#include <metal_stdlib>

using namespace metal;

kernel void compute(texture2d<float, access::write> output [[texture(0)]],
                    texture2d<float, access::sample> input [[texture(1)]],
                    constant float &timer [[buffer(0)]],
                    uint2 gid [[thread_position_in_grid]])
{
  int width = input.get_width();
  int height = input.get_height();
  float2 uv = float2(gid) / float2(width, height);
  uv = uv * 2.0 - 0.5;
  uv = uv * 4;
  float radius = 1;
  float distance = length(uv) - radius;
  constexpr sampler textureSampler(coord::normalized,
                                   address::repeat,
                                   min_filter::linear,
                                   mag_filter::linear,
                                   mip_filter::linear);
  float3 norm = float3(uv, sqrt(1.0 - dot(uv, uv)));
  float pi = 3.14;
  float s = atan2( norm.z, norm.x ) / (2 * pi);
  float t = asin( norm.y ) / (2 * pi);
  t += 0.5;
  float4 color = input.sample(textureSampler, float2(s + timer * 0.1, t));
  output.write(distance < 0 ? color : float4(0), gid);
}