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/// \file 
/// Reverb parameter estimation.

#pragma once

#include <AK/SoundEngine/Common/AkTypes.h>
#include <AK/SpatialAudio/Common/AkSpatialAudio.h>
#include <AK/SoundEngine/Common/AkVirtualAcoustics.h>

namespace AK {
namespace SpatialAudio {

/// Audiokinetic reverb estimation namespace
namespace ReverbEstimation
{
	////////////////////////////////////////////////////////////////////////
	/// @name Reverb estimation. 
	/// These functions can be used to estimate the reverb parameters of a physical environment, using its volume and surface area
	//@{

	/// This is used to estimate the line of best fit through the absorption values of an Acoustic Texture.
	/// This value is what's known as the HFDamping.
	/// return Gradient of line of best fit through y = mx + c.
	float CalculateSlope(const AkAcousticTexture& texture)
	{
		// constants for the mean and standard deviation of the acoustic texture indices 0,1,2,3
		const float meanX = 1.5f;
		const float sdX = 1.11803399;

		const int N = 4;
		float meanY = (texture.fAbsorptionLow + texture.fAbsorptionMidLow + texture.fAbsorptionMidHigh + texture.fAbsorptionHigh) / (float)N;
		float absorptions[4] = { texture.fAbsorptionLow, texture.fAbsorptionMidLow, texture.fAbsorptionMidHigh, texture.fAbsorptionHigh };
		float sdY = 0.0f;
		float meanDiffDotProd = 0.0f;
		for (int i = 0; i < N; ++i)
		{
			const float yMeanDiff = absorptions[i] - meanY;
			const float xMeanDiff = (float)i - meanX;
			meanDiffDotProd += yMeanDiff * xMeanDiff;
			sdY += yMeanDiff * yMeanDiff;
		}
		if (sdY == 0.0f)
			return 0.0f;
		sdY = sqrtf(sdY / (float)N);
		const float correlationCoeff = (1.0f / (N - 1.0f)) * (meanDiffDotProd / (sdY * sdX));
		return correlationCoeff * (sdY / sdX);
	}

	/// Calculate average absorption values using each of the textures in in_textures, weighted by their corresponding surface area.
	void GetAverageAbsorptionValues(AkAcousticTexture* in_textures, float* in_surfaceAreas, int in_numTextures, AkAcousticTexture& out_average)
	{
		float surfaceArea = 0.0f;
		float totalSurfaceArea = 0.0f;
		out_average.fAbsorptionLow = 0.0f;
		out_average.fAbsorptionMidLow = 0.0f;
		out_average.fAbsorptionMidHigh = 0.0f;
		out_average.fAbsorptionHigh = 0.0f;
		for (int textureIndex = 0; textureIndex < in_numTextures; ++textureIndex)
		{
			AkAcousticTexture& texture = in_textures[textureIndex];
			surfaceArea = in_surfaceAreas[textureIndex];
			out_average.fAbsorptionLow += texture.fAbsorptionLow * surfaceArea;
			out_average.fAbsorptionMidLow += texture.fAbsorptionMidLow * surfaceArea;
			out_average.fAbsorptionMidHigh += texture.fAbsorptionMidHigh * surfaceArea;
			out_average.fAbsorptionHigh += texture.fAbsorptionHigh * surfaceArea;
			totalSurfaceArea += surfaceArea;
		}
		if (totalSurfaceArea > 0.0f)
		{
			out_average.fAbsorptionLow = out_average.fAbsorptionLow / totalSurfaceArea;
			out_average.fAbsorptionMidLow = out_average.fAbsorptionMidLow / totalSurfaceArea;
			out_average.fAbsorptionMidHigh = out_average.fAbsorptionMidHigh / totalSurfaceArea;
			out_average.fAbsorptionHigh = out_average.fAbsorptionHigh / totalSurfaceArea;
		}
	}

	/// Estimate the time taken (in seconds) for the sound reverberation in a physical environment to decay by 60 dB.
	/// This is estimated using the Sabine equation. The T60 decay time can be used to drive the decay parameter of a reverb effect.
	AK_EXTERNAPIFUNC(AKRESULT, EstimateT60Decay)(
		AkReal32 in_volumeCubicMeters,				///< The volume (in cubic meters) of the physical environment. 0 volume or negative volume will give a decay estimate of 0.
		AkReal32 in_surfaceAreaSquaredMeters,		///< The surface area (in squared meters) of the physical environment. Must be >= AK_SA_MIN_ENVIRONMENT_SURFACE_AREA
		AkReal32 in_environmentAverageAbsorption,	///< The average absorption amount of the surfaces in the environment. Must be between AK_SA_MIN_ENVIRONMENT_ABSORPTION and 1.
		AkReal32& out_decayEstimate					///< Returns the time taken (in seconds) for the reverberation to decay bu 60 dB.
		)
	{
		if (in_volumeCubicMeters <= 0.0f)
		{
			out_decayEstimate = 0.0f;
			return AKRESULT::AK_Success;
		}
		if (in_surfaceAreaSquaredMeters < AK_SA_MIN_ENVIRONMENT_SURFACE_AREA)
		{
			AKASSERT(false && "AK::SpatialAudio::ReverbEstimation::EstimateT60Decay: Invalid surface area. in_SurfaceAreaSquaredMeters Must be >= AK_SA_MIN_ENVIRONMENT_SURFACE_AREA");
			return AKRESULT::AK_Fail;
		}
		if (in_environmentAverageAbsorption < AK_SA_MIN_ENVIRONMENT_ABSORPTION || in_environmentAverageAbsorption > 1.0f)
		{
			AKASSERT(false && "AK::SpatialAudio::ReverbEstimation::EstimateT60Decay: Invalid absorption value. in_EnvironmentAverageAbsorption Must be between AK_SA_MIN_ENVIRONMENT_ABSORPTION and 1");
			return AKRESULT::AK_Fail;
		}
		// The Sabine equation.
		out_decayEstimate = (0.161f * in_volumeCubicMeters) / (in_surfaceAreaSquaredMeters * in_environmentAverageAbsorption);
		return AKRESULT::AK_Success;
	}

	/// Estimate the time taken (in milliseconds) for the first reflection to reach the listener.
	/// This assumes the emitter and listener are both positioned in the centre of the environment.
	AK_EXTERNAPIFUNC(AKRESULT, EstimateTimeToFirstReflection)(
		AkVector in_environmentExtentMeters,	///< Defines the dimensions of the environment (in meters) relative to the center; all components must be positive numbers.
		AkReal32& out_timeToFirstReflectionMs,	///< Returns the time taken (in milliseconds) for the first reflection to reach the listener.
		AkReal32 in_speedOfSound = 343.0f		///< Defaults to 343.0 - the speed of sound in dry air. Must be > 0.
		)
	{
		if (in_speedOfSound <= 0.0f)
		{
			AKASSERT(false && "AK::SpatialAudio::ReverbEstimation::EstimateTimeToFirstReflection: Invalid speed of sound. in_speedOfSound must be greater than 0.");
			return AKRESULT::AK_Fail;
		}
		if (in_environmentExtentMeters.X < 0.0f || in_environmentExtentMeters.Y < 0.0f || in_environmentExtentMeters.Z < 0.0f)
		{
			AKASSERT(false && "AK::SpatialAudio::ReverbEstimation::EstimateTimeToFirstReflection: Invalid extent. All components must be positive numbers.");
			return AKRESULT::AK_Fail;
		}
		const float minDimension = AkMin(AkMin(in_environmentExtentMeters.X, in_environmentExtentMeters.Y), in_environmentExtentMeters.Z);
		out_timeToFirstReflectionMs = (minDimension / in_speedOfSound) * 1000.0f;
		return AKRESULT::AK_Success;
	}

	/// Estimate the high frequency damping from a collection of AkAcousticTextures and corresponding surface areas. 
	/// The high frequency damping is a measure of how much high frequencies are dampened compared to low frequencies. 
	/// The value is comprised between -1 and 1. A value > 0 indicates more high frequency damping than low frequency damping. < 0 indicates more low frequency damping than high frequency damping. 0 indicates uniform damping.
	/// The average absorption values are calculated using each of the textures in the collection, weighted by their corresponding surface area.
	/// The HFDamping is then calculated as the line-of-best-fit through the average absorption values.
	AK_EXTERNAPIFUNC(AkReal32, EstimateHFDamping)(
		AkAcousticTexture* in_textures,	///< A collection of AkAcousticTexture structs from which to calculate the average high frequency damping.
		float* in_surfaceAreas,			///< Surface area values for each of the textures in in_textures.
		int in_numTextures				///< The number of textures in in_textures (and the number of surface area values in in_surfaceAreas).
		)
	{
		if (in_textures == nullptr || in_surfaceAreas == nullptr || in_numTextures == 0)
		{
			return 0.0f;
		}
		AkAcousticTexture averageAbsorptionValues;
		GetAverageAbsorptionValues(in_textures, in_surfaceAreas, in_numTextures, averageAbsorptionValues);
		return CalculateSlope(averageAbsorptionValues) * 2.f; // Multiply by 2 so that the Hf Damping value is between -1 and 1.
	}

	//@}
}
}
}