layout (location = 0) in vec2 a_pos;

out vec2 v_uv;
out vec2 v_world;

layout (std140) uniform ParticleGpuValidationUBO {
    highp vec4 u_pass;
    highp vec4 u_marker;
    highp vec4 u_mask;
    highp mat4 u_matrix;
};

uniform sampler2D u_state_texture;

void main() {
    v_uv = a_pos;
    v_world = a_pos;
    if (u_pass.x < 1.5) {
        gl_Position = vec4(a_pos * 2.0 - 1.0, 0.0, 1.0);
    } else if (u_pass.x > 3.5) {
        // 40-03: a_pos is a static State texel index generated only when the
        // particle capacity changes. The position itself remains GPU-owned.
        vec2 stateUV = (a_pos + vec2(0.5)) / u_mask.xy;
        vec2 position = texture(u_state_texture, stateUV).rg;
        v_uv = position;
        v_world = position;
        gl_PointSize = 3.0;
        gl_Position = vec4(position * 2.0 - 1.0, 0.0, 1.0);
    } else {
        // VALIDATION ONLY: a_pos is canonical z0 world geometry; u_matrix is
        // PaintParameters::matrixForTile() for the selected MapLibre world copy.
        gl_Position = u_matrix * vec4(a_pos * 8192.0, 0.0, 1.0);
    }
}