From 5b0e7e506510237aa1a079cae410219f06f591b8 Mon Sep 17 00:00:00 2001
From: rjo12 <119473038+rjo12@users.noreply.github.com>
Date: Thu, 11 May 2023 22:49:23 +0930
Subject: [PATCH] Bugfix:   Off-by-one bug in subtracting UINT.   Fixes #1557

---
 .../opentracks/sensors/UintUtilsTest.java     | 39 ++++++++++++++++---
 .../opentracks/sensors/UintUtils.java         |  3 +-
 2 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/src/androidTest/java/de/dennisguse/opentracks/sensors/UintUtilsTest.java b/src/androidTest/java/de/dennisguse/opentracks/sensors/UintUtilsTest.java
index 13926fcb6..63d40834a 100644
--- a/src/androidTest/java/de/dennisguse/opentracks/sensors/UintUtilsTest.java
+++ b/src/androidTest/java/de/dennisguse/opentracks/sensors/UintUtilsTest.java
@@ -11,15 +11,44 @@ public class UintUtilsTest {
         assertEquals(0, UintUtils.diff(1, 1, UintUtils.UINT16_MAX));
         assertEquals(1, UintUtils.diff(2, 1, UintUtils.UINT16_MAX));
         assertEquals(3, UintUtils.diff(5, 2, UintUtils.UINT16_MAX));
-        assertEquals(65534, UintUtils.diff(1, 2, UintUtils.UINT16_MAX));
-//        assertEquals(65535, UintUtils.diff(0, 1, UintUtils.UINT16_MAX));
-        assertEquals(65535, UintUtils.UINT16_MAX);
-
+        assertEquals(65535, UintUtils.diff(1, 2, UintUtils.UINT16_MAX));     /* unsigned 16 arithmetic is modulo UINT16_MAX + 1, not modulo UINT16_MAX */
         assertEquals(65530, UintUtils.diff(UintUtils.UINT16_MAX, 5, UintUtils.UINT16_MAX));
+
+        /*   Test modulo arithmetic for arguments that are out of range */
+
+        if (false) {   /*  false means UintUtils.diff() throws if arguments out of range */
+            assertEquals(0, UintUtils.diff(65537, 1, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(1, 65537, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(65537, 65537, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(-65535, 1, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(1, -65535, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(-65535, -65535, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(-65535, 65537, UintUtils.UINT16_MAX));
+            assertEquals(0, UintUtils.diff(65537, -65535, UintUtils.UINT16_MAX));
+        }
+
+        /*  The following tests are the above, but for 32-bit unsigned. */
+
+        assertEquals(0, UintUtils.diff(1, 1, UintUtils.UINT32_MAX));
+        assertEquals(1, UintUtils.diff(2, 1, UintUtils.UINT32_MAX));
+        assertEquals(3, UintUtils.diff(5, 2, UintUtils.UINT32_MAX));
+        assertEquals(4294967295L, UintUtils.diff(1, 2, UintUtils.UINT32_MAX));
+        assertEquals(4294967290L, UintUtils.diff(UintUtils.UINT32_MAX, 5, UintUtils.UINT32_MAX));
+
+        if (false) { /*  false means UintUtils.diff() throws if arguments out of range */
+            assertEquals(0, UintUtils.diff(4294967297L, 1, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(1, 4294967297L, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(4294967297L, 4294967297L, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(-4294967295L, 1, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(1, -4294967295L, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(-4294967295L, -4294967295L, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(-4294967295L, 4294967297L, UintUtils.UINT32_MAX));
+            assertEquals(0, UintUtils.diff(65537, -4294967297L, UintUtils.UINT32_MAX));
+        }
     }
 
     @Test
     public void realData() {
         assertEquals(1, UintUtils.diff(381616, 381615, UintUtils.UINT32_MAX));
     }
-}
\ No newline at end of file
+}
diff --git a/src/main/java/de/dennisguse/opentracks/sensors/UintUtils.java b/src/main/java/de/dennisguse/opentracks/sensors/UintUtils.java
index c4610cd22..de76428f2 100644
--- a/src/main/java/de/dennisguse/opentracks/sensors/UintUtils.java
+++ b/src/main/java/de/dennisguse/opentracks/sensors/UintUtils.java
@@ -14,6 +14,7 @@ public class UintUtils {
      * @return diff
      */
     public static long diff(long a, long b, final long UINT_MAX) {
+
         if (a < 0 || b < 0) {
             throw new RuntimeException("a or b cannot be less than zero.");
         }
@@ -25,6 +26,6 @@ public class UintUtils {
             return a - b;
         }
 
-        return (UINT_MAX - b) + a;
+        return (UINT_MAX + 1 - b) + a;
     }
 }