add class for online centroid computation

nominatim/utils/centroid.py

@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2022 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Functions for computation of centroids.
+"""
+from collections.abc import Collection
+
+class PointsCentroid:
+    """ Centroid computation from single points using an online algorithm.
+        More points may be added at any time.
+
+        Coordinates are internally treated as a 7-digit fixed-point float
+        (i.e. in OSM style).
+    """
+
+    def __init__(self):
+        self.sum_x = 0
+        self.sum_y = 0
+        self.count = 0
+
+    def centroid(self):
+        """ Return the centroid of all points collected so far.
+        """
+        if self.count == 0:
+            raise ValueError("No points available for centroid.")
+
+        return (float(self.sum_x/self.count)/10000000,
+                float(self.sum_y/self.count)/10000000)
+
+
+    def __len__(self):
+        return self.count
+
+
+    def __iadd__(self, other):
+        if isinstance(other, Collection) and len(other) == 2:
+            if all(isinstance(p, (float, int)) for p in other):
+                x, y = other
+                self.sum_x += int(x * 10000000)
+                self.sum_y += int(y * 10000000)
+                self.count += 1
+                return self
+
+        raise ValueError("Can only add 2-element tuples to centroid.")

test/python/utils/test_centroid.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2022 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Tests for centroid computation.
+"""
+import pytest
+
+from nominatim.utils.centroid import PointsCentroid
+
+def test_empty_set():
+    c = PointsCentroid()
+
+    with pytest.raises(ValueError, match='No points'):
+        c.centroid()
+
+
+@pytest.mark.parametrize("centroid", [(0,0), (-1, 3), [0.0000032, 88.4938]])
+def test_one_point_centroid(centroid):
+    c = PointsCentroid()
+
+    c += centroid
+
+    assert len(c.centroid()) == 2
+    assert c.centroid() == (pytest.approx(centroid[0]), pytest.approx(centroid[1]))
+
+
+def test_multipoint_centroid():
+    c = PointsCentroid()
+
+    c += (20.0, -10.0)
+    assert c.centroid() == (pytest.approx(20.0), pytest.approx(-10.0))
+    c += (20.2, -9.0)
+    assert c.centroid() == (pytest.approx(20.1), pytest.approx(-9.5))
+    c += (20.2, -9.0)
+    assert c.centroid() == (pytest.approx(20.13333), pytest.approx(-9.333333))
+
+
+def test_manypoint_centroid():
+    c = PointsCentroid()
+
+    for _ in range(10000):
+        c += (4.564732, -0.000034)
+
+    assert c.centroid() == (pytest.approx(4.564732), pytest.approx(-0.000034))
+
+
+@pytest.mark.parametrize("param", ["aa", None, 5, [1, 2, 3], (3, None), ("a", 3.9)])
+def test_add_non_tuple(param):
+    c = PointsCentroid()
+
+    with pytest.raises(ValueError, match='2-element tuples'):
+        c += param