/**
 * @author zz85 / https://github.com/zz85
 *
 * Based on "A Practical Analytic Model for Daylight"
 * aka The Preetham Model, the de facto standard analytic skydome model
 * http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
 *
 * First implemented by Simon Wallner
 * http://www.simonwallner.at/projects/atmospheric-scattering
 *
 * Improved by Martin Upitis
 * http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
 *
 * Three.js integration by zz85 http://twitter.com/blurspline
*/

THREE.ShaderLib[ 'sky' ] = {

	uniforms: {

		luminance: { value: 1 },
		turbidity: { value: 2 },
		reileigh: { value: 1 },
		mieCoefficient: { value: 0.005 },
		mieDirectionalG: { value: 0.8 },
		sunPosition: { value: new THREE.Vector3() }

	},

	vertexShader: [

		"varying vec3 vWorldPosition;",

		"void main() {",

			"vec4 worldPosition = modelMatrix * vec4( position, 1.0 );",
			"vWorldPosition = worldPosition.xyz;",

			"gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

		"}",

	].join( "\n" ),

	fragmentShader: [

		"uniform sampler2D skySampler;",
		"uniform vec3 sunPosition;",
		"varying vec3 vWorldPosition;",

		"vec3 cameraPos = vec3(0., 0., 0.);",
		"// uniform sampler2D sDiffuse;",
		"// const float turbidity = 10.0; //",
		"// const float reileigh = 2.; //",
		"// const float luminance = 1.0; //",
		"// const float mieCoefficient = 0.005;",
		"// const float mieDirectionalG = 0.8;",

		"uniform float luminance;",
		"uniform float turbidity;",
		"uniform float reileigh;",
		"uniform float mieCoefficient;",
		"uniform float mieDirectionalG;",

		"// constants for atmospheric scattering",
		"const float e = 2.71828182845904523536028747135266249775724709369995957;",
		"const float pi = 3.141592653589793238462643383279502884197169;",

		"const float n = 1.0003; // refractive index of air",
		"const float N = 2.545E25; // number of molecules per unit volume for air at",
								"// 288.15K and 1013mb (sea level -45 celsius)",
		"const float pn = 0.035;	// depolatization factor for standard air",

		"// wavelength of used primaries, according to preetham",
		"const vec3 lambda = vec3(680E-9, 550E-9, 450E-9);",

		"// mie stuff",
		"// K coefficient for the primaries",
		"const vec3 K = vec3(0.686, 0.678, 0.666);",
		"const float v = 4.0;",

		"// optical length at zenith for molecules",
		"const float rayleighZenithLength = 8.4E3;",
		"const float mieZenithLength = 1.25E3;",
		"const vec3 up = vec3(0.0, 1.0, 0.0);",

		"const float EE = 1000.0;",
		"const float sunAngularDiameterCos = 0.999956676946448443553574619906976478926848692873900859324;",
		"// 66 arc seconds -> degrees, and the cosine of that",

		"// earth shadow hack",
		"const float cutoffAngle = pi/1.95;",
		"const float steepness = 1.5;",


		"vec3 totalRayleigh(vec3 lambda)",
		"{",
			"return (8.0 * pow(pi, 3.0) * pow(pow(n, 2.0) - 1.0, 2.0) * (6.0 + 3.0 * pn)) / (3.0 * N * pow(lambda, vec3(4.0)) * (6.0 - 7.0 * pn));",
		"}",

		// see http://blenderartists.org/forum/showthread.php?321110-Shaders-and-Skybox-madness
		"// A simplied version of the total Reayleigh scattering to works on browsers that use ANGLE",
		"vec3 simplifiedRayleigh()",
		"{",
			"return 0.0005 / vec3(94, 40, 18);",
			// return 0.00054532832366 / (3.0 * 2.545E25 * pow(vec3(680E-9, 550E-9, 450E-9), vec3(4.0)) * 6.245);
		"}",

		"float rayleighPhase(float cosTheta)",
		"{	 ",
			"return (3.0 / (16.0*pi)) * (1.0 + pow(cosTheta, 2.0));",
		"//	return (1.0 / (3.0*pi)) * (1.0 + pow(cosTheta, 2.0));",
		"//	return (3.0 / 4.0) * (1.0 + pow(cosTheta, 2.0));",
		"}",

		"vec3 totalMie(vec3 lambda, vec3 K, float T)",
		"{",
			"float c = (0.2 * T ) * 10E-18;",
			"return 0.434 * c * pi * pow((2.0 * pi) / lambda, vec3(v - 2.0)) * K;",
		"}",

		"float hgPhase(float cosTheta, float g)",
		"{",
			"return (1.0 / (4.0*pi)) * ((1.0 - pow(g, 2.0)) / pow(1.0 - 2.0*g*cosTheta + pow(g, 2.0), 1.5));",
		"}",

		"float sunIntensity(float zenithAngleCos)",
		"{",
		// This function originally used `exp(n)`, but it returns an incorrect value
		// on Samsung S6 phones. So it has been replaced with the equivalent `pow(e, n)`.
		// See https://github.com/mrdoob/three.js/issues/8382
			"return EE * max(0.0, 1.0 - pow(e, -((cutoffAngle - acos(zenithAngleCos))/steepness)));",
		"}",

		"// float logLuminance(vec3 c)",
		"// {",
		"// 	return log(c.r * 0.2126 + c.g * 0.7152 + c.b * 0.0722);",
		"// }",

		"// Filmic ToneMapping http://filmicgames.com/archives/75",
		"float A = 0.15;",
		"float B = 0.50;",
		"float C = 0.10;",
		"float D = 0.20;",
		"float E = 0.02;",
		"float F = 0.30;",
		"float W = 1000.0;",

		"vec3 Uncharted2Tonemap(vec3 x)",
		"{",
		   "return ((x*(A*x+C*B)+D*E)/(x*(A*x+B)+D*F))-E/F;",
		"}",


		"void main() ",
		"{",
			"float sunfade = 1.0-clamp(1.0-exp((sunPosition.y/450000.0)),0.0,1.0);",

			"// luminance =  1.0 ;// vWorldPosition.y / 450000. + 0.5; //sunPosition.y / 450000. * 1. + 0.5;",

			 "// gl_FragColor = vec4(sunfade, sunfade, sunfade, 1.0);",

			"float reileighCoefficient = reileigh - (1.0* (1.0-sunfade));",

			"vec3 sunDirection = normalize(sunPosition);",

			"float sunE = sunIntensity(dot(sunDirection, up));",

			"// extinction (absorbtion + out scattering) ",
			"// rayleigh coefficients",

			// "vec3 betaR = totalRayleigh(lambda) * reileighCoefficient;",
			"vec3 betaR = simplifiedRayleigh() * reileighCoefficient;",

			"// mie coefficients",
			"vec3 betaM = totalMie(lambda, K, turbidity) * mieCoefficient;",

			"// optical length",
			"// cutoff angle at 90 to avoid singularity in next formula.",
			"float zenithAngle = acos(max(0.0, dot(up, normalize(vWorldPosition - cameraPos))));",
			"float sR = rayleighZenithLength / (cos(zenithAngle) + 0.15 * pow(93.885 - ((zenithAngle * 180.0) / pi), -1.253));",
			"float sM = mieZenithLength / (cos(zenithAngle) + 0.15 * pow(93.885 - ((zenithAngle * 180.0) / pi), -1.253));",



			"// combined extinction factor	",
			"vec3 Fex = exp(-(betaR * sR + betaM * sM));",

			"// in scattering",
			"float cosTheta = dot(normalize(vWorldPosition - cameraPos), sunDirection);",

			"float rPhase = rayleighPhase(cosTheta*0.5+0.5);",
			"vec3 betaRTheta = betaR * rPhase;",

			"float mPhase = hgPhase(cosTheta, mieDirectionalG);",
			"vec3 betaMTheta = betaM * mPhase;",


			"vec3 Lin = pow(sunE * ((betaRTheta + betaMTheta) / (betaR + betaM)) * (1.0 - Fex),vec3(1.5));",
			"Lin *= mix(vec3(1.0),pow(sunE * ((betaRTheta + betaMTheta) / (betaR + betaM)) * Fex,vec3(1.0/2.0)),clamp(pow(1.0-dot(up, sunDirection),5.0),0.0,1.0));",

			"//nightsky",
			"vec3 direction = normalize(vWorldPosition - cameraPos);",
			"float theta = acos(direction.y); // elevation --> y-axis, [-pi/2, pi/2]",
			"float phi = atan(direction.z, direction.x); // azimuth --> x-axis [-pi/2, pi/2]",
			"vec2 uv = vec2(phi, theta) / vec2(2.0*pi, pi) + vec2(0.5, 0.0);",
			"// vec3 L0 = texture2D(skySampler, uv).rgb+0.1 * Fex;",
			"vec3 L0 = vec3(0.1) * Fex;",

			"// composition + solar disc",
			"//if (cosTheta > sunAngularDiameterCos)",
			"float sundisk = smoothstep(sunAngularDiameterCos,sunAngularDiameterCos+0.00002,cosTheta);",
			"// if (normalize(vWorldPosition - cameraPos).y>0.0)",
			"L0 += (sunE * 19000.0 * Fex)*sundisk;",


			"vec3 whiteScale = 1.0/Uncharted2Tonemap(vec3(W));",

			"vec3 texColor = (Lin+L0);   ",
			"texColor *= 0.04 ;",
			"texColor += vec3(0.0,0.001,0.0025)*0.3;",

			"float g_fMaxLuminance = 1.0;",
			"float fLumScaled = 0.1 / luminance;     ",
			"float fLumCompressed = (fLumScaled * (1.0 + (fLumScaled / (g_fMaxLuminance * g_fMaxLuminance)))) / (1.0 + fLumScaled); ",

			"float ExposureBias = fLumCompressed;",

			"vec3 curr = Uncharted2Tonemap((log2(2.0/pow(luminance,4.0)))*texColor);",
			"vec3 color = curr*whiteScale;",

			"vec3 retColor = pow(color,vec3(1.0/(1.2+(1.2*sunfade))));",


			"gl_FragColor.rgb = retColor;",

			"gl_FragColor.a = 1.0;",
		"}",

	].join( "\n" )

};

THREE.Sky = function () {

	var skyShader = THREE.ShaderLib[ "sky" ];
	var skyUniforms = THREE.UniformsUtils.clone( skyShader.uniforms );

	var skyMat = new THREE.ShaderMaterial( {
		fragmentShader: skyShader.fragmentShader,
		vertexShader: skyShader.vertexShader,
		uniforms: skyUniforms,
		side: THREE.BackSide
	} );

	var skyGeo = new THREE.SphereBufferGeometry( 450000, 32, 15 );
	var skyMesh = new THREE.Mesh( skyGeo, skyMat );


	// Expose variables
	this.mesh = skyMesh;
	this.uniforms = skyUniforms;

};
