replace datrie library with a more simple pure-Python class

src/nominatim_db/tokenizer/token_analysis/generic.py

@@ -2,7 +2,7 @@
 #
 # This file is part of Nominatim. (https://nominatim.org)
 #
-# Copyright (C) 2024 by the Nominatim developer community.
+# Copyright (C) 2025 by the Nominatim developer community.
 # For a full list of authors see the git log.
 """
 Generic processor for names that creates abbreviation variants.
@@ -10,12 +10,11 @@ Generic processor for names that creates abbreviation variants.
 from typing import Mapping, Dict, Any, Iterable, Iterator, Optional, List, cast
 import itertools
 
-import datrie
-
 from ...errors import UsageError
 from ...data.place_name import PlaceName
 from .config_variants import get_variant_config
 from .generic_mutation import MutationVariantGenerator
+from .simple_trie import SimpleTrie
 
 # Configuration section
 
@@ -25,8 +24,7 @@ def configure(rules: Mapping[str, Any], normalizer: Any, _: Any) -> Dict[str, An
     """
     config: Dict[str, Any] = {}
 
-    config['replacements'], config['chars'] = get_variant_config(rules.get('variants'),
-                                                                 normalizer)
+    config['replacements'], _ = get_variant_config(rules.get('variants'), normalizer)
     config['variant_only'] = rules.get('mode', '') == 'variant-only'
 
     # parse mutation rules
@@ -68,12 +66,8 @@ class GenericTokenAnalysis:
         self.variant_only = config['variant_only']
 
         # Set up datrie
-        if config['replacements']:
-            self.replacements = datrie.Trie(config['chars'])
-            for src, repllist in config['replacements']:
-                self.replacements[src] = repllist
-        else:
-            self.replacements = None
+        self.replacements: Optional[SimpleTrie[List[str]]] = \
+            SimpleTrie(config['replacements']) if config['replacements'] else None
 
         # set up mutation rules
         self.mutations = [MutationVariantGenerator(*cfg) for cfg in config['mutations']]
@@ -116,10 +110,10 @@ class GenericTokenAnalysis:
             pos = 0
             force_space = False
             while pos < baselen:
-                full, repl = self.replacements.longest_prefix_item(baseform[pos:],
-                                                                   (None, None))
-                if full is not None:
-                    done = baseform[startpos:pos]
+                frm = pos
+                repl, pos = self.replacements.longest_prefix(baseform, pos)
+                if repl is not None:
+                    done = baseform[startpos:frm]
                     partials = [v + done + r
                                 for v, r in itertools.product(partials, repl)
                                 if not force_space or r.startswith(' ')]
@@ -128,11 +122,10 @@ class GenericTokenAnalysis:
                         # to be helpful. Only use the original term.
                         startpos = 0
                         break
-                    startpos = pos + len(full)
-                    if full[-1] == ' ':
-                        startpos -= 1
+                    if baseform[pos - 1] == ' ':
+                        pos -= 1
                         force_space = True
-                    pos = startpos
+                    startpos = pos
                 else:
                     pos += 1
                     force_space = False

src/nominatim_db/tokenizer/token_analysis/simple_trie.py

@@ -0,0 +1,84 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2025 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Simple dict-based implementation of a trie structure.
+"""
+from typing import TypeVar, Generic, Tuple, Optional, List, Dict
+from collections import defaultdict
+
+T = TypeVar('T')
+
+
+class SimpleTrie(Generic[T]):
+    """ A simple read-only trie structure.
+        This structure supports examply one lookup operation,
+        which is longest-prefix lookup.
+    """
+
+    def __init__(self, data: Optional[List[Tuple[str, T]]] = None) -> None:
+        self._tree: Dict[str, 'SimpleTrie[T]'] = defaultdict(SimpleTrie[T])
+        self._value: Optional[T] = None
+        self._prefix = ''
+
+        if data:
+            for key, value in data:
+                self._add(key, 0, value)
+
+            self._make_compact()
+
+    def _add(self, word: str, pos: int, value: T) -> None:
+        """ (Internal) Add a sub-word to the trie.
+            The word is added from index 'pos'. If the sub-word to add
+            is empty, then the trie saves the given value.
+        """
+        if pos < len(word):
+            self._tree[word[pos]]._add(word, pos + 1, value)
+        else:
+            self._value = value
+
+    def _make_compact(self) -> None:
+        """ (Internal) Compress tree where there is exactly one subtree
+            and no value.
+
+            Compression works recursively starting at the leaf.
+        """
+        for t in self._tree.values():
+            t._make_compact()
+
+        if len(self._tree) == 1 and self._value is None:
+            assert not self._prefix
+            for k, v in self._tree.items():
+                self._prefix = k + v._prefix
+                self._tree = v._tree
+                self._value = v._value
+
+    def longest_prefix(self, word: str, start: int = 0) -> Tuple[Optional[T], int]:
+        """ Return the longest prefix match for the given word starting at
+            the position 'start'.
+
+            The function returns a tuple with the value for the longest match and
+            the position of the word after the match. If no match was found at
+            all, the function returns (None, start).
+        """
+        cur = self
+        pos = start
+        result: Tuple[Optional[T], int] = None, start
+
+        while True:
+            if cur._prefix:
+                if not word.startswith(cur._prefix, pos):
+                    return result
+                pos += len(cur._prefix)
+
+            if cur._value:
+                result = cur._value, pos
+
+            if pos >= len(word) or word[pos] not in cur._tree:
+                return result
+
+            cur = cur._tree[word[pos]]
+            pos += 1