#version 420 core
layout(r32ui) coherent uniform uimage2D img2D_0;
layout(RGBA32F) coherent uniform image2D img2D_1;
vec4 insert(vec4 data, float new_data) {
    if      (new_data<data.x) return vec4(      new_data,data.xyz);
    else if (new_data<data.y) return vec4(data.x,new_data,data.yz);
    else if (new_data<data.z) return vec4(data.xy,new_data,data.z);
    else if (new_data<data.w) return vec4(data.xyz,new_data      );
    else                      return data;
}
void main() {
    ivec2 coord = ivec2(gl_FragCoord.xy);
    bool have_written = false;
    while (!have_written) {
        bool can_write = (imageAtomicExchange(img2D_0,coord,1u) != 1u);
        vec4 depths = imageLoad(img2D_1,coord);
        depths = insert(depths,gl_FragCoord.z);
        if (can_write) {
            imageStore(img2D_1,coord,depths);
            have_written = true;
        }
        memoryBarrier();
        imageAtomicExchange(img2D_0,coord,0);
        memoryBarrier();
    }
}