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#extension GL_EXT_gpu_shader4 : enable

uniform int key;
uniform sampler2D _input; //_input texture
uniform sampler2D stateTex;
uniform float dt; //time
uniform vec3 camPos;
uvec4 whiteNoise(in uvec4 _input, in unsigned int key);

vec4 convertToR0_R1(in uvec4 _input);
const float twopi = 6.283185;
const float EXCITERANGE = 20.0;
const float EPSILON = 2.4414e-4;
//This program updates the director texture of GPUEntities.
//we randomly pick a direction.
//Precondition:
//We assume that _input.xy contains the directional vector that we want to update. 
//Where x is the e1 axis and y e2 axis.
void main()
{
	//gl_TexCoord[0].t = 1.0 - gl_TexCoord[0].t;
	vec4 state = texture2D(_input,gl_TexCoord[0].st);
	if(state.z > state.w)
	{
		state.z = 0.0-dt;
		//next throw dice to determine whether we want to change
		uvec4 coord;
		unsigned int uintkey = unsigned int(key);
		coord.x = uint(gl_TexCoord[0].x*20000.0);
		coord.y = uint(gl_TexCoord[0].y*20000.0);
		coord.z = uintkey;
		coord.w = uintkey;
		
		uvec4 noise = whiteNoise(coord,uintkey);
		vec4 rand = convertToR0_R1(noise);
		
		state.w = rand.w*30.00+1.0; //change sometime in 1 to 5 sec
		
		if(rand.x < rand.y) //do our probability thing
		{
			
			rand.z = rand.z*twopi; //theta
			state.x = sin(rand.z);
			state.y = cos(rand.z);
		}
		
		vec3 pos = texture2D(stateTex,gl_TexCoord[0].st).xyz;
		vec3 camPosG = camPos;
		camPosG.y = 0.0;
		vec3 vecToPlayer = camPosG - pos;
		float dis = length(vecToPlayer); //distance to player.
		camPosG.y = 0.0;
		vecToPlayer = camPosG - pos;
		vecToPlayer = normalize(vecToPlayer);
		
		if(dis > EPSILON)
			dis = min(EXCITERANGE/dis,1.0);
		else
			dis = 1.0;
			
		
		state.xy = (1.0-dis)*state.xy+dis*vecToPlayer.xz;
		
	}
	
	
	
	state.z += dt;
	gl_FragColor = state;
}