remove Ubuntu 18 installation from TOC

[nominatim.git] / docs / develop / ICU-Tokenizer-Modules.md

diff --git a/docs/develop/ICU-Tokenizer-Modules.md b/docs/develop/ICU-Tokenizer-Modules.md
index 51b189f137f0ddc7a14c996ec87214122d62da89..2cf30a5699f7863db7db1f7eb04f7fa2ae3bf1b5 100644 (file)
--- a/docs/develop/ICU-Tokenizer-Modules.md
+++ b/docs/develop/ICU-Tokenizer-Modules.md
@@ -5,7 +5,14 @@ highly customizable method to pre-process and normalize the name information
 of the input data before it is added to the search index. It comes with a
 selection of sanitizers and token analyzers which you can use to adapt your
 installation to your needs. If the provided modules are not enough, you can
 of the input data before it is added to the search index. It comes with a
 selection of sanitizers and token analyzers which you can use to adapt your
 installation to your needs. If the provided modules are not enough, you can

-also provide your own implementations. This section describes how to do that.
+also provide your own implementations. This section describes the API
+of sanitizers and token analysis.
+
+!!! warning
+    This API is currently in early alpha status. While this API is meant to
+    be a public API on which other sanitizers and token analyzers may be
+    implemented, it is not guaranteed to be stable at the moment.
+

 
 ## Using non-standard sanitizers and token analyzers
 
 
 ## Using non-standard sanitizers and token analyzers
 


@@ -50,9 +57,9 @@ the function.
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-### The sanitation function
+### The main filter function of the sanitizer

 
 

-The sanitation function receives a single object of type `ProcessInfo`
+The filter function receives a single object of type `ProcessInfo`

 which has with three members:
 
  * `place`: read-only information about the place being processed.
 which has with three members:
 
  * `place`: read-only information about the place being processed.


@@ -82,11 +89,60 @@ adding extra attributes) or completely replace the list with a different one.
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-## Custom token analysis module

 
 

-Setup of a token analyser is split into two parts: configuration and
-analyser factory. A token analysis module must therefore implement two
-functions:
+### Example: Filter for US street prefixes
+
+The following sanitizer removes the directional prefixes from street names
+in the US:
+
+``` python
+import re
+
+def _filter_function(obj):
+    if obj.place.country_code == 'us' \
+       and obj.place.rank_address >= 26 and obj.place.rank_address <= 27:
+        for name in obj.names:
+            name.name = re.sub(r'^(north|south|west|east) ',
+                               '',
+                               name.name,
+                               flags=re.IGNORECASE)
+
+def create(config):
+    return _filter_function
+```
+
+This is the most simple form of a sanitizer module. If defines a single
+filter function and implements the required `create()` function by returning
+the filter.
+
+The filter function first checks if the object is interesting for the
+sanitizer. Namely it checks if the place is in the US (through `country_code`)
+and it the place is a street (a `rank_address` of 26 or 27). If the
+conditions are met, then it goes through all available names and
+removes any leading directional prefix using a simple regular expression.
+
+Save the source code in a file in your project directory, for example as
+`us_streets.py`. Then you can use the sanitizer in your `icu_tokenizer.yaml`:
+
+``` yaml
+...
+sanitizers:
+    - step: us_streets.py
+...
+```
+
+!!! warning
+    This example is just a simplified show case on how to create a sanitizer.
+    It is not really read for real-world use: while the sanitizer would
+    correcly transform `West 5th Street` into `5th Street`. it would also
+    shorten a simple `North Street` to `Street`.
+
+For more sanitizer examples, have a look at the sanitizers provided by Nominatim.
+They can be found in the directory
+[`nominatim/tokenizer/sanitizers`](https://github.com/osm-search/Nominatim/tree/master/nominatim/tokenizer/sanitizers).
+
+
+## Custom token analysis module

 
 ::: nominatim.tokenizer.token_analysis.base.AnalysisModule
     rendering:
 
 ::: nominatim.tokenizer.token_analysis.base.AnalysisModule
     rendering:


@@ -98,3 +154,74 @@ functions:
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+
+### Example: Creating acronym variants for long names
+
+The following example of a token analysis module creates acronyms from
+very long names and adds them as a variant:
+
+``` python
+class AcronymMaker:
+    """ This class is the actual analyzer.
+    """
+    def __init__(self, norm, trans):
+        self.norm = norm
+        self.trans = trans
+
+
+    def get_canonical_id(self, name):
+        # In simple cases, the normalized name can be used as a canonical id.
+        return self.norm.transliterate(name.name).strip()
+
+
+    def compute_variants(self, name):
+        # The transliterated form of the name always makes up a variant.
+        variants = [self.trans.transliterate(name)]
+
+        # Only create acronyms from very long words.
+        if len(name) > 20:
+            # Take the first letter from each word to form the acronym.
+            acronym = ''.join(w[0] for w in name.split())
+            # If that leds to an acronym with at least three letters,
+            # add the resulting acronym as a variant.
+            if len(acronym) > 2:
+                # Never forget to transliterate the variants before returning them.
+                variants.append(self.trans.transliterate(acronym))
+
+        return variants
+
+# The following two functions are the module interface.
+
+def configure(rules, normalizer, transliterator):
+    # There is no configuration to parse and no data to set up.
+    # Just return an empty configuration.
+    return None
+
+
+def create(normalizer, transliterator, config):
+    # Return a new instance of our token analysis class above.
+    return AcronymMaker(normalizer, transliterator)
+```
+
+Given the name `Trans-Siberian Railway`, the code above would return the full
+name `Trans-Siberian Railway` and the acronym `TSR` as variant, so that
+searching would work for both.
+
+## Sanitizers vs. Token analysis - what to use for variants?
+
+It is not always clear when to implement variations in the sanitizer and
+when to write a token analysis module. Just take the acronym example
+above: it would also have been possible to write a sanitizer which adds the
+acronym as an additional name to the name list. The result would have been
+similar. So which should be used when?
+
+The most important thing to keep in mind is that variants created by the
+token analysis are only saved in the word lookup table. They do not need
+extra space in the search index. If there are many spelling variations, this
+can mean quite a significant amount of space is saved.
+
+When creating additional names with a sanitizer, these names are completely
+independent. In particular, they can be fed into different token analysis
+modules. This gives a much greater flexibility but at the price that the
+additional names increase the size of the search index.
+