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Cubic spline code cleanup

--- a/src/mixer/ft2_cubic_spline.c

+++ b/src/mixer/ft2_cubic_spline.c

@@ -18,13 +18,13 @@

 	float *fPtr = fCubicSplineLUT;

 	for (int32_t i = 0; i < CUBIC_SPLINE_PHASES; i++)

 	{

-		const double x  = i * (1.0 / CUBIC_SPLINE_PHASES); // x = i / CUBIC_SPLINE_PHASES

-		const double x2 = x * x;  // x^2

-		const double x3 = x2 * x; // x^3

+		const double x1 = i * (1.0 / (double)CUBIC_SPLINE_PHASES); // i / CUBIC_SPLINE_PHASES

+		const double x2 = x1 * x1; // x^2

+		const double x3 = x2 * x1; // x^3

-		*fPtr++ = (float)(-0.5 * x3 + 1.0 * x2 - 0.5 * x);

+		*fPtr++ = (float)(-0.5 * x3 + 1.0 * x2 - 0.5 * x1);

 		*fPtr++ = (float)( 1.5 * x3 - 2.5 * x2 + 1.0);

-		*fPtr++ = (float)(-1.5 * x3 + 2.0 * x2 + 0.5 * x);

+		*fPtr++ = (float)(-1.5 * x3 + 2.0 * x2 + 0.5 * x1);

 		*fPtr++ = (float)( 0.5 * x3 - 0.5 * x2);

 	}

--- a/src/mixer/ft2_cubic_spline.h

+++ b/src/mixer/ft2_cubic_spline.h

@@ -5,7 +5,7 @@

 #include "ft2_mix.h" // MIXER_FRAC_BITS

 #define CUBIC_SPLINE_TAPS 4

-#define CUBIC_WIDTH_BITS 2

+#define CUBIC_WIDTH_BITS 2 // log2(CUBIC_SPLINE_TAPS)

 // 8192 is a good compromise

 #define CUBIC_SPLINE_PHASES 8192