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Diffstat (limited to 'resources/libraries/python/jumpavg/bit_counting_stats.py')
| -rw-r--r-- | resources/libraries/python/jumpavg/bit_counting_stats.py | 157 |
1 files changed, 157 insertions, 0 deletions
diff --git a/resources/libraries/python/jumpavg/bit_counting_stats.py b/resources/libraries/python/jumpavg/bit_counting_stats.py new file mode 100644 index 0000000000..caece2c8ca --- /dev/null +++ b/resources/libraries/python/jumpavg/bit_counting_stats.py @@ -0,0 +1,157 @@ +# Copyright (c) 2023 Cisco and/or its affiliates. +# 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 holding BitCountingStats class.""" + +import dataclasses +import math +import typing + +from .avg_stdev_stats import AvgStdevStats + + +@dataclasses.dataclass +class BitCountingStats(AvgStdevStats): + """Class for statistics which include information content of a group. + + The information content is based on an assumption that the data + consists of independent random values from a normal distribution. + + Instances are only statistics, the data itself is stored elsewhere. + + The coding needs to know the previous average, and a maximal value + so both values are required as inputs. + + This is a subclass of AvgStdevStats, even though all methods are overriden. + Only for_runs method calls the parent implementation, without using super(). + """ + + max_value: float = None + """Maximal sample value (real or estimated). + Default value is there just for argument ordering reasons, + leaving None leads to exceptions.""" + unit: float = 1.0 + """Typical resolution of the values.""" + prev_avg: typing.Optional[float] = None + """Population average of the previous group (if any).""" + bits: float = None + """The computed information content of the group. + It is formally an argument to init function, just to keep repr string + a valid call. ut the init value is ignored and always recomputed. + """ + + def __post_init__(self): + """Construct the stats object by computing from the values needed. + + The None values are allowed for stats for zero size data, + but such stats can report arbitrary avg and max_value. + Stats for nonzero size data cannot contain None, + else ValueError is raised. + + The max_value needs to be numeric for nonzero size, + but its relations to avg and prev_avg are not examined. + + The bit count is not real, as that would depend on numeric precision + (number of significant bits in values). + The difference is assumed to be constant per value, + which is consistent with Gauss distribution + (but not with floating point mechanic). + The hope is the difference will have + no real impact on the classification procedure. + """ + # Zero size should in principle have non-zero bits (coding zero size), + # but zero allows users to add empty groups without affecting bits. + self.bits = 0.0 + if self.size < 1: + return + if self.max_value <= 0.0: + raise ValueError(f"Invalid max value: {self!r}") + max_value = self.max_value / self.unit + avg = self.avg / self.unit + # Length of the sequence must be also counted in bits, + # otherwise the message would not be decodable. + # Model: probability of k samples is 1/k - 1/(k+1) == 1/k/(k+1) + # This is compatible with zero size leading to zero bits. + self.bits += math.log(self.size * (self.size + 1), 2) + if self.prev_avg is None: + # Avg is considered to be uniformly distributed + # from zero to max_value. + self.bits += math.log(max_value + 1, 2) + else: + # Opposite triangle distribution with minimum. + prev_avg = self.prev_avg / self.unit + norm = prev_avg * prev_avg + norm -= (prev_avg - 1) * max_value + norm += max_value * max_value / 2 + self.bits -= math.log((abs(avg - prev_avg) + 1) / norm, 2) + if self.size < 2: + return + stdev = self.stdev / self.unit + # Stdev is considered to be uniformly distributed + # from zero to max_value. That is quite a bad expectation, + # but resilient to negative samples etc. + self.bits += math.log(max_value + 1, 2) + # Now we know the samples lie on sphere in size-1 dimensions. + # So it is (size-2)-sphere, with radius^2 == stdev^2 * size. + # https://en.wikipedia.org/wiki/N-sphere + sphere_area_ln = math.log(2) + sphere_area_ln += math.log(math.pi) * ((self.size - 1) / 2) + sphere_area_ln -= math.lgamma((self.size - 1) / 2) + sphere_area_ln += math.log(stdev + 1) * (self.size - 2) + sphere_area_ln += math.log(self.size) * ((self.size - 2) / 2) + self.bits += sphere_area_ln / math.log(2) + + @classmethod + def for_runs_and_params( + cls, + runs: typing.Iterable[typing.Union[float, AvgStdevStats]], + max_value: float, + unit: float = 1.0, + prev_avg: typing.Optional[float] = None, + ): + """Return new stats instance describing the sequence of runs. + + If you want to append data to existing stats object, + you can simply use the stats object as the first run. + + Instead of a verb, "for" is used to start this method name, + to signify the result contains less information than the input data. + + The two optional values can come from outside of the runs provided. + + The max_value cannot be None for non-zero size data. + The implementation does not check if no datapoint exceeds max_value. + + TODO: Document the behavior for zero size result. + + :param runs: Sequence of data to describe by the new metadata. + :param max_value: Maximal expected value. + :param unit: Typical resolution of the values. + :param prev_avg: Population average of the previous group, if any. + :type runs: Iterable[Union[float, AvgStdevStats]] + :type max_value: Union[float, NoneType] + :type unit: float + :type prev_avg: Union[float, NoneType] + :returns: The new stats instance. + :rtype: cls + """ + asd = AvgStdevStats.for_runs(runs) + ret_obj = cls( + size=asd.size, + avg=asd.avg, + stdev=asd.stdev, + max_value=max_value, + unit=unit, + prev_avg=prev_avg, + ) + return ret_obj |