[GitHub] [solr] danrosher commented on a diff in pull request #940: SOLR-16292 : NVector for alternative great-circle distance calculations

[GitBox <gi...@apache.org>]

danrosher commented on code in PR #940:
URL: https://github.com/apache/solr/pull/940#discussion_r922320371


##########
solr/core/src/test/org/apache/solr/util/FastInvTrigTest.java:
##########
@@ -0,0 +1,75 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+package org.apache.solr.util;
+
+import org.apache.solr.SolrTestCase;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Random;
+
+import static org.apache.solr.util.NVectorUtil.EARTH_RADIUS;
+
+public class FastInvTrigTest extends SolrTestCase {
+
+    final static int num_points = 100000;
+    final static double EPSILON = 0.0001;
+    static final Random r = new Random();
+    private static final double TEN_METERS = 0.01;
+
+    static double[][] points = new double[num_points][2];
+
+    @Before
+    public void initAll() {
+        for (int i = 0; i < num_points; i++) {
+            points[i] = generateRandomPoint();
+        }
+    }
+
+    public static double deg2rad(double deg) {
+        return deg * (Math.PI / 180);
+    }
+
+    public static double[] generateRandomPoint() {
+        double u = r.nextDouble();
+        double v = r.nextDouble();
+
+        double latitude = deg2rad(Math.toDegrees(Math.acos(u * 2 - 1)) - 90);
+        double longitude = deg2rad(360 * v - 180);
+        return new double[]{latitude, longitude};
+    }
+
+    @Test
+    public void acos() {
+        for (double i = -1; i <= 1; i = i + 0.00001) {
+            assertTrue(FastInvTrig.acos(i) - Math.acos(i) <= EPSILON);

Review Comment:
   changed to assertEquals



##########
solr/core/src/java/org/apache/solr/util/FastInvTrig.java:
##########
@@ -0,0 +1,108 @@
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+package org.apache.solr.util;
+
+public class FastInvTrig {

Review Comment:
   Yes I wrote this, original in my github repo. It's a Maclaurin series expansion of acos. TABLE stores the coefficients that can be re-used for subsequent calculations, it also reuses x^2 for x^3,x^5 etc. Also initially my implementation required a lot of terms for convergence, until I found this https://stackoverflow.com/questions/20196000/own-asin-function-with-taylor-series-not-accurate which allows for faster convergence near -1,1. I have a benchmark in my repo to show it more performant than Math.acos or FastMath.acos, accuracy appears OK in my tests. NVector with acos faster than Haversine.



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