#version 430 core

out vec4 FragColor;

uniform vec2  u_resolution;   // framebuffer size in pixels
uniform ivec2 u_gridSize;     // sand grid size
uniform usampler2D u_state;   // GL_R8UI texture
uniform vec2  u_brushPos;
uniform float u_brushRadius;
uniform int   u_showBrush;
void main() {
    vec2 fragCoord = gl_FragCoord.xy;

    // Normalized coordinates
    vec2 uv = fragCoord / u_resolution;
    uv.y = 1.0 - uv.y;
    // Map to grid coordinates
    ivec2 cell = ivec2(uv * vec2(u_gridSize));
    cell = clamp(cell, ivec2(0), u_gridSize - ivec2(1));

    uint v = texelFetch(u_state, cell, 0).r;

    vec3 color = (v == 1u)
        ? vec3(0.9, 0.8, 0.1)        // sand
        : vec3(0.05, 0.05, 0.10);    // background
    if (u_showBrush != 0) {
        // center of this cell in grid coordinates
        vec2 cellCenter = vec2(cell) + vec2(0.5);
        float dist = length(cellCenter - u_brushPos);

        // how thick the ring is, in cells
        float thickness = 1.0; // 1-cell-thick ring

        // abs(dist - radius) < thickness => inside ring
        float d = abs(dist - u_brushRadius);

        // Smooth edge: edge = 1 at center of ring, 0 outside
        float edge = smoothstep(thickness, 0.0, d);

        vec3 ringColor = vec3(1.0);  // white ring
        color = mix(color, ringColor, edge);
    }

    FragColor = vec4(color, 1.0);
}