#include "io_registers.h"
#include "bits.h"

void main()
{
  // power control
  io_registers.a.POWCNT = 0
    | POWCNT__lcd_output_destination__a_to_upper__b_to_lower
    | POWCNT__geometry_engine__enable
    | POWCNT__rendering_engine__enable
    | POWCNT__lcd__enable;

  // enable bg0 and 3d graphics
  io_registers.a.DISPCNT = 0
    | DISPCNT__display_mode__graphics_display
    | DISPCNT__bg0__enable
    | DISPCNT__display_selection_for_bg0__3d_graphics
    ;

  // disable all 3d effects
  io_registers.a.DISP3DCNT = 0
    | DISP3DCNT__clear_image__disable
    | DISP3DCNT__fog_master__disable
    | DISP3DCNT__edge_marking__disable
    | DISP3DCNT__anti_aliasing__disable
    | DISP3DCNT__alpha_blending__disable
    | DISP3DCNT__alpha_test__disable
    | DISP3DCNT__texture_mapping__disable;

  // clear matrix stack status
  io_registers.a.GXSTAT |= GXSTAT__matrix_stack_status__overflow_or_underflow;

  // load identity matrices
  io_registers.a.MTX_MODE = MTX_MODE__matrix_mode__projection;
  io_registers.a.MTX_IDENTITY = 0;

  io_registers.a.MTX_MODE = MTX_MODE__matrix_mode__position;
  io_registers.a.MTX_IDENTITY = 0;

  io_registers.a.MTX_MODE = MTX_MODE__matrix_mode__position_and_vector;
  io_registers.a.MTX_IDENTITY = 0;

  // set the 3d clear color to a dark red
  io_registers.a.CLEAR_COLOR = 0
    | CLEAR_COLOR__clear_polygon_id(31)
    | CLEAR_COLOR__alpha_value(31)
    | CLEAR_COLOR__blue(1)
    | CLEAR_COLOR__green(1)
    | CLEAR_COLOR__red(10);

  // set the depth buffer clear value to the maximum value
  io_registers.a.CLEAR_DEPTH = CLEAR_DEPTH__value(0x7fff);

  // the following polygons are fully opaque and are not
  // backface-culled
  io_registers.a.POLYGON_ATTR = 0
    | POLYGON_ATTR__alpha_value(31)
    | POLYGON_ATTR__render_front_surface__enable
    | POLYGON_ATTR__render_back_surface__enable;

  // the 3d viewport is the entire display area
  io_registers.a.VIEWPORT = 0
    | VIEWPORT__y2(191)
    | VIEWPORT__x2(255)
    | VIEWPORT__y1(0)
    | VIEWPORT__x1(0);

  // VTX_10 uses signed 4.6 floating point (10 bit)
  // | 9 | 8 7 6 | 5 4 3 2 1 0 |
  // | s |  int  | decimal     |
  int fixed_point_divisor = 64; // == 2⁶

  // equilateral triangle; centered around the display origin in NDC
  /*
        A
       / \
      B---C

   */

  // fixed point constants; these are converted from floating-point to
  // integer at compile-time
  int ax =  0.0           * fixed_point_divisor;
  int ay =  1.0           * fixed_point_divisor;

  int bx = -0.86602540378 * fixed_point_divisor; // - sqrt(3) / 2
  int by = -0.5           * fixed_point_divisor;

  int cx =  0.86602540378 * fixed_point_divisor; // + sqrt(3) / 2
  int cy = -0.5           * fixed_point_divisor;

  int  z =  1.0           * fixed_point_divisor;

  // the following vertices are a triangle
  io_registers.a.BEGIN_VTXS = BEGIN_VTXS__type__triangle;

  io_registers.a.COLOR = COLOR__blue(31);
  io_registers.a.VTX_10 = 0
    | VTX_10__z_coordinate(z)
    | VTX_10__y_coordinate(ay)
    | VTX_10__x_coordinate(ax);

  io_registers.a.COLOR = COLOR__green(31);
  io_registers.a.VTX_10 = 0
    | VTX_10__z_coordinate(z)
    | VTX_10__y_coordinate(by)
    | VTX_10__x_coordinate(bx);

  io_registers.a.COLOR = COLOR__red(31);
  io_registers.a.VTX_10 = 0
    | VTX_10__z_coordinate(z)
    | VTX_10__y_coordinate(cy)
    | VTX_10__x_coordinate(cx);

  // end of the triangle
  io_registers.a.END_VTXS = 0;

  // wait for the geometry engine
  while (io_registers.a.GXSTAT & GXSTAT__geometry_engine_busy);

  // swap buffers
  io_registers.a.SWAP_BUFFERS = 0;
}