recursion when
processing text with repetitive Chinese characters (e.g.,
"一一一一一十一十一十一..." or "一一一一一一十十十十十十十二十二十二..."), causing memory leaks.
### Type of change
Implemented three optimizations to the dfs_() function:
1.Added memoization with _memo dictionary to cache computed results
2.Added recursion depth limiting with _depth parameter (max 10 levels)
3.Implemented special handling for repetitive character sequences
- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/nlp/rag_tokenizer.py

@@ -116,55 +116,86 @@ class RagTokenizer:
     def _tradi2simp(self, line):
         return HanziConv.toSimplified(line)
 
-    def dfs_(self, chars, s, preTks, tkslist):
+    def dfs_(self, chars, s, preTks, tkslist, _depth=0, _memo=None):
+        if _memo is None:
+            _memo = {}
+        MAX_DEPTH = 10
+        if _depth > MAX_DEPTH:
+            if s < len(chars):
+                copy_pretks = copy.deepcopy(preTks)
+                remaining = "".join(chars[s:])
+                copy_pretks.append((remaining, (-12, '')))
+                tkslist.append(copy_pretks)
+            return s
+    
+        state_key = (s, tuple(tk[0] for tk in preTks)) if preTks else (s, None)
+        if state_key in _memo:
+            return _memo[state_key]
+        
         res = s
-        if len(tkslist) >= 2048:
-            return res
-        # if s > MAX_L or s>= len(chars):
         if s >= len(chars):
             tkslist.append(preTks)
-            return res
-
-        # pruning
+            _memo[state_key] = s
+            return s
+        if s < len(chars) - 4:
+            is_repetitive = True
+            char_to_check = chars[s]
+            for i in range(1, 5):
+                if s + i >= len(chars) or chars[s + i] != char_to_check:
+                    is_repetitive = False
+                    break
+            if is_repetitive:
+                end = s
+                while end < len(chars) and chars[end] == char_to_check:
+                    end += 1
+                mid = s + min(10, end - s)
+                t = "".join(chars[s:mid])
+                k = self.key_(t)
+                copy_pretks = copy.deepcopy(preTks)
+                if k in self.trie_:
+                    copy_pretks.append((t, self.trie_[k]))
+                else:
+                    copy_pretks.append((t, (-12, '')))
+                next_res = self.dfs_(chars, mid, copy_pretks, tkslist, _depth + 1, _memo)
+                res = max(res, next_res)
+                _memo[state_key] = res
+                return res
+    
         S = s + 1
         if s + 2 <= len(chars):
-            t1, t2 = "".join(chars[s:s + 1]), "".join(chars[s:s + 2])
-            if self.trie_.has_keys_with_prefix(self.key_(t1)) and not self.trie_.has_keys_with_prefix(
-                    self.key_(t2)):
+            t1 = "".join(chars[s:s + 1])
+            t2 = "".join(chars[s:s + 2])
+            if self.trie_.has_keys_with_prefix(self.key_(t1)) and not self.trie_.has_keys_with_prefix(self.key_(t2)):
                 S = s + 2
-        if len(preTks) > 2 and len(
-                preTks[-1][0]) == 1 and len(preTks[-2][0]) == 1 and len(preTks[-3][0]) == 1:
+        if len(preTks) > 2 and len(preTks[-1][0]) == 1 and len(preTks[-2][0]) == 1 and len(preTks[-3][0]) == 1:
             t1 = preTks[-1][0] + "".join(chars[s:s + 1])
             if self.trie_.has_keys_with_prefix(self.key_(t1)):
                 S = s + 2
-
-        ################
+    
         for e in range(S, len(chars) + 1):
             t = "".join(chars[s:e])
             k = self.key_(t)
-
             if e > s + 1 and not self.trie_.has_keys_with_prefix(k):
                 break
-
             if k in self.trie_:
                 pretks = copy.deepcopy(preTks)
-                if k in self.trie_:
-                    pretks.append((t, self.trie_[k]))
-                else:
-                    pretks.append((t, (-12, '')))
-                res = max(res, self.dfs_(chars, e, pretks, tkslist))
-
+                pretks.append((t, self.trie_[k]))
+                res = max(res, self.dfs_(chars, e, pretks, tkslist, _depth + 1, _memo))
+        
         if res > s:
+            _memo[state_key] = res
             return res
-
+    
         t = "".join(chars[s:s + 1])
         k = self.key_(t)
+        copy_pretks = copy.deepcopy(preTks)
         if k in self.trie_:
-            preTks.append((t, self.trie_[k]))
+            copy_pretks.append((t, self.trie_[k]))
         else:
-            preTks.append((t, (-12, '')))
-
-        return self.dfs_(chars, s + 1, preTks, tkslist)
+            copy_pretks.append((t, (-12, '')))
+        result = self.dfs_(chars, s + 1, copy_pretks, tkslist, _depth + 1, _memo)
+        _memo[state_key] = result
+        return result
 
     def freq(self, tk):
         k = self.key_(tk)