Fixed issue #5734 (FreeBoy Division by zero) (#6053)

* Fixed issue #5734 (FreeBoy Division by zero).

Added comments and used more descriptive variable names for noise
channel initialization block.

Also indented the nested for loop to improve code clarity.
The reasons for doing this can be found in this answer:
https://softwareengineering.stackexchange.com/a/362796

* Better initial div_ratio guess

Allows us to skip r = 0 and a conditional in the loop below.

---------

Co-authored-by: Spekular <[email protected]>

plugins/FreeBoy/FreeBoy.cpp

@@ -358,30 +358,34 @@ void FreeBoyInstrument::playNote(NotePlayHandle* nph, sampleFrame* workingBuffer
 
 	if (tfp == 0)
 	{
-		//PRNG Frequency = (1048576 Hz / (ratio + 1)) / 2 ^ (shiftclockfreq + 1)
-		char sopt = 0;
-		char ropt = 1;
-		float fopt = 524288.0 / (ropt * std::pow(2.0, sopt + 1.0));
-		float f;
+		// Initialize noise channel...
+		// PRNG Frequency = (1048576 Hz / (ratio + 1)) / 2 ^ (shiftclockfreq + 1)
+		// When div_ratio = 0 the ratio should be 0.5. Since s = 0 is the only case where r = 0 gives
+		// a unique frequency, we can start by guessing s = r = 0 here and then skip r = 0 in the loop.
+		char clock_freq = 0;
+		char div_ratio = 0;
+		float closest_freq = 524288.0 / (0.5 * std::pow(2.0, clock_freq + 1.0));
+		// This nested for loop iterates over all possible combinations of clock frequency and dividing
+		// ratio and chooses the combination whose resulting frequency is closest to the note frequency
 		for (char s = 0; s < 16; ++s)
 		{
-			for (char r = 0; r < 8; ++r)
+			for (char r = 1; r < 8; ++r)
 			{
-				f = 524288.0 / (r * std::pow(2.0, s + 1.0));
-				if (std::fabs(freq - fopt) > std::fabs(freq - f))
+				float f = 524288.0 / (r * std::pow(2.0, s + 1.0));
+				if (std::fabs(freq - closest_freq) > std::fabs(freq - f))
 				{
-					fopt = f;
-					ropt = r;
-					sopt = s;
+					closest_freq = f;
+					div_ratio = r;
+					clock_freq = s;
 				}
 			}
 		}
 
-		data = sopt;
+		data = clock_freq;
 		data = data << 1;
 		data += m_ch4ShiftRegWidthModel.value();
 		data = data << 3;
-		data += ropt;
+		data += div_ratio;
 		papu->writeRegister(0xff22, data);
 
 		//channel 4 init