mod_base: add filter round_significant (0.x) (#3143)

doc/ref/filters/filter_round_significant.rst

@@ -0,0 +1,33 @@
+.. highlight:: django
+.. include:: meta-round_significant.rst
+
+Round a number value to a number of significant digits.
+The significance defaults to two digits.
+
+Example::
+
+  {{ 1256|round_significant }}
+  {{ 1256|round_significant:1 }}
+  {{ 1256|round_significant:3 }}
+
+Results in::
+
+  1300
+  1000
+  1260
+
+Floating point values are also rounded to a number of digits. For example
+if ``n`` is set to ``1256.78`` then::
+
+  {{ n|round_significant }}
+  {{ n|round_significant:3 }}
+  {{ n|round_significant:5 }}
+
+Results in::
+
+  1300.0
+  1260.0
+  1256.8
+
+Input values that are not a number are converted to a floating point value. If the
+conversion fails then ``undefined`` is returned.

doc/ref/filters/numbers/index.rst

@@ -14,4 +14,5 @@ Numbers
    ../filter_minmax
    ../filter_rand
    ../filter_round
+   ../filter_round_significant
    ../filter_to_integer

modules/mod_base/filters/filter_round_significant.erl

@@ -0,0 +1,61 @@
+%% @author Marc Worrell <marc@worrell.nl>
+%% @copyright 2022 Marc Worrell
+%% @doc Round a value to the give significant digits. If you need to
+%% round to a number of digits after the decimal point then use the
+%% "round" filter.
+
+%% Copyright 2022 Marc Worrell
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%% 
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%% 
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+
+-module(filter_round_significant).
+
+-export([
+    round_significant/2,
+    round_significant/3
+    ]).
+
+round_significant(Value, Context) ->
+    round_significant(Value, 2, Context).
+
+round_significant(undefined, _Significance, _Context) ->
+    undefined;
+round_significant(N, Significance, _Context) when is_integer(N) ->
+    Digits = digits(N),
+    if
+        Digits =< Significance ->
+            N;
+        true ->
+            Delta = Digits - Significance,
+            Divider = math:pow(10, Delta),
+            erlang:trunc(erlang:round(N / Divider) * Divider)
+    end;
+round_significant(N, Significance, _Context) ->
+    try
+        F = z_convert:to_float(N),
+        Digits = digits(N),
+        Delta = abs(Digits - Significance),
+        Divider = math:pow(10, Delta),
+        if
+            Digits =< Significance ->
+                erlang:round(F * Divider) / Divider;
+            true ->
+                erlang:round(F / Divider) * Divider
+        end
+    catch
+        _:_ -> undefined
+    end.
+
+digits(N) ->
+    1 + erlang:trunc(math:log10(N)).
+