// Copyright Epic Games, Inc. All Rights Reserved.

#include "DSP/Noise.h"
#include "DSP/Dsp.h"
#include "HAL/PlatformTime.h"

namespace Audio
{
	FWhiteNoise::FWhiteNoise(int32 InRandomSeed)
		: RandomStream{ InRandomSeed }
	{}

	FWhiteNoise::FWhiteNoise()
		: FWhiteNoise{ static_cast<int32>(FPlatformTime::Cycles()) }
	{}

	static const float DefaultFilterGain = Audio::ConvertToLinear(-3.f);

	FPinkNoise::FPinkNoise(int32 InRandomSeed)
		: Noise{ InRandomSeed }
		, X_Z{ 0,0,0,0 }
		, Y_Z{ 0,0.0,0 }
		, A0{ DefaultFilterGain }
	{
		static_assert(UE_ARRAY_COUNT(X_Z) == 4, "sizeof(X_Z)==4");
		static_assert(UE_ARRAY_COUNT(Y_Z) == 4, "sizeof(Y_Z)==4");
	}

	FPinkNoise::FPinkNoise()
		: FPinkNoise{static_cast<int32>(FPlatformTime::Cycles())}
	{}

	float FPinkNoise::Generate()
	{
		// Filter Coefficients based on:
		// https://ccrma.stanford.edu/~jos/sasp/Example_Synthesis_1_F_Noise.html
		static constexpr float A[4] { 1.0f, -2.494956002f,2.017265875f, -0.522189400f };
		static constexpr float B[4] { 0.049922035f,-0.095993537f,0.050612699f,-0.004408786f};	

		static_assert(UE_ARRAY_COUNT(A) == UE_ARRAY_COUNT(X_Z), "A Coefficients and X need to be the same size");
		static_assert(UE_ARRAY_COUNT(B) == UE_ARRAY_COUNT(Y_Z), "B Coefficients and Y need to be the same size");

		X_Z[0] = Noise.Generate(); // Xn

		float Yn =
				A0 * X_Z[0]		// a0 * (x) (inject non-const filter-gain A0 in here).
			+ A[1] * X_Z[1]		// a1 * (x-1)
			+ A[2] * X_Z[2]		// a2 * (x-2)
			+ A[3] * X_Z[3]		// a3 * (x-3) 

			- B[0] * Y_Z[0]		// b1 * (y-1)
			- B[1] * Y_Z[1]		// b2 * (y-2)
			- B[2] * Y_Z[2]		// b3 * (y-3)
			- B[3] * Y_Z[3];	// b4 * (y-4)

		// Shuffle feed-forward state by one.
		X_Z[3] = X_Z[2];
		X_Z[2] = X_Z[1];
		X_Z[1] = X_Z[0];

		// Shuffle feed-back state by one.
		Y_Z[3] = Y_Z[2]; 
		Y_Z[2] = Y_Z[1];
		Y_Z[1] = Y_Z[0];
		Y_Z[0] = Yn;

		return Yn;
	}
}