Optimized version of the Ashima simplex noise 2D

optimizedSnoise2d.glsl

// Optimized by @makio64 - FabriceNeyret2
// Original post on shadertoy: https://www.shadertoy.com/view/4sdGD8
// Original : https://github.com/ashima/webgl-noise/blob/master/src/noise2D.glsl

vec3 permute(vec3 x) { return mod( x*x*34.+x, 289.); }
float snoise(vec2 v) {
  vec2 i = floor(v + (v.x+v.y)*.36602540378443),
      x0 = v -   i + (i.x+i.y)*.211324865405187,
       j = step(x0.yx,x0),
      x1 = x0-j+.211324865405187,
      x3 = x0-.577350269189626;
  i = mod(i,289.);
  vec3 p = permute( permute( i.y + vec3(0., j.y, 1.))
                           + i.x + vec3(0., j.x, 1.)),
       m = max( .5 - vec3(dot(x0,x0), dot(x1,x1), dot(x3,x3)), 0.),
       x = 2. * fract(p * .024390243902439) - 1.,
       h = abs(x) - .5,
      a0 = x - floor(x + .5),
       g = a0 * vec3(x0.x,x1.x,x3.x)
          + h * vec3(x0.y,x1.y,x3.y);
  m = m*m*m*m* ( 1.79284291400159 - 0.85373472095314 * ( a0*a0 + h*h ) );  
  return .5 + 65. * dot(m, g);
}