#2516 DMR Decoder enhancements. Architecture now shares common C4FM framework like P25P1 and NXDN. Adds soft decoding and enhanced/corrected error detection and correction. (#2517)

Co-authored-by: denny <dennis.sheirer@gmail.com>

src/main/java/io/github/dsheirer/dsp/c4fm/C4FMMessageFramer.java

@@ -7,6 +7,26 @@ import io.github.dsheirer.dsp.symbol.Dibit;
  */
 public abstract class C4FMMessageFramer
 {
+    private static final float SOFT_SYMBOL_SCALE = (float)(4.0 / Math.PI); //Radians to symbol units (+/-1, +/-3)
+
+    /**
+     * Converts the soft symbol into a pair of soft bit values using max-log likelihood ratios for the Gray mapped
+     * 4-level symbol (D01=+3, D00=+1, D10=-1, D11=-3), where a positive soft bit value favors a 1 bit.
+     * @param softSymbol in radians
+     * @param softBits array to receive the two soft bit values
+     * @param index for the first bit (sign bit), followed by the second (magnitude) bit
+     */
+    protected static void setSoftBits(float softSymbol, float[] softBits, int index)
+    {
+        float u = softSymbol * SOFT_SYMBOL_SCALE;
+        float magnitude = Math.abs(u);
+        //First bit is 1 for negative symbols
+        float favorZero = magnitude <= 2.0f ? 4.0f * u : 8.0f * u - 8.0f * Math.signum(u);
+        softBits[index] = -favorZero;
+        //Second bit is 1 for the outer (+/-3) symbols
+        softBits[index + 1] = 4.0f * (magnitude - 2.0f);
+    }
+
     /**
      * Process the symbol
      * @param symbol in radians