housenumber analyzer: avoid creating too many variants

Housenumber fields with lots of text are likely bad data. So is
data with many changes from letter to digit. Exclude them from adding
optional spaces.

nominatim/tokenizer/token_analysis/housenumbers.py

@@ -15,17 +15,18 @@ from nominatim.tokenizer.token_analysis.generic_mutation import MutationVariantG
 RE_NON_DIGIT = re.compile('[^0-9]')
 RE_DIGIT_ALPHA = re.compile(r'(\d)\s*([^\d\s␣])')
 RE_ALPHA_DIGIT = re.compile(r'([^\s\d␣])\s*(\d)')
+RE_NAMED_PART = re.compile(r'[a-z]{4}')
 
 ### Configuration section
 
-def configure(rules, normalization_rules):
+def configure(rules, normalization_rules): # pylint: disable=W0613
     """ All behaviour is currently hard-coded.
     """
     return None
 
 ### Analysis section
 
-def create(normalizer, transliterator, config):
+def create(normalizer, transliterator, config): # pylint: disable=W0613
     """ Create a new token analysis instance for this module.
     """
     return HousenumberTokenAnalysis(normalizer, transliterator)
@@ -48,8 +49,14 @@ class HousenumberTokenAnalysis:
             return name
 
         norm = self.trans.transliterate(self.norm.transliterate(name))
-        norm = RE_DIGIT_ALPHA.sub(r'\1␣\2', norm)
-        norm = RE_ALPHA_DIGIT.sub(r'\1␣\2', norm)
+        # If there is a significant non-numeric part, use as is.
+        if RE_NAMED_PART.search(norm) is None:
+            # Otherwise add optional spaces between digits and letters.
+            (norm_opt, cnt1) = RE_DIGIT_ALPHA.subn(r'\1␣\2', norm)
+            (norm_opt, cnt2) = RE_ALPHA_DIGIT.subn(r'\1␣\2', norm_opt)
+            # Avoid creating too many variants per number.
+            if cnt1 + cnt2 <= 4:
+                return norm_opt
 
         return norm