varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform vec2  dimension;
uniform float seed;
uniform float scale;
uniform int   detail;
uniform float flowRate;

uniform int   useExternal;
uniform float externalStrength;
uniform sampler2D externalForce;

float random (in vec2 st) { return fract(sin(dot(st.xy + vec2(21.456, 46.856), vec2(12.989, 78.233))) * 43758.545); }
float grey   (vec4 col)   { return (col.r + col.g + col.b) / 3.; }

vec3 mod289(vec3 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec4 mod289(vec4 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec4 permute(vec4 x) { return mod289(((x * 34.0) + 10.0) * x); }
vec4 taylorInvSqrt(vec4 r) { return 1.79284291400159 - 0.85373472095314 * r; }

float snoise(vec3 vec) { #region
	vec3 v = vec * 4.;
	
	const vec2 C = vec2(1.0 / 6.0, 1.0 / 3.0);
	const vec4 D = vec4(0.0, 0.5, 1.0, 2.0);

	// First corner
	vec3 i  = floor(v + dot(v, C.yyy));
	vec3 x0 =   v - i + dot(i, C.xxx);

	// Other corners
	vec3 g = step(x0.yzx, x0.xyz);
	vec3 l = 1.0 - g;
	vec3 i1 = min( g.xyz, l.zxy );
	vec3 i2 = max( g.xyz, l.zxy );
	
	//   x0 = x0 - 0.0 + 0.0 * C.xxx;
	//   x1 = x0 - i1  + 1.0 * C.xxx;
	//   x2 = x0 - i2  + 2.0 * C.xxx;
	//   x3 = x0 - 1.0 + 3.0 * C.xxx;
	vec3 x1 = x0 - i1 + C.xxx;
	vec3 x2 = x0 - i2 + C.yyy; // 2.0*C.x = 1/3 = C.y
	vec3 x3 = x0 - D.yyy;      // -1.0+3.0*C.x = -0.5 = -D.y

	// Permutations
	i = mod289(i); 
	vec4 p = permute( permute( permute( 
             i.z + vec4(0.0, i1.z, i2.z, 1.0 ))
           + i.y + vec4(0.0, i1.y, i2.y, 1.0 )) 
           + i.x + vec4(0.0, i1.x, i2.x, 1.0 ));

	// Gradients: 7x7 points over a square, mapped onto an octahedron.
	// The ring size 17*17 = 289 is close to a multiple of 49 (49*6 = 294)
	float n_ = 0.142857142857; // 1.0/7.0
	vec3  ns = n_ * D.wyz - D.xzx;
	
	vec4 j = p - 49.0 * floor(p * ns.z * ns.z);  //  mod(p,7*7)
	
	vec4 x_ = floor(j * ns.z);
	vec4 y_ = floor(j - 7.0 * x_ );    // mod(j,N)
	
	vec4 x = x_ * ns.x + ns.yyyy;
	vec4 y = y_ * ns.x + ns.yyyy;
	vec4 h = 1.0 - abs(x) - abs(y);
	
	vec4 b0 = vec4( x.xy, y.xy );
	vec4 b1 = vec4( x.zw, y.zw );
	
	//vec4 s0 = vec4(lessThan(b0,0.0))*2.0 - 1.0;
	//vec4 s1 = vec4(lessThan(b1,0.0))*2.0 - 1.0;
	vec4 s0 = floor(b0) * 2.0 + 1.0;
	vec4 s1 = floor(b1) * 2.0 + 1.0;
	vec4 sh = -step(h, vec4(0.0));
	
	vec4 a0 = b0.xzyw + s0.xzyw * sh.xxyy ;
	vec4 a1 = b1.xzyw + s1.xzyw * sh.zzww ;
	
	vec3 p0 = vec3(a0.xy, h.x);
	vec3 p1 = vec3(a0.zw, h.y);
	vec3 p2 = vec3(a1.xy, h.z);
	vec3 p3 = vec3(a1.zw, h.w);

	//Normalise gradients
	vec4 norm = taylorInvSqrt(vec4(dot(p0, p0), dot(p1, p1), dot(p2, p2), dot(p3, p3)));
	p0 *= norm.x;
	p1 *= norm.y;
	p2 *= norm.z;
	p3 *= norm.w;

	// Mix final noise value
	vec4 m = max(0.5 - vec4(dot(x0, x0), dot(x1, x1), dot(x2, x2), dot(x3, x3)), 0.0);
	m = m * m;
	
	float n = 105.0 * dot( m * m, vec4( dot(p0, x0), dot(p1, x1), dot(p2, x2), dot(p3, x3) ) );
	n = mix(0.0, 0.5 + 0.5 * n, smoothstep(0.0, 0.003, vec.z));
	return n;
} #endregion

float simplex(in vec2 st, in int itr) { #region
    vec2 p   = st * 2.0 - 1.0;
    vec3 xyz = vec3(p, seed);
    
	float amp = pow(2., float(itr) - 1.)  / (pow(2., float(itr)) - 1.);
    float n = 0.;
	
	for(int i = 0; i < 32; i++) {
		if(i >= itr) break;
		
		n += snoise(xyz) * amp;
		
		amp *= .5;
		xyz *= 2.;
	}
	
	return n;
} #endregion

void main() {
	vec2  tx   = 1. / dimension;
	
	float x0 = simplex((v_vTexcoord + vec2(-tx.x, 0.)) / scale, detail);
	float x1 = simplex((v_vTexcoord + vec2( tx.x, 0.)) / scale, detail);
	float y0 = simplex((v_vTexcoord + vec2(0., -tx.y)) / scale, detail);
	float y1 = simplex((v_vTexcoord + vec2(0.,  tx.y)) / scale, detail);
	
	vec2 flow = vec2(x1 - x0, y1 - y0);
	if(useExternal == 1) {
		vec4 _forc = texture2D( externalForce, v_vTexcoord );
		flow -= _forc.xy * externalStrength;
	}
	
    gl_FragColor = texture2D( gm_BaseTexture, v_vTexcoord - flow * flowRate );
}