doc: add math_abs document

doc/source/reference/functions/math_abs.rst

@@ -0,0 +1,80 @@
+.. -*- rst -*-
+
+.. highlightlang:: none
+
+.. groonga-command
+.. database: math_abs
+
+``math_abs``
+===============
+
+Summary
+-------
+
+.. versionadded:: 7.0.4
+
+``math_abs`` returns the absolute value of value.
+
+To enable this function, register ``functions/math`` plugin by following the command::
+
+  plugin_register functions/math
+
+Syntax
+------
+
+``math_abs`` requires one argument - ``target``.
+
+::
+
+  math_abs(target)
+
+Usage
+-----
+
+Here is a schema definition and sample data.
+
+Sample schema:
+
+.. groonga-command
+.. plugin_register functions/math
+.. include:: ../../example/reference/functions/math_abs/usage_setup_schema.log
+.. table_create Shops TABLE_HASH_KEY ShortText
+.. column_create Shops from_station COLUMN_SCALAR Int32
+.. column_create Shops from_office COLUMN_SCALAR Int32
+
+Sample data:
+
+.. groonga-command
+.. include:: ../../example/reference/functions/math_abs/usage_setup_data.log
+.. load --table Shops
+.. [
+.. {"_key": "Coffee Shop",         "from_station":  50},
+.. {"_key": "Donut & Coffee Shop", "from_station": 400},
+.. {"_key": "Cake & Coffee Shop",  "from_station": 200}
+.. ]
+
+Here is the simple usage of ``math_abs`` function which returns nearest shops from office.
+
+To detect nearest shops, we need to calculate distance.
+If the distance of your office from station is 250 meters, you can calculate it by ``math_abs(250 - from_station)``.
+
+.. groonga-command
+.. include:: ../../example/reference/functions/math_abs/nearest_shops.log
+.. select Shops --filter true --output_columns '_key, from_office' --scorer 'from_office = math_abs(250 - from_station)' --sort_keys from_office
+
+By specifying ``--sort_keys from_office``, you can show nearest shops by ascending order.
+
+Parameters
+----------
+
+There is one required parameter, ``target``.
+
+``target``
+^^^^^^^^^^
+
+Specifies a column of table that is specified by ``table`` parameter in ``select``.
+
+Return value
+------------
+
+``math_abs`` returns the absolute value of target column value.