simplexNoise2D

fn snoise2D_permute3(x: vec3<f32>) -> vec3<f32> {
    return ((x * 34.0 + 1.0) * x) % vec3<f32>(289.0);
}

fn snoise2D(v: vec2<f32>) -> f32 {
    let C = vec4<f32>(0.211324865405187, 0.366025403784439, -0.577350269189626, 0.024390243902439);
    var i = floor(v + dot(v, C.yy));
    let x0 = v - i + dot(i, C.xx);
    
    var i1: vec2<f32>;
    if (x0.x > x0.y) {
        i1 = vec2<f32>(1.0, 0.0);
    } else {
        i1 = vec2<f32>(0.0, 1.0);
    }
    
    var x12 = x0.xyxy + C.xxzz;
    x12 = vec4<f32>(x12.xy - i1, x12.zw);
    
    i = i % vec2<f32>(289.0);
    let p = snoise2D_permute3(snoise2D_permute3(i.y + vec3<f32>(0.0, i1.y, 1.0)) + i.x + vec3<f32>(0.0, i1.x, 1.0));
    
    var m = max(0.5 - vec3<f32>(dot(x0, x0), dot(x12.xy, x12.xy), dot(x12.zw, x12.zw)), vec3<f32>(0.0));
    m = m * m;
    m = m * m;
    
    let x = 2.0 * fract(p * C.www) - 1.0;
    let h = abs(x) - 0.5;
    let ox = floor(x + 0.5);
    let a0 = x - ox;
    
    m *= 1.79284291400159 - 0.85373472095314 * (a0 * a0 + h * h);
    
    var g: vec3<f32>;
    g.x = a0.x * x0.x + h.x * x0.y;
    g.y = a0.y * x12.x + h.y * x12.y;
    g.z = a0.z * x12.z + h.z * x12.w;
    
    return 130.0 * dot(m, g);
}