scripts: add oe-build-perf-report script

A new tool for pretty-printing build perf test results stored in a Git
repository. The scripts is able to produce either simple plaintext
report showing the difference between two commits, or, an html report
that also displays trendcharts of the test results. The script uses
Jinja2 templates for generating HTML reports so it requires
python3-jinja2 to be installed on the system.

[YOCTO #10931]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

5 files changed, 651 insertions, 0 deletions

diff --git a/scripts/lib/build_perf/__init__.py b/scripts/lib/build_perf/__init__.py
new file mode 100644
index 0000000000..1f8b729078
--- /dev/null
+++ b/scripts/lib/build_perf/__init__.py
@@ -0,0 +1,31 @@
+#
+# Copyright (c) 2017, Intel Corporation.
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms and conditions of the GNU General Public License,
+# version 2, as published by the Free Software Foundation.
+#
+# This program is distributed in the hope it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+"""Build performance test library functions"""
+
+def print_table(rows, row_fmt=None):
+    """Print data table"""
+    if not rows:
+        return
+    if not row_fmt:
+        row_fmt = ['{:{wid}} '] * len(rows[0])
+
+    # Go through the data to get maximum cell widths
+    num_cols = len(row_fmt)
+    col_widths = [0] * num_cols
+    for row in rows:
+        for i, val in enumerate(row):
+            col_widths[i] = max(col_widths[i], len(str(val)))
+
+    for row in rows:
+        print(*[row_fmt[i].format(col, wid=col_widths[i]) for i, col in enumerate(row)])
+
diff --git a/scripts/lib/build_perf/html.py b/scripts/lib/build_perf/html.py
new file mode 100644
index 0000000000..578bb162ee
--- /dev/null
+++ b/scripts/lib/build_perf/html.py
@@ -0,0 +1,19 @@
+#
+# Copyright (c) 2017, Intel Corporation.
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms and conditions of the GNU General Public License,
+# version 2, as published by the Free Software Foundation.
+#
+# This program is distributed in the hope it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+"""Helper module for HTML reporting"""
+from jinja2 import Environment, PackageLoader
+
+
+env = Environment(loader=PackageLoader('build_perf', 'html'))
+
+template = env.get_template('report.html')
diff --git a/scripts/lib/build_perf/html/measurement_chart.html b/scripts/lib/build_perf/html/measurement_chart.html
new file mode 100644
index 0000000000..26fe1453c0
--- /dev/null
+++ b/scripts/lib/build_perf/html/measurement_chart.html
@@ -0,0 +1,50 @@
+<script type="text/javascript">
+  google.charts.setOnLoadCallback(drawChart_{{ chart_elem_id }});
+  function drawChart_{{ chart_elem_id }}() {
+    var data = new google.visualization.DataTable();
+
+    // Chart options
+    var options = {
+      theme : 'material',
+      legend: 'none',
+      hAxis: { format: '', title: 'Commit number',
+               minValue: {{ chart_opts.haxis.min }},
+               maxValue: {{ chart_opts.haxis.max }} },
+      {% if measurement.type == 'time' %}
+      vAxis: { format: 'h:mm:ss' },
+      {% else %}
+      vAxis: { format: '' },
+      {% endif %}
+      pointSize: 5,
+      chartArea: { left: 80, right: 15 },
+    };
+
+    // Define data columns
+    data.addColumn('number', 'Commit');
+    data.addColumn('{{ measurement.value_type.gv_data_type }}',
+                   '{{ measurement.value_type.quantity }}');
+    // Add data rows
+    data.addRows([
+      {% for sample in measurement.samples %}
+        [{{ sample.commit_num }}, {{ sample.mean.gv_value() }}],
+      {% endfor %}
+    ]);
+
+    // Finally, draw the chart
+    chart_div = document.getElementById('{{ chart_elem_id }}');
+    var chart = new google.visualization.LineChart(chart_div);
+    google.visualization.events.addListener(chart, 'ready', function () {
+      //chart_div = document.getElementById('{{ chart_elem_id }}');
+      //chart_div.innerHTML = '<img src="' + chart.getImageURI() + '">';
+      png_div = document.getElementById('{{ chart_elem_id }}_png');
+      png_div.outerHTML = '<a id="{{ chart_elem_id }}_png" href="' + chart.getImageURI() + '">PNG</a>';
+      console.log("CHART READY: {{ chart_elem_id }}");
+      {% if last_chart == true %}
+        console.log("ALL CHARTS READY");
+      {% endif %}
+      //console.log(chart_div.innerHTML);
+    });
+    chart.draw(data, options);
+}
+</script>
+
diff --git a/scripts/lib/build_perf/html/report.html b/scripts/lib/build_perf/html/report.html
new file mode 100644
index 0000000000..e42871177d
--- /dev/null
+++ b/scripts/lib/build_perf/html/report.html
@@ -0,0 +1,209 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+{# Scripts, for visualization#}
+<!--START-OF-SCRIPTS-->
+<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>
+<script type="text/javascript">
+google.charts.load('current', {'packages':['corechart']});
+</script>
+
+{# Render measurement result charts #}
+{% for test in test_data %}
+  {% set test_loop = loop %}
+  {% if test.status == 'SUCCESS' %}
+    {% for measurement in test.measurements %}
+      {% set chart_elem_id = test.name + '_' + measurement.name + '_chart' %}
+      {% if test_loop.last and loop.last %}
+        {% set last_chart = true %}
+      {% endif %}
+      {% include 'measurement_chart.html' %}
+    {% endfor %}
+  {% endif %}
+{% endfor %}
+
+<!--END-OF-SCRIPTS-->
+
+{# Styles #}
+<style>
+.meta-table {
+  font-size: 14px;
+  text-align: left;
+  border-collapse: collapse;
+}
+.meta-table tr:nth-child(even){background-color: #f2f2f2}
+meta-table th, .meta-table td {
+  padding: 4px;
+}
+.summary {
+  margin: 0;
+  font-size: 14px;
+  text-align: left;
+  border-collapse: collapse;
+}
+summary th, .meta-table td {
+  padding: 4px;
+}
+.measurement {
+  padding: 8px 0px 8px 8px;
+  border: 2px solid #f0f0f0;
+  margin-bottom: 10px;
+}
+.details {
+  margin: 0;
+  font-size: 12px;
+  text-align: left;
+  border-collapse: collapse;
+}
+.details th {
+  font-weight: normal;
+  padding-right: 8px;
+}
+.preformatted {
+  font-family: monospace;
+  white-space: pre-wrap;
+  background-color: #f0f0f0;
+  margin-left: 10px;
+}
+hr {
+  color: #f0f0f0;
+}
+h2 {
+  font-size: 20px;
+  margin-bottom: 0px;
+  color: #707070;
+}
+h3 {
+  font-size: 16px;
+  margin: 0px;
+  color: #707070;
+}
+</style>
+
+<title>{{ title }}</title>
+</head>
+
+{% macro poky_link(commit) -%}
+    <a href="http://git.yoctoproject.org/cgit/cgit.cgi/poky/log/?id={{ commit }}">{{ commit[0:11] }}</a>
+{%- endmacro %}
+
+<body><div style="width: 700px">
+  {# Test metadata #}
+  <h2>General</h2>
+  <hr>
+  <table class="meta-table" style="width: 100%">
+    <tr>
+      <th></th>
+      <th>Current commit</th>
+      <th>Comparing with</th>
+    </tr>
+    {% for key, item in metadata.items() %}
+    <tr>
+      <th>{{ item.title }}</th>
+      {%if key == 'commit' %}
+        <td>{{ poky_link(item.value) }}</td>
+        <td>{{ poky_link(item.value_old) }}</td>
+      {% else %}
+        <td>{{ item.value }}</td>
+        <td>{{ item.value_old }}</td>
+      {% endif %}
+    </tr>
+    {% endfor %}
+  </table>
+
+  {# Test result summary #}
+  <h2>Test result summary</h2>
+  <hr>
+  <table class="summary" style="width: 100%">
+    {% for test in test_data %}
+      {% if loop.index is even %}
+        {% set row_style = 'style="background-color: #f2f2f2"' %}
+      {% else %}
+        {% set row_style = 'style="background-color: #ffffff"' %}
+      {% endif %}
+      <tr {{ row_style }}><td>{{ test.name }}: {{ test.description }}</td>
+      {% if test.status == 'SUCCESS' %}
+        {% for measurement in test.measurements %}
+          {# add empty cell in place of the test name#}
+          {% if loop.index > 1 %}<td></td>{% endif %}
+          {% if measurement.absdiff > 0 %}
+            {% set result_style = "color: red" %}
+          {% elif measurement.absdiff == measurement.absdiff %}
+            {% set result_style = "color: green" %}
+          {% else %}
+            {% set result_style = "color: orange" %}
+          {%endif %}
+          <td>{{ measurement.description }}</td>
+          <td style="font-weight: bold">{{ measurement.value.mean }}</td>
+          <td style="{{ result_style }}">{{ measurement.absdiff_str }}</td>
+          <td style="{{ result_style }}">{{ measurement.reldiff }}</td>
+          </tr><tr {{ row_style }}>
+        {% endfor %}
+      {% else %}
+        <td style="font-weight: bold; color: red;">{{test.status }}</td>
+        <td></td> <td></td> <td></td> <td></td>
+      {% endif %}
+      </tr>
+    {% endfor %}
+  </table>
+
+  {# Detailed test results #}
+  {% for test in test_data %}
+  <h2>{{ test.name }}: {{ test.description }}</h2>
+  <hr>
+    {% if test.status == 'SUCCESS' %}
+      {% for measurement in test.measurements %}
+        <div class="measurement">
+          <h3>{{ measurement.description }}</h3>
+          <div style="font-weight:bold;">
+            <span style="font-size: 23px;">{{ measurement.value.mean }}</span>
+            <span style="font-size: 20px; margin-left: 12px">
+            {% if measurement.absdiff > 0 %}
+            <span style="color: red">
+            {% elif measurement.absdiff == measurement.absdiff %}
+            <span style="color: green">
+            {% else %}
+            <span style="color: orange">
+            {% endif %}
+            {{ measurement.absdiff_str }} ({{measurement.reldiff}})
+            </span></span>
+          </div>
+          <table style="width: 100%">
+            <tr>
+              <td style="width: 75%">
+                {# Linechart #}
+                <div id="{{ test.name }}_{{ measurement.name }}_chart"></div>
+              </td>
+              <td>
+                {# Measurement statistics #}
+                <table class="details">
+                  <tr>
+                    <th>Test runs</th><td>{{ measurement.value.sample_cnt }}</td>
+                  </tr><tr>
+                    <th>-/+</th><td>-{{ measurement.value.minus }} / +{{ measurement.value.plus }}</td>
+                  </tr><tr>
+                    <th>Min</th><td>{{ measurement.value.min }}</td>
+                  </tr><tr>
+                    <th>Max</th><td>{{ measurement.value.max }}</td>
+                  </tr><tr>
+                    <th>Stdev</th><td>{{ measurement.value.stdev }}</td>
+                  </tr><tr>
+                    <th><div id="{{ test.name }}_{{ measurement.name }}_chart_png"></div></th>
+                  </tr>
+                </table>
+              </td>
+            </tr>
+          </table>
+        </div>
+      {% endfor %}
+    {# Unsuccessful test #}
+    {% else %}
+      <span style="font-size: 150%; font-weight: bold; color: red;">{{ test.status }}
+      {% if test.err_type %}<span style="font-size: 75%; font-weight: normal">({{ test.err_type }})</span>{% endif %}
+      </span>
+      <div class="preformatted">{{ test.message }}</div>
+    {% endif %}
+  {% endfor %}
+</div></body>
+</html>
+
diff --git a/scripts/lib/build_perf/report.py b/scripts/lib/build_perf/report.py
new file mode 100644
index 0000000000..eb00ccca2d
--- /dev/null
+++ b/scripts/lib/build_perf/report.py
@@ -0,0 +1,342 @@
+#
+# Copyright (c) 2017, Intel Corporation.
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms and conditions of the GNU General Public License,
+# version 2, as published by the Free Software Foundation.
+#
+# This program is distributed in the hope it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+"""Handling of build perf test reports"""
+from collections import OrderedDict, Mapping
+from datetime import datetime, timezone
+from numbers import Number
+from statistics import mean, stdev, variance
+
+
+def isofmt_to_timestamp(string):
+    """Convert timestamp string in ISO 8601 format into unix timestamp"""
+    if '.' in string:
+        dt = datetime.strptime(string, '%Y-%m-%dT%H:%M:%S.%f')
+    else:
+        dt = datetime.strptime(string, '%Y-%m-%dT%H:%M:%S')
+    return dt.replace(tzinfo=timezone.utc).timestamp()
+
+
+def metadata_xml_to_json(elem):
+    """Convert metadata xml into JSON format"""
+    assert elem.tag == 'metadata', "Invalid metadata file format"
+
+    def _xml_to_json(elem):
+        """Convert xml element to JSON object"""
+        out = OrderedDict()
+        for child in elem.getchildren():
+            key = child.attrib.get('name', child.tag)
+            if len(child):
+                out[key] = _xml_to_json(child)
+            else:
+                out[key] = child.text
+        return out
+    return _xml_to_json(elem)
+
+
+def results_xml_to_json(elem):
+    """Convert results xml into JSON format"""
+    rusage_fields = ('ru_utime', 'ru_stime', 'ru_maxrss', 'ru_minflt',
+                     'ru_majflt', 'ru_inblock', 'ru_oublock', 'ru_nvcsw',
+                     'ru_nivcsw')
+    iostat_fields = ('rchar', 'wchar', 'syscr', 'syscw', 'read_bytes',
+                     'write_bytes', 'cancelled_write_bytes')
+
+    def _read_measurement(elem):
+        """Convert measurement to JSON"""
+        data = OrderedDict()
+        data['type'] = elem.tag
+        data['name'] = elem.attrib['name']
+        data['legend'] = elem.attrib['legend']
+        values = OrderedDict()
+
+        # SYSRES measurement
+        if elem.tag == 'sysres':
+            for subel in elem:
+                if subel.tag == 'time':
+                    values['start_time'] = isofmt_to_timestamp(subel.attrib['timestamp'])
+                    values['elapsed_time'] = float(subel.text)
+                elif subel.tag == 'rusage':
+                    rusage = OrderedDict()
+                    for field in rusage_fields:
+                        if 'time' in field:
+                            rusage[field] = float(subel.attrib[field])
+                        else:
+                            rusage[field] = int(subel.attrib[field])
+                    values['rusage'] = rusage
+                elif subel.tag == 'iostat':
+                    values['iostat'] = OrderedDict([(f, int(subel.attrib[f]))
+                        for f in iostat_fields])
+                elif subel.tag == 'buildstats_file':
+                    values['buildstats_file'] = subel.text
+                else:
+                    raise TypeError("Unknown sysres value element '{}'".format(subel.tag))
+        # DISKUSAGE measurement
+        elif elem.tag == 'diskusage':
+            values['size'] = int(elem.find('size').text)
+        else:
+            raise Exception("Unknown measurement tag '{}'".format(elem.tag))
+        data['values'] = values
+        return data
+
+    def _read_testcase(elem):
+        """Convert testcase into JSON"""
+        assert elem.tag == 'testcase', "Expecting 'testcase' element instead of {}".format(elem.tag)
+
+        data = OrderedDict()
+        data['name'] = elem.attrib['name']
+        data['description'] = elem.attrib['description']
+        data['status'] = 'SUCCESS'
+        data['start_time'] = isofmt_to_timestamp(elem.attrib['timestamp'])
+        data['elapsed_time'] = float(elem.attrib['time'])
+        measurements = OrderedDict()
+
+        for subel in elem.getchildren():
+            if subel.tag == 'error' or subel.tag == 'failure':
+                data['status'] = subel.tag.upper()
+                data['message'] = subel.attrib['message']
+                data['err_type'] = subel.attrib['type']
+                data['err_output'] = subel.text
+            elif subel.tag == 'skipped':
+                data['status'] = 'SKIPPED'
+                data['message'] = subel.text
+            else:
+                measurements[subel.attrib['name']] = _read_measurement(subel)
+        data['measurements'] = measurements
+        return data
+
+    def _read_testsuite(elem):
+        """Convert suite to JSON"""
+        assert elem.tag == 'testsuite', \
+                "Expecting 'testsuite' element instead of {}".format(elem.tag)
+
+        data = OrderedDict()
+        if 'hostname' in elem.attrib:
+            data['tester_host'] = elem.attrib['hostname']
+        data['start_time'] = isofmt_to_timestamp(elem.attrib['timestamp'])
+        data['elapsed_time'] = float(elem.attrib['time'])
+        tests = OrderedDict()
+
+        for case in elem.getchildren():
+            tests[case.attrib['name']] = _read_testcase(case)
+        data['tests'] = tests
+        return data
+
+    # Main function
+    assert elem.tag == 'testsuites', "Invalid test report format"
+    assert len(elem) == 1, "Too many testsuites"
+
+    return _read_testsuite(elem.getchildren()[0])
+
+
+def aggregate_metadata(metadata):
+    """Aggregate metadata into one, basically a sanity check"""
+    mutable_keys = ('pretty_name', 'version_id')
+
+    def aggregate_obj(aggregate, obj, assert_str=True):
+        """Aggregate objects together"""
+        assert type(aggregate) is type(obj), \
+                "Type mismatch: {} != {}".format(type(aggregate), type(obj))
+        if isinstance(obj, Mapping):
+            assert set(aggregate.keys()) == set(obj.keys())
+            for key, val in obj.items():
+                aggregate_obj(aggregate[key], val, key not in mutable_keys)
+        elif isinstance(obj, list):
+            assert len(aggregate) == len(obj)
+            for i, val in enumerate(obj):
+                aggregate_obj(aggregate[i], val)
+        elif not isinstance(obj, str) or (isinstance(obj, str) and assert_str):
+            assert aggregate == obj, "Data mismatch {} != {}".format(aggregate, obj)
+
+    if not metadata:
+        return {}
+
+    # Do the aggregation
+    aggregate = metadata[0].copy()
+    for testrun in metadata[1:]:
+        aggregate_obj(aggregate, testrun)
+    aggregate['testrun_count'] = len(metadata)
+    return aggregate
+
+
+def aggregate_data(data):
+    """Aggregate multiple test results JSON structures into one"""
+
+    mutable_keys = ('status', 'message', 'err_type', 'err_output')
+
+    class SampleList(list):
+        """Container for numerical samples"""
+        pass
+
+    def new_aggregate_obj(obj):
+        """Create new object for aggregate"""
+        if isinstance(obj, Number):
+            new_obj = SampleList()
+            new_obj.append(obj)
+        elif isinstance(obj, str):
+            new_obj = obj
+        else:
+            # Lists and and dicts are kept as is
+            new_obj = obj.__class__()
+            aggregate_obj(new_obj, obj)
+        return new_obj
+
+    def aggregate_obj(aggregate, obj, assert_str=True):
+        """Recursive "aggregation" of JSON objects"""
+        if isinstance(obj, Number):
+            assert isinstance(aggregate, SampleList)
+            aggregate.append(obj)
+            return
+
+        assert type(aggregate) == type(obj), \
+                "Type mismatch: {} != {}".format(type(aggregate), type(obj))
+        if isinstance(obj, Mapping):
+            for key, val in obj.items():
+                if not key in aggregate:
+                    aggregate[key] = new_aggregate_obj(val)
+                else:
+                    aggregate_obj(aggregate[key], val, key not in mutable_keys)
+        elif isinstance(obj, list):
+            for i, val in enumerate(obj):
+                if i >= len(aggregate):
+                    aggregate[key] = new_aggregate_obj(val)
+                else:
+                    aggregate_obj(aggregate[i], val)
+        elif isinstance(obj, str):
+            # Sanity check for data
+            if assert_str:
+                assert aggregate == obj, "Data mismatch {} != {}".format(aggregate, obj)
+        else:
+            raise Exception("BUG: unable to aggregate '{}' ({})".format(type(obj), str(obj)))
+
+    if not data:
+        return {}
+
+    # Do the aggregation
+    aggregate = data[0].__class__()
+    for testrun in data:
+        aggregate_obj(aggregate, testrun)
+    return aggregate
+
+
+class MeasurementVal(float):
+    """Base class representing measurement values"""
+    gv_data_type = 'number'
+
+    def gv_value(self):
+        """Value formatting for visualization"""
+        if self != self:
+            return "null"
+        else:
+            return self
+
+
+class TimeVal(MeasurementVal):
+    """Class representing time values"""
+    quantity = 'time'
+    gv_title = 'elapsed time'
+    gv_data_type = 'timeofday'
+
+    def hms(self):
+        """Split time into hours, minutes and seconeds"""
+        hhh = int(abs(self) / 3600)
+        mmm = int((abs(self) % 3600) / 60)
+        sss = abs(self) % 60
+        return hhh, mmm, sss
+
+    def __str__(self):
+        if self != self:
+            return "nan"
+        hh, mm, ss = self.hms()
+        sign = '-' if self < 0 else ''
+        if hh > 0:
+            return '{}{:d}:{:02d}:{:02.0f}'.format(sign, hh, mm, ss)
+        elif mm > 0:
+            return '{}{:d}:{:04.1f}'.format(sign, mm, ss)
+        elif ss > 1:
+            return '{}{:.1f} s'.format(sign, ss)
+        else:
+            return '{}{:.2f} s'.format(sign, ss)
+
+    def gv_value(self):
+        """Value formatting for visualization"""
+        if self != self:
+            return "null"
+        hh, mm, ss = self.hms()
+        return [hh, mm, int(ss), int(ss*1000) % 1000]
+
+
+class SizeVal(MeasurementVal):
+    """Class representing time values"""
+    quantity = 'size'
+    gv_title = 'size in MiB'
+    gv_data_type = 'number'
+
+    def __str__(self):
+        if self != self:
+            return "nan"
+        if abs(self) < 1024:
+            return '{:.1f} kiB'.format(self)
+        elif abs(self) < 1048576:
+            return '{:.2f} MiB'.format(self / 1024)
+        else:
+            return '{:.2f} GiB'.format(self / 1048576)
+
+    def gv_value(self):
+        """Value formatting for visualization"""
+        if self != self:
+            return "null"
+        return self / 1024
+
+def measurement_stats(meas, prefix=''):
+    """Get statistics of a measurement"""
+    if not meas:
+        return {prefix + 'sample_cnt': 0,
+                prefix + 'mean': MeasurementVal('nan'),
+                prefix + 'stdev': MeasurementVal('nan'),
+                prefix + 'variance': MeasurementVal('nan'),
+                prefix + 'min': MeasurementVal('nan'),
+                prefix + 'max': MeasurementVal('nan'),
+                prefix + 'minus': MeasurementVal('nan'),
+                prefix + 'plus': MeasurementVal('nan')}
+
+    stats = {'name': meas['name']}
+    if meas['type'] == 'sysres':
+        val_cls = TimeVal
+        values = meas['values']['elapsed_time']
+    elif meas['type'] == 'diskusage':
+        val_cls = SizeVal
+        values = meas['values']['size']
+    else:
+        raise Exception("Unknown measurement type '{}'".format(meas['type']))
+    stats['val_cls'] = val_cls
+    stats['quantity'] = val_cls.quantity
+    stats[prefix + 'sample_cnt'] = len(values)
+
+    mean_val = val_cls(mean(values))
+    min_val = val_cls(min(values))
+    max_val = val_cls(max(values))
+
+    stats[prefix + 'mean'] = mean_val
+    if len(values) > 1:
+        stats[prefix + 'stdev'] = val_cls(stdev(values))
+        stats[prefix + 'variance'] = val_cls(variance(values))
+    else:
+        stats[prefix + 'stdev'] = float('nan')
+        stats[prefix + 'variance'] = float('nan')
+    stats[prefix + 'min'] = min_val
+    stats[prefix + 'max'] = max_val
+    stats[prefix + 'minus'] = val_cls(mean_val - min_val)
+    stats[prefix + 'plus'] = val_cls(max_val - mean_val)
+
+    return stats
+