edac: correct Golay(24,12) words that are not at the start of the message (#2631)

Golay24.checkAndCorrect indexed the word from bit 0 (getInt(0, 22), xor(12, ...),
flip(index)) and took the parity over the whole message, so it was only right for a
message that is exactly one 24-bit word. P25 TDULC words 2-12 and the DMR voice
superframe words at 24 and 48 were "corrected" in the wrong bits. Decode any other
word in a 24-bit copy and write it back.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/main/java/io/github/dsheirer/edac/Golay24.java

@@ -68,6 +68,29 @@ public class Golay24
      */
     public static int checkAndCorrect(CorrectedBinaryMessage message, int startIndex)
     {
+        //The correction below indexes the word from bit 0 and takes the parity over the whole message, so it is only
+        //correct for a message that is exactly the 24-bit word.  Decode any other word in a 24-bit copy.
+        if(startIndex != 0 || message.size() != 24)
+        {
+            CorrectedBinaryMessage word = new CorrectedBinaryMessage(24);
+            int original = message.getInt(startIndex, startIndex + 23);
+            word.load(0, 24, original);
+            int result = checkAndCorrectWord(word);
+            int corrected = word.getInt(0, 23);
+            message.load(startIndex, 24, corrected);
+            message.incrementCorrectedBitCount(Integer.bitCount(original ^ corrected));
+            return result;
+        }
+
+        return checkAndCorrectWord(message);
+    }
+
+    /**
+     * Error detection and correction of a message that is exactly one 24-bit word.
+     */
+    private static int checkAndCorrectWord(CorrectedBinaryMessage message)
+    {
+        int startIndex = 0;
         boolean parityError = message.cardinality() % 2 != 0;
 
         int syndrome = getSyndrome(message, startIndex);

src/test/java/io/github/dsheirer/edac/Golay24OffsetTest.java

@@ -0,0 +1,85 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2026 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ * ****************************************************************************
+ */
+
+package io.github.dsheirer.edac;
+
+import io.github.dsheirer.bits.CorrectedBinaryMessage;
+import java.util.Random;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Tests Golay(24,12) correction of the data bits of words that are not at the start of a 24-bit message.
+ */
+public class Golay24OffsetTest
+{
+    private static int encode(int data)
+    {
+        int checksum = 0;
+
+        for(int bit = 0; bit < 12; bit++)
+        {
+            if((data & (1 << (11 - bit))) != 0)
+            {
+                checksum ^= Golay24.CHECKSUMS[bit];
+            }
+        }
+
+        int word23 = (data << 11) | checksum;
+        return (word23 << 1) | (Integer.bitCount(word23) & 1);
+    }
+
+    @Test
+    public void correctsUpToThreeErrorsInEveryWordOfALongerMessage()
+    {
+        Random random = new Random(1);
+
+        for(int trial = 0; trial < 2000; trial++)
+        {
+            int[] words = new int[3];
+            CorrectedBinaryMessage message = new CorrectedBinaryMessage(72);
+
+            for(int w = 0; w < 3; w++)
+            {
+                words[w] = encode(random.nextInt(4096));
+                int received = words[w];
+
+                for(int e = random.nextInt(4); e > 0; e--)
+                {
+                    received ^= 1 << random.nextInt(24);
+                }
+
+                message.load(w * 24, 24, received);
+            }
+
+            for(int w = 0; w < 3; w++)
+            {
+                Golay24.checkAndCorrect(message, w * 24);
+            }
+
+            for(int w = 0; w < 3; w++)
+            {
+                //The 12 data bits.  (The core correction can leave the overall parity bit wrong when it corrects other
+                //bits, which no caller reads.)
+                assertEquals(words[w] >> 12, message.getInt(w * 24, w * 24 + 11), "trial " + trial + " word " + w);
+            }
+        }
+    }
+}