C4FM: bounds check the start of the buffer in the sync score (#2622)

A lagging sync detect on the first symbol after a buffer reload, followed by a
coarse timing search that steps all the way left, scores a candidate whose first
sync symbol is one sample before the buffer start. score() only checked the end
of the buffer, so it threw ArrayIndexOutOfBoundsException: Index -1.

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

src/main/java/io/github/dsheirer/dsp/c4fm/C4FMSymbolProcessor.java

@@ -1313,7 +1313,7 @@ public abstract class C4FMSymbolProcessor
 
         for(int x = 0; x < patternSymbols.length; x++)
         {
-            if(bufferPointer < maxPointer)
+            if(bufferPointer >= 0 && bufferPointer < maxPointer)
             {
                 softSymbol = interpolate(mBuffer[bufferPointer], mBuffer[bufferPointer + 1], fractional) * gain + balance;
             }

src/test/java/io/github/dsheirer/dsp/c4fm/C4FMSymbolProcessorScoreTest.java

@@ -0,0 +1,50 @@
+/*
+ * *****************************************************************************
+ * 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.dsp.c4fm;
+
+import io.github.dsheirer.module.decode.p25.phase1.P25P1SymbolProcessorC4FM;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+/**
+ * JUnit tests for the sync correlation score in the C4FM symbol processor.
+ */
+public class C4FMSymbolProcessorScoreTest
+{
+    /**
+     * A lagging sync detect on the first symbol after a buffer reload, followed by a coarse timing search that steps
+     * all the way left, scores a candidate whose first sync symbol starts just before the start of the buffer.
+     */
+    @Test
+    public void scoreBeforeBufferStart()
+    {
+        float samplesPerSymbol = 25000f / 4800f;
+        P25P1SymbolProcessorC4FM processor = new P25P1SymbolProcessorC4FM(null, null);
+        processor.setSamplesPerSymbol(samplesPerSymbol);
+        //Sync workspace, sized as in setSamplesPerSymbol(), plus a 1024 sample buffer
+        int syncWorkspaceLength = (int)Math.ceil((processor.mSyncDetector.getDibitLength() + 1) * samplesPerSymbol);
+        processor.mBuffer = new float[syncWorkspaceLength + 1024];
+
+        //Offset to the final sync symbol that puts the first sync symbol at buffer index -0.3
+        double offset = (processor.mSyncDetector.getDibitLength() - 1) * samplesPerSymbol - 0.3;
+        assertDoesNotThrow(() -> processor.score(offset, samplesPerSymbol));
+    }
+}