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diff --git a/resources/libraries/python/MLRsearch/PerDurationDatabase.py b/resources/libraries/python/MLRsearch/PerDurationDatabase.py
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+++ b/resources/libraries/python/MLRsearch/PerDurationDatabase.py
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+# Copyright (c) 2021 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 defining PerDurationDatabase class."""
+
+import copy
+
+
+class PerDurationDatabase:
+    """List-like structure holding measurement results for one duration.
+
+    This is a building block for MeasurementDatabase.
+
+    This class hold measurements for single target duration value only,
+    so the logic is quite simple.
+
+    Several utility methods are added, accomplishing tasks useful for MLRsearch
+    (to be called by MeasurementDatabade).
+    """
+
+    def __init__(self, duration, measurements):
+        """Store (deep copy of) measurement results and normalize them.
+
+        The results have to have the corresponding target duration,
+        and there should be no duplicate target_tr values.
+        Empty iterable (zero measurements) is an acceptable input.
+
+        :param duration: All measurements have to have this target duration [s].
+        :param measurements: The measurement results to store.
+        :type duration: float
+        :type measurements: Iterable[ReceiveRateMeasurement]
+        :raises ValueError: If duration does not match or if TR duplicity.
+        """
+        self.duration = duration
+        self.measurements = [copy.deepcopy(meas) for meas in measurements]
+        self._normalize()
+
+    def __repr__(self):
+        """Return string executable to get equivalent instance.
+
+        :returns: Code to construct equivalent instance.
+        :rtype: str
+        """
+        return (
+            u"PerDurationDatabase("
+            f"duration={self.duration!r},"
+            f"measurements={self.measurements!r})"
+        )
+
+    def _normalize(self):
+        """Sort by target_tr, fail on detecting duplicate target_tr.
+
+        Also set effective loss ratios.
+        """
+        measurements = self.measurements
+        measurements.sort(key=lambda measurement: measurement.target_tr)
+        # Detect duplicated TRs.
+        previous_tr = None
+        for measurement in measurements:
+            current_tr = measurement.target_tr
+            if current_tr == previous_tr:
+                raise ValueError(
+                    u"Transmit rate conflict:"
+                    f" {measurement!r} {previous_tr!r}"
+                )
+            previous_tr = current_tr
+        # Update effective ratios.
+        ratio_previous = None
+        for measurement in measurements:
+            if ratio_previous is None:
+                ratio_previous = measurement.loss_ratio
+                measurement.effective_loss_ratio = ratio_previous
+                continue
+            ratio_previous = max(ratio_previous, measurement.loss_ratio)
+            measurement.effective_loss_ratio = ratio_previous
+
+    def add(self, measurement):
+        """Add measurement and normalize.
+
+        :param measurement: Measurement result to add to the database.
+        :type measurement: ReceiveRateMeasurement
+        """
+        # TODO: We should deepcopy either everywhere or nowhere.
+        self.measurements.append(measurement)
+        self._normalize()
+
+    def get_valid_bounds(self, ratio):
+        """Return None or a valid measurement for two tightest bounds.
+
+        The validity of a measurement to act as a bound is determined
+        by comparing the argument ratio with measurement's effective loss ratio.
+
+        Both lower and upper bounds are returned, both tightest and second
+        tightest. If some value is not available, None is returned instead.
+
+        :param ratio: Target ratio, valid has to be lower or equal.
+        :type ratio: float
+        :returns: Tightest lower bound, tightest upper bound,
+            second tightest lower bound, second tightest upper bound.
+        :rtype: 4-tuple of Optional[ReceiveRateMeasurement]
+        """
+        lower_1, upper_1, lower_2, upper_2 = None, None, None, None
+        for measurement in self.measurements:
+            if measurement.effective_loss_ratio > ratio:
+                if upper_1 is None:
+                    upper_1 = measurement
+                    continue
+                upper_2 = measurement
+                break
+            lower_1, lower_2 = measurement, lower_1
+        return lower_1, upper_1, lower_2, upper_2