#include "shader.h"
#include "miaux.h"

struct ground_fog_layers {
    miColor fog_color;
    miColor fog_density;
    miScalar fade_start;
    miScalar fade_end;
    miScalar unit_density;
    miScalar march_increment;
    miBoolean show_layers;
    miColor full_fog_marker;
    miColor partial_fog_marker;
};

DLLEXPORT
int ground_fog_layers_version(void) { return 1; }

DLLEXPORT
miBoolean ground_fog_layers (
     miColor *result, miState *state, struct ground_fog_layers *params  )
{
    miScalar fade_start = *mi_eval_scalar(&params->fade_start);
    miScalar fade_end = *mi_eval_scalar(&params->fade_end);
    miVector world_point, march_point;

    if (state->dist == 0.0) 
        return miTRUE;
    else if (*mi_eval_boolean(&params->show_layers)) {
        miColor* full_fog_marker = 
            mi_eval_color(&params->full_fog_marker);
        miColor* partial_fog_marker = 
            mi_eval_color(&params->partial_fog_marker);

        mi_point_to_world(state, &world_point, &state->point);
        if (world_point.y < fade_start)
            miaux_multiply_colors(result, result, full_fog_marker);
        else if (world_point.y < fade_end)
            miaux_multiply_colors(result, result, partial_fog_marker);
    } else {
        miScalar fog_density = *mi_eval_scalar(&params->fog_density); 
        miScalar march_increment = *mi_eval_scalar(&params->march_increment);
        miScalar unit_density = *mi_eval_scalar(&params->unit_density);
        miScalar accumulated_density = 0.0, distance, density_factor;
        
        for (distance = 0; distance < state->dist; distance += march_increment) {
            miaux_world_space_march_point(&march_point, state, distance);
            density_factor = miaux_fit_clamp(
                march_point.y, fade_start, fade_end, fog_density, 0.0);
            accumulated_density += 
                density_factor * march_increment * unit_density;
            if (accumulated_density > 1.0) {
                *result = *mi_eval_color(&params->fog_color);
                return miTRUE;
            }
        }
        if (accumulated_density > 0.0) {
            miaux_blend_colors(result, result, mi_eval_color(&params->fog_color),
                               1.0 - accumulated_density);
        }
    }
    return miTRUE;
}