Add geom::simplify() function

For linestrings only. For polygons we'll need something else.

Author: joto

src/geom-functions.cpp

@@ -562,4 +562,25 @@ geometry_t centroid(geometry_t const &geom)
     return output;
 }
 
+void simplify(geometry_t *output, geometry_t const &geom, double tolerance)
+{
+    if (!geom.is_linestring()) {
+        output->reset();
+        return;
+    }
+
+    output->set<linestring_t>();
+    output->set_srid(geom.srid());
+
+    boost::geometry::simplify(geom.get<linestring_t>(),
+                              output->get<linestring_t>(), tolerance);
+}
+
+geometry_t simplify(geometry_t const &geom, double tolerance)
+{
+    geom::geometry_t output{linestring_t{}, geom.srid()};
+    simplify(&output, geom, tolerance);
+    return output;
+}
+
 } // namespace geom

src/geom-functions.hpp

@@ -126,6 +126,36 @@ geometry_t line_merge(geometry_t geom);
  */
 geometry_t centroid(geometry_t const &geom);
 
+/**
+ * Simplify a linestring geometry using Douglas-Peucker algorithm.
+ *
+ * Geometries might become invalid by using simplify. The simplification
+ * process might create self-intersections.
+ *
+ * Returns null geometry for anything but linestring geometries!
+ *
+ * \param output Pointer to output geometry.
+ * \param geom Input geometry.
+ * \param tolerance Max distance (in units of input coordinates) of a point
+ *                  to other segments to be removed.
+ */
+void simplify(geometry_t *output, geometry_t const &geom, double tolerance);
+
+/**
+ * Simplify a linestring geometry using Douglas-Peucker algorithm.
+ *
+ * Geometries might become invalid by using simplify. The simplification
+ * process might create self-intersections.
+ *
+ * Returns null geometry for anything but linestring geometries!
+ *
+ * \param geom Input geometry.
+ * \param tolerance Max distance (in units of input coordinates) of a point
+ *                  to other segments to be removed.
+ * \returns Simplified geometry.
+ */
+geometry_t simplify(geometry_t const &geom, double tolerance);
+
 } // namespace geom
 
 #endif // OSM2PGSQL_GEOM_FUNCTIONS_HPP

tests/test-geom-linestrings.cpp

@@ -192,3 +192,30 @@ TEST_CASE("geom::segmentize with split 1.0 at end", "[NoDB]")
     REQUIRE(ml[2] == expected[2]);
     REQUIRE(ml[3] == expected[3]);
 }
+
+TEST_CASE("geom::simplify", "[NoDB]")
+{
+    geom::geometry_t const input{
+        geom::linestring_t{{0, 0}, {1, 1}, {2, 0}, {3, 1}, {4, 0}, {5, 1}}};
+
+    SECTION("small tolerance leaves linestring as is")
+    {
+        auto const geom = geom::simplify(input, 0.5);
+
+        REQUIRE(geom.is_linestring());
+        auto const &l = geom.get<geom::linestring_t>();
+        REQUIRE(l.size() == 6);
+        REQUIRE(l == input.get<geom::linestring_t>());
+    }
+
+    SECTION("large tolerance simplifies linestring")
+    {
+        auto const geom = geom::simplify(input, 2.0);
+
+        REQUIRE(geom.is_linestring());
+        auto const &l = geom.get<geom::linestring_t>();
+        REQUIRE(l.size() == 2);
+        REQUIRE(l[0] == input.get<geom::linestring_t>()[0]);
+        REQUIRE(l[1] == input.get<geom::linestring_t>()[5]);
+    }
+}