feat: add custom background rendering and speed scaling features for snake gameplay

include/consts.h

@@ -3,11 +3,18 @@
 #endif
 
 const int START_LENGTH = 4;
-const int START_LENGTH_HARD =2;
+const int START_LENGTH_HARD = 2;
 const int APPLE_SCORE = 10;
 const int CURSE_DURATION_TICKS = 40;
 
-//Screen size
+// Speed scaling configuration (gradual acceleration based on consumed apples)
+const int SPEED_BASE_NORMAL_MHZ = 5000;  // 5.0 Hz (~200 ms per step)
+const int SPEED_BASE_HARD_MHZ   = 9000;  // 9.0 Hz (~111 ms per step)
+const int SPEED_MAX_NORMAL_MHZ  = 13000; // 13.0 Hz (~77 ms per step)
+const int SPEED_MAX_HARD_MHZ    = 17000; // 17.0 Hz (~59 ms per step)
+const int SPEED_APPLES_CAP      = 40;    // Apples consumed to reach max intensity & speed
+
+// Screen size
 const int X_MAX = 31;
 const int Y_MAX = 23;
 

include/game_bg.h

@@ -0,0 +1,34 @@
+#ifndef GAME_BG_H
+#define GAME_BG_H
+
+#include <nds.h>
+#include "game_state.h"
+
+struct BackgroundContext {
+    int bg_main_id = -1;
+    int bg_sub_id = -1;
+
+    // Fixed-point (8.8) scroll accumulators for smooth 60fps drift
+    uint16_t scroll_main_x_fp = 0;
+    uint16_t scroll_main_y_fp = 0;
+    uint16_t scroll_sub_x_fp = 0;
+    uint16_t scroll_sub_y_fp = 0;
+
+    // Animation frame counter & timers
+    uint8_t anim_phase = 0;
+    uint16_t anim_phase_fp = 0; // Fixed-point (8.8) phase accumulator
+    uint8_t pulse_timer = 0; // Triggered when an apple is eaten
+
+    // Cached state to optimize updates
+    bool hard_mode = false;
+    bool is_cursed = false;
+    bool game_over = false;
+    Scene scene = Scene::Menu;
+};
+
+void bg_setup(BackgroundContext &bg_ctx, int top_console_bg_id, int bottom_console_bg_id);
+void bg_update(BackgroundContext &bg_ctx, const GameState &state, uint8_t frame);
+void bg_trigger_pulse(BackgroundContext &bg_ctx);
+void bg_set_scene(BackgroundContext &bg_ctx, Scene scene, bool hard_mode);
+
+#endif // GAME_BG_H

include/game_logic.h

@@ -13,4 +13,8 @@ void spawn_apples_rotten(GameState &state);
 void spawn_apples_cursed(GameState &state);
 void tick_status_effects(GameState &state);
 
+uint8_t calculate_intensity_31(const GameState &state);
+uint16_t calculate_timer_freq_mhz(const GameState &state);
+uint16_t calculate_timer_reload(const GameState &state);
+
 #endif

include/game_render.h

@@ -15,9 +15,12 @@ struct HudCache {
     bool dirty = true;
 };
 
+struct BackgroundContext;
+
 struct RenderAssets {
     PrintConsole *topScreen = nullptr;
     PrintConsole *bottomScreen = nullptr;
+    BackgroundContext *bg_context = nullptr;
 
     u16 *gfx_segment = nullptr;
     u16 *gfx_segment_small = nullptr;

include/game_state.h

@@ -18,6 +18,7 @@ struct GameState {
     volatile uint16_t snakelength = 0;
     volatile uint16_t snakeDirection = 1;
     volatile uint16_t score = 0;
+    volatile uint16_t applesEaten = 0;
 
     bool gameOver = false;
     bool paused = false;