From 25a4bde61f640791bf86a2a2b4b9bc9fa047c28e Mon Sep 17 00:00:00 2001
From: Isaac Brodsky <isaac@uber.com>
Date: Mon, 4 Jun 2018 11:20:21 -0700
Subject: [PATCH] Add the formula for number of indexes per resolution

---
 CMakeLists.txt                          |  2 +
 src/apps/miscapps/generateNumHexagons.c | 55 +++++++++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 src/apps/miscapps/generateNumHexagons.c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index fc7dd4543..6aacb10c2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -133,6 +133,7 @@ set(OTHER_SOURCE_FILES
     src/apps/miscapps/h3ToGeoBoundaryHier.c
     src/apps/miscapps/h3ToGeoHier.c
     src/apps/miscapps/generateBaseCellNeighbors.c
+    src/apps/miscapps/generateNumHexagons.c
     src/apps/miscapps/h3ToHier.c
     src/apps/benchmarks/benchmarkPolyfill.c
     src/apps/benchmarks/benchmarkH3Api.c)
@@ -249,6 +250,7 @@ add_h3_executable(h3ToGeoBoundary src/apps/filters/h3ToGeoBoundary.c ${APP_SOURC
 add_h3_executable(hexRange src/apps/filters/hexRange.c ${APP_SOURCE_FILES})
 add_h3_executable(kRing src/apps/filters/kRing.c ${APP_SOURCE_FILES})
 add_h3_executable(generateBaseCellNeighbors src/apps/miscapps/generateBaseCellNeighbors.c ${APP_SOURCE_FILES})
+add_h3_executable(generateNumHexagons src/apps/miscapps/generateNumHexagons.c ${APP_SOURCE_FILES})
 add_h3_executable(h3ToGeoBoundaryHier src/apps/miscapps/h3ToGeoBoundaryHier.c ${APP_SOURCE_FILES})
 add_h3_executable(h3ToGeoHier src/apps/miscapps/h3ToGeoHier.c ${APP_SOURCE_FILES})
 add_h3_executable(h3ToHier src/apps/miscapps/h3ToHier.c ${APP_SOURCE_FILES})
diff --git a/src/apps/miscapps/generateNumHexagons.c b/src/apps/miscapps/generateNumHexagons.c
new file mode 100644
index 000000000..41326dc29
--- /dev/null
+++ b/src/apps/miscapps/generateNumHexagons.c
@@ -0,0 +1,55 @@
+/*
+ * Copyright 2018 Uber Technologies, Inc.
+ *
+ * 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.
+ */
+/** @file generateNumHexagons.c
+ * @brief Generates the tables for numHexagons.
+ *
+ *  usage: `generateNumHexagons`
+ *
+ *  This program generates the number of unique indexes (not necessarily
+ *  hexagons) at each H3 resolution. It assumes aperture 7 and 12 pentagons
+ *  per resolution.
+ */
+
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include "constants.h"
+
+#define NUM_PENTAGONS 12
+
+/**
+ * Generates and prints the numHexagons table.
+ */
+static void generate() {
+    printf("static const int64_t nums[] = {\n");
+    int64_t count = NUM_BASE_CELLS;
+    for (int i = 0; i <= MAX_H3_RES; i++) {
+        printf("    %" PRId64 "L,\n", count);
+        // Each hexagon has 7 children, and each pentagon has 6 children
+        count = ((count - NUM_PENTAGONS) * 7) + (NUM_PENTAGONS * 6);
+    }
+    printf("};\n");
+}
+
+int main(int argc, char* argv[]) {
+    // check command line args
+    if (argc > 1) {
+        fprintf(stderr, "usage: %s\n", argv[0]);
+        exit(1);
+    }
+
+    generate();
+}