#include <metal_stdlib>
using namespace metal;

struct Uniforms {
  float2 direction;  // (1,0) for horizontal, (0,1) for vertical
  float2 texel_size; // 1.0 / texture_size
};

struct FragmentIn {
  float4 position [[position]];
  float2 uv;
};

// 9-tap Gaussian blur
fragment float4 fragment_main(
  FragmentIn in [[stage_in]],
  constant Uniforms &uniforms [[buffer(0)]],
  texture2d<float> tex [[texture(0)]],
  sampler samp [[sampler(0)]]
) {
  float2 offset = uniforms.direction * uniforms.texel_size;
  
  // Gaussian weights for 9 samples
  float weights[5] = {0.227027, 0.1945946, 0.1216216, 0.054054, 0.016216};
  
  float4 result = tex.sample(samp, in.uv) * weights[0];
  
  for (int i = 1; i < 5; i++) {
    result += tex.sample(samp, in.uv + offset * float(i)) * weights[i];
    result += tex.sample(samp, in.uv - offset * float(i)) * weights[i];
  }
  
  return result;
}