#version 430 core

layout(local_size_x = 16, local_size_y = 16) in;

// Read-write access to the current grid
layout(r8ui, binding = 0) coherent uniform uimage2D state;

uniform ivec2 u_gridSize;
uniform vec2  u_brushPos;     // in grid-space (cells)
uniform float u_brushRadius;  // in cells
uniform int   u_mode;         // 0 = none, 1 = add sand, -1 = erase

void main()
{
    ivec2 pos = ivec2(gl_GlobalInvocationID.xy);
    if (pos.x >= u_gridSize.x || pos.y >= u_gridSize.y) {
        return;
    }

    if (u_mode == 0) {
        return;
    }

    // Center of this cell in grid coords
    vec2 cellCenter = vec2(pos) + vec2(0.5);
    float dist = length(cellCenter - u_brushPos);

    if (dist > u_brushRadius) {
        return;
    }

    const uint AIR  = 0u;
    const uint SAND = 1u;

    if (u_mode > 0) {
        // left click: fill with sand
        imageStore(state, pos, uvec4(SAND, 0u, 0u, 0u));
    } else if (u_mode < 0) {
        // right click: erase sand
        imageStore(state, pos, uvec4(AIR, 0u, 0u, 0u));
    }
}