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

source/game_logic.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <cstdlib>
 
+#include <nds.h>
 #include <consts.h>
 
 void update_snake(GameState &state) {
@@ -68,6 +69,7 @@ bool check_collision_apples(GameState &state) {
 
   state.score += APPLE_SCORE;
   state.snakelength++;
+  state.applesEaten++;
   state.apples.erase(it);
   return true;
 }
@@ -140,3 +142,33 @@ void tick_status_effects(GameState &state) {
     state.curseTicksRemaining--;
   }
 }
+
+uint8_t calculate_intensity_31(const GameState &state) {
+  const uint16_t current_apples = state.applesEaten;
+  const uint16_t apples =
+      std::min<uint16_t>(current_apples, static_cast<uint16_t>(SPEED_APPLES_CAP));
+  return static_cast<uint8_t>((static_cast<uint32_t>(apples) * 31U) /
+                              static_cast<uint32_t>(SPEED_APPLES_CAP));
+}
+
+uint16_t calculate_timer_freq_mhz(const GameState &state) {
+  const uint16_t base_mhz =
+      state.hardMode ? SPEED_BASE_HARD_MHZ : SPEED_BASE_NORMAL_MHZ;
+  const uint16_t max_mhz =
+      state.hardMode ? SPEED_MAX_HARD_MHZ : SPEED_MAX_NORMAL_MHZ;
+  const uint16_t current_apples = state.applesEaten;
+  const uint16_t apples =
+      std::min<uint16_t>(current_apples, static_cast<uint16_t>(SPEED_APPLES_CAP));
+  const uint32_t diff = static_cast<uint32_t>(max_mhz - base_mhz);
+  return static_cast<uint16_t>(
+      base_mhz + ((static_cast<uint32_t>(apples) * diff) /
+                  static_cast<uint32_t>(SPEED_APPLES_CAP)));
+}
+
+uint16_t calculate_timer_reload(const GameState &state) {
+  const uint32_t freq_mhz = calculate_timer_freq_mhz(state);
+  // Timer clock with ClockDivider_1024 is BUS_CLOCK >> 10 (32728 Hz)
+  const uint32_t timer_clock_mhz = (BUS_CLOCK >> 10U) * 1000U;
+  const uint32_t period_ticks = timer_clock_mhz / freq_mhz;
+  return static_cast<uint16_t>(-static_cast<int32_t>(period_ticks));
+}