vec3 mod289(vec3 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec2 mod289(vec2 x) { return x - floor(x * (1.0 / 289.0)) * 289.0; }
vec3 permute(vec3 x) { return mod289(((x*34.0) + 1.0)*x); }

float snoise(vec2 v)
{
	// Precompute values for skewed triangular grid
	const vec4 C = vec4(0.211324865405187,  // (3.0-sqrt(3.0))/6.0
		0.366025403784439,	// 0.5*(sqrt(3.0)-1.0)
		-0.577350269189626, // -1.0 + 2.0 * C.x
		0.024390243902439	// 1.0 / 41.0
		);


	// First corner (x0)
	vec2 i = floor(v + dot(v, C.yy));
	vec2 x0 = v - i + dot(i, C.xx);

	// Other two corners (x1, x2)
	vec2 i1;
	i1 = (x0.x > x0.y) ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
	vec2 x1 = x0.xy + C.xx - i1;
	vec2 x2 = x0.xy + C.zz;

	// Do some permutations to avoid
	// truncation effects in permutation
	i = mod289(i);
	vec3 p = permute(
		permute(i.y + vec3(0.0, i1.y, 1.0))
		+ i.x + vec3(0.0, i1.x, 1.0));

	vec3 m = max(0.5 - vec3(
		dot(x0, x0),
		dot(x1, x1),
		dot(x2, x2)
		), 0.0);

	m = m*m;
	m = m*m;

	// Gradients: 
	//  41 pts uniformly over a line, mapped onto a diamond
	//  The ring size 17*17 = 289 is close to a multiple of 41 (41*7 = 287)

	vec3 x = 2.0 * fract(p * C.www) - 1.0;
	vec3 h = abs(x) - 0.5;
	vec3 ox = floor(x + 0.5);
	vec3 a0 = x - ox;

	// Normalise gradients implicitly by scaling m
	// Approximation of: m *= inversesqrt(a0*a0 + h*h);
	m *= 1.79284291400159 - 0.85373472095314 *(a0*a0 + h*h);

	// Compute final noise value at P
	vec3 g;
	g.x = a0.x  * x0.x + h.x  * x0.y;
	g.yz = a0.yz * vec2(x1.x, x2.x) + h.yz * vec2(x1.y, x2.y);
	return 130.0 * dot(m, g);
}