From 14c7339ed0060eeca9c3725ff2cc07e6f57b92e8 Mon Sep 17 00:00:00 2001
From: liamcottle <liam@liamcottle.com>
Date: Thu, 3 Oct 2024 09:41:57 +1300
Subject: [PATCH] implement function to set integer position precision in bits

---
 src/utils/position_util.js      | 22 ++++++++++++++++++++++
 src/utils/position_util.test.js | 29 +++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/src/utils/position_util.js b/src/utils/position_util.js
index a3e4f05..6824b0b 100644
--- a/src/utils/position_util.js
+++ b/src/utils/position_util.js
@@ -1,5 +1,27 @@
 class PositionUtil {
 
+    /**
+     * Obfuscates the provided latitude or longitude down to the provided precision in bits.
+     * This is based on the same logic in the official meshtastic firmware.
+     * https://github.com/meshtastic/firmware/blob/0a93261c0646f93aea518cc0599e547e9dc0e997/src/modules/PositionModule.cpp#L187
+     */
+    static setPositionPrecision(latitudeOrLongitudeInteger, precision) {
+
+        // check if we should use the provided precision
+        if(precision > 0 && precision < 32){
+
+            // apply bitmask to reduce precision of position to wanted bits
+            latitudeOrLongitudeInteger = latitudeOrLongitudeInteger & (0xFFFFFFFF << (32 - precision));
+
+            // we want the imprecise position to be the middle of the possible location
+            latitudeOrLongitudeInteger += (1 << (31 - precision));
+
+        }
+
+        return latitudeOrLongitudeInteger;
+
+    }
+
     /**
      * Truncates the provided latitude or longitude to a maximum of x decimal places
      * e.g: 12.3456789 with 2 decimal places would be 12.34
diff --git a/src/utils/position_util.test.js b/src/utils/position_util.test.js
index 70cf615..f00d3bd 100644
--- a/src/utils/position_util.test.js
+++ b/src/utils/position_util.test.js
@@ -16,3 +16,32 @@ test('can truncate string position to provided decimal places', () => {
     expect(PositionUtil.truncateDecimalPlaces("123", 2)).toBe("123");
     expect(PositionUtil.truncateDecimalPlaces("1234.", 2)).toBe("1234");
 });
+
+test('can set integer position precision to provided bits', () => {
+
+    // these tests are using the auckland sky tower position
+    // auckland sky tower: -36.84844007222091, 174.76221115261924
+    // the outputs we are expecting, are the same values returned by the code in the meshtastic firmware
+    // https://github.com/meshtastic/firmware/blob/0a93261c0646f93aea518cc0599e547e9dc0e997/src/modules/PositionModule.cpp#L187
+
+    // set precision to 32 bits (within 0 meters)
+    // -36.8484400, 174.7622111 -> -36.8484400, 174.7622111
+    expect(PositionUtil.setPositionPrecision(-368484400, 32)).toBe(-368484400);
+    expect(PositionUtil.setPositionPrecision(1747622111, 32)).toBe(1747622111);
+
+    // set precision to 16 bits (within ~364 meters)
+    // -36.8484400, 174.7622111 -> -36.8476160, 174.7615744
+    expect(PositionUtil.setPositionPrecision(-368484400, 16)).toBe(-368476160);
+    expect(PositionUtil.setPositionPrecision(1747622111, 16)).toBe(1747615744);
+
+    // set precision to 13 bits (within ~2.9 kilometers)
+    // -36.8484400, 174.7622111 -> -36.8312320, 174.7714048
+    expect(PositionUtil.setPositionPrecision(-368484400, 13)).toBe(-368312320);
+    expect(PositionUtil.setPositionPrecision(1747622111, 13)).toBe(1747714048);
+
+    // set precision to 11 bits (within ~11.6 kilometers)
+    // -36.8484400, 174.7622111 -> -36.8050176, 174.7976192
+    expect(PositionUtil.setPositionPrecision(-368484400, 11)).toBe(-368050176);
+    expect(PositionUtil.setPositionPrecision(1747622111, 11)).toBe(1747976192);
+
+});