refactor: Replace LFSR with Xorshift PRNG, store menu strings in PROGMEM, and ensure atomic CV value updates.

firmware/Rhythm/Rhythm.ino

@@ -81,8 +81,16 @@ volatile int cv1_val = 0;
 volatile int cv2_val = 0;
 int last_mapped_slot = -1;
 
-// LFSR State for Chaos
+// PRNG State for Chaos
-uint16_t lfsr = 0xACE1;
+uint16_t xorshift_seed = 0xACE1;
+
+// Fast 16-bit xorshift PRNG
+uint16_t xorshift16() {
+  xorshift_seed ^= xorshift_seed << 7;
+  xorshift_seed ^= xorshift_seed >> 9;
+  xorshift_seed ^= xorshift_seed << 8;
+  return xorshift_seed;
+}
 
 // Math Helper: 1D Linear Interpolation between two bytes
 uint8_t lerp(uint8_t a, uint8_t b, uint8_t t) {
@@ -95,21 +103,17 @@ uint8_t GetThreshold(int inst, int step, int x_pos, int y_pos) {
   // x_pos is 0-255 mapped across 4 nodes (0, 1, 2, 3). Distance is 85 (255 / 3)
   // y_pos is 0-255 mapped across 4 nodes (0, 1, 2, 3). Distance is 85 (255 / 3)
 
-  int x_idx = x_pos / 85; 
+  int x_idx = 0;
-  int y_idx = y_pos / 85;
+  uint8_t x_frac = 0;
+  if (x_pos < 85) { x_idx = 0; x_frac = x_pos * 3; }
+  else if (x_pos < 170) { x_idx = 1; x_frac = (x_pos - 85) * 3; }
+  else { x_idx = 2; x_frac = x_pos == 255 ? 255 : (x_pos - 170) * 3; }
 
-  uint8_t x_frac = (x_pos % 85) * 3; // scale remainder 0-84 up to 0-255
+  int y_idx = 0;
-  uint8_t y_frac = (y_pos % 85) * 3;
+  uint8_t y_frac = 0;
-
+  if (y_pos < 85) { y_idx = 0; y_frac = y_pos * 3; }
-  // Guard against out of bounds if exactly 255
+  else if (y_pos < 170) { y_idx = 1; y_frac = (y_pos - 85) * 3; }
-  if (x_idx >= 3) {
+  else { y_idx = 2; y_frac = y_pos == 255 ? 255 : (y_pos - 170) * 3; }
-    x_idx = 2;
-    x_frac = 255;
-  }
-  if (y_idx >= 3) {
-    y_idx = 2;
-    y_frac = 255;
-  }
 
   // Read 4 corners from PROGMEM
   uint8_t p00 = pgm_read_byte(&PATTERN_MAPS[x_idx][y_idx][inst][step]);
@@ -138,20 +142,13 @@ void SaveState() {
   EEPROM.put(EEPROM_PATTERNS, patterns);
 }
 
-// LFSR random bit generator (returns 0 or 1, fast)
 uint8_t GetRandomBit() {
-  uint8_t bit = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 5)) & 1;
+  return xorshift16() & 1;
-  lfsr = (lfsr >> 1) | (bit << 15);
-  return bit;
 }
 
 // Get 8-bit pseudo-random number
 uint8_t GetRandomByte() {
-  uint8_t r = 0;
+  return xorshift16() & 0xFF;
-  for (int i = 0; i < 8; i++) {
-    r = (r << 1) | GetRandomBit();
-  }
-  return r;
 }
 
 void ProcessSequencerTick(uint32_t tick) {
@@ -251,6 +248,23 @@ void ProcessSequencerTick(uint32_t tick) {
   }
 }
 
+void HandleExtClockTick() {
+  switch (selected_source) {
+  case Clock::SOURCE_INTERNAL:
+  case Clock::SOURCE_EXTERNAL_MIDI:
+    // Use EXT as Reset when not used for clock source.
+    for (int i = 0; i < 6; i++) {
+      gravity.outputs[i].Low();
+    }
+    gravity.clock.Reset();
+    current_step = 0;
+    break;
+  default:
+    // Register EXT clock tick.
+    gravity.clock.Tick();
+  }
+}
+
 void OnPlayPress() {
   if (!gravity.clock.IsPaused()) {
     gravity.clock.Stop();
@@ -411,7 +425,21 @@ void drawMainSelection() {
   gravity.display.drawLine(0, mainHeight, 0, mainHeight - tickSize);
 }
 
-void drawMenuItems(String menu_items[], int menu_size, int current_item) {
+const char str_kick[] PROGMEM = "KICK";
+const char str_snare[] PROGMEM = "SNARE";
+const char str_hhat[] PROGMEM = "HHAT";
+const char str_chaos[] PROGMEM = "CHAOS";
+const char str_mapx[] PROGMEM = "MAP X";
+const char str_mapy[] PROGMEM = "MAP Y";
+const char* const param_menu_items[] PROGMEM = {str_kick, str_snare, str_hhat, str_chaos, str_mapx, str_mapy};
+
+const char str_source[] PROGMEM = "SOURCE";
+const char str_tempo[] PROGMEM = "TEMPO";
+const char str_cv1dest[] PROGMEM = "CV1 DEST";
+const char str_cv2dest[] PROGMEM = "CV2 DEST";
+const char* const global_menu_items[] PROGMEM = {str_source, str_tempo, str_cv1dest, str_cv2dest};
+
+void drawMenuItems(const char* const menu_items[], int menu_size, int current_item) {
   gravity.display.setFont(TEXT_FONT);
 
   int selectedBoxY = 0;
@@ -444,8 +472,9 @@ void drawMenuItems(String menu_items[], int menu_size, int current_item) {
 
   for (int i = 0; i < min(menu_size, (int)VISIBLE_MENU_ITEMS); ++i) {
     int idx = start_index + i;
-    drawRightAlignedText(menu_items[idx].c_str(),
+    char buffer[16];
-                         MENU_ITEM_HEIGHT * (i + 1) - 1);
+    strcpy_P(buffer, (char*)pgm_read_ptr(&(menu_items[idx])));
+    drawRightAlignedText(buffer, MENU_ITEM_HEIGHT * (i + 1) - 1);
   }
 }
 
@@ -453,103 +482,120 @@ void DisplayMainArea() {
   gravity.display.setFontMode(1);
   gravity.display.setDrawColor(2);
 
-  String mainText;
+  char mainText[16] = "";
-  String subText;
+  char subText[16] = "";
 
   if (selected_slot > 0) {
     PatternState &p = patterns[active_pattern];
+    int act_val = 0;
+    switch (current_param) {
+      case PARAM_KICK_DENS: act_val = p.inst_density[0]; break;
+      case PARAM_SNARE_DENS: act_val = p.inst_density[1]; break;
+      case PARAM_HIHAT_DENS: act_val = p.inst_density[2]; break;
+      case PARAM_CHAOS: act_val = p.chaos_amount; break;
+      case PARAM_MAP_X: act_val = p.map_x; break;
+      case PARAM_MAP_Y: act_val = p.map_y; break;
+      default: break;
+    }
+
+    if (!editing_param) {
+      if ((int)cv1_dest == (int)current_param + 1) act_val += cv1_val / 4;
+      if ((int)cv2_dest == (int)current_param + 1) act_val += cv2_val / 4;
+      act_val = constrain(act_val, 0, 255);
+    }
+
     switch (current_param) {
     case PARAM_KICK_DENS:
-      mainText = String(map(p.inst_density[0], 0, 255, 0, 100)) + "%";
+      itoa(map(act_val, 0, 255, 0, 100), mainText, 10);
-      subText = "KICK DENS";
+      strcat(mainText, "%");
+      strcpy(subText, "KICK DENS");
       break;
     case PARAM_SNARE_DENS:
-      mainText = String(map(p.inst_density[1], 0, 255, 0, 100)) + "%";
+      itoa(map(act_val, 0, 255, 0, 100), mainText, 10);
-      subText = "SNAR DENS";
+      strcat(mainText, "%");
+      strcpy(subText, "SNAR DENS");
       break;
     case PARAM_HIHAT_DENS:
-      mainText = String(map(p.inst_density[2], 0, 255, 0, 100)) + "%";
+      itoa(map(act_val, 0, 255, 0, 100), mainText, 10);
-      subText = "HHAT DENS";
+      strcat(mainText, "%");
+      strcpy(subText, "HHAT DENS");
       break;
     case PARAM_CHAOS:
-      mainText = String(map(p.chaos_amount, 0, 255, 0, 100)) + "%";
+      itoa(map(act_val, 0, 255, 0, 100), mainText, 10);
-      subText = "CHAOS";
+      strcat(mainText, "%");
+      strcpy(subText, "CHAOS");
       break;
     case PARAM_MAP_X:
-      mainText = String(p.map_x);
+      itoa(act_val, mainText, 10);
-      subText = "MAP X";
+      strcpy(subText, "MAP X");
       break;
     case PARAM_MAP_Y:
-      mainText = String(p.map_y);
+      itoa(act_val, mainText, 10);
-      subText = "MAP Y";
+      strcpy(subText, "MAP Y");
       break;
     default:
       break;
     }
 
-    drawCenteredText(mainText.c_str(), MAIN_TEXT_Y, LARGE_FONT);
+    drawCenteredText(mainText, MAIN_TEXT_Y, LARGE_FONT);
-    drawCenteredText(subText.c_str(), SUB_TEXT_Y, TEXT_FONT);
+    drawCenteredText(subText, SUB_TEXT_Y, TEXT_FONT);
 
-    String menu_items[PARAM_LAST] = {
+    drawMenuItems(param_menu_items, PARAM_LAST, (int)current_param);
-        "KICK", "SNARE", "HHAT", "CHAOS", "MAP X", "MAP Y"};
-    drawMenuItems(menu_items, PARAM_LAST, (int)current_param);
     
   } else {
     switch (current_global_param) {
     case PARAM_GLOBAL_CLK_SRC:
       switch (selected_source) {
       case Clock::SOURCE_INTERNAL:
-        mainText = F("INT");
+        strcpy_P(mainText, PSTR("INT"));
-        subText = F("CLOCK");
+        strcpy_P(subText, PSTR("CLOCK"));
         break;
       case Clock::SOURCE_EXTERNAL_PPQN_24:
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
-        subText = F("24 PPQN");
+        strcpy_P(subText, PSTR("24 PPQN"));
         break;
       case Clock::SOURCE_EXTERNAL_PPQN_4:
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
-        subText = F("4 PPQN");
+        strcpy_P(subText, PSTR("4 PPQN"));
         break;
       case Clock::SOURCE_EXTERNAL_PPQN_2:
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
-        subText = F("2 PPQN");
+        strcpy_P(subText, PSTR("2 PPQN"));
         break;
       case Clock::SOURCE_EXTERNAL_PPQN_1:
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
-        subText = F("1 PPQN");
+        strcpy_P(subText, PSTR("1 PPQN"));
         break;
       case Clock::SOURCE_EXTERNAL_MIDI:
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
-        subText = F("MIDI");
+        strcpy_P(subText, PSTR("MIDI"));
         break;
       }
       break;
     case PARAM_GLOBAL_BPM:
       if (gravity.clock.ExternalSource()) {
-        mainText = F("EXT");
+        strcpy_P(mainText, PSTR("EXT"));
       } else {
-        mainText = String(gravity.clock.Tempo());
+        itoa(gravity.clock.Tempo(), mainText, 10);
       }
-      subText = F("BPM");
+      strcpy_P(subText, PSTR("BPM"));
       break;
     case PARAM_GLOBAL_CV1_DEST:
-      mainText = F("CV1");
+      strcpy_P(mainText, PSTR("CV1"));
-      subText = GetCvDestName(cv1_dest);
+      strcpy(subText, GetCvDestName(cv1_dest));
       break;
     case PARAM_GLOBAL_CV2_DEST:
-      mainText = F("CV2");
+      strcpy_P(mainText, PSTR("CV2"));
-      subText = GetCvDestName(cv2_dest);
+      strcpy(subText, GetCvDestName(cv2_dest));
       break;
     default:
       break;
     }
   
-    drawCenteredText(mainText.c_str(), MAIN_TEXT_Y, LARGE_FONT);
+    drawCenteredText(mainText, MAIN_TEXT_Y, LARGE_FONT);
-    drawCenteredText(subText.c_str(), SUB_TEXT_Y, TEXT_FONT);
+    drawCenteredText(subText, SUB_TEXT_Y, TEXT_FONT);
 
-    String menu_items[PARAM_GLOBAL_LAST] = {
+    drawMenuItems(global_menu_items, PARAM_GLOBAL_LAST, (int)current_global_param);
-        "SOURCE", "TEMPO", "CV1 DEST", "CV2 DEST"};
-    drawMenuItems(menu_items, PARAM_GLOBAL_LAST, (int)current_global_param);
   }
 }
 
@@ -604,6 +650,7 @@ void setup() {
   gravity.encoder.AttachPressRotateHandler(OnEncoderPressRotate);
 
   gravity.clock.AttachIntHandler(ProcessSequencerTick);
+  gravity.clock.AttachExtHandler(HandleExtClockTick);
   // Default to 120 BPM internal
   gravity.clock.SetTempo(120);
   gravity.clock.SetSource(Clock::SOURCE_INTERNAL);
@@ -615,10 +662,37 @@ void setup() {
 void loop() {
   gravity.Process();
 
-  // Apply CV modulation
+  static int last_cv1_val = 0;
-  // CV1 modulates Map X, CV2 modulates Chaos
+  static int last_cv2_val = 0;
-  cv1_val = gravity.cv1.Read(); // -512 to 512
+
-  cv2_val = gravity.cv2.Read();
+  int cv1_temp = gravity.cv1.Read(); // -512 to 512
+  int cv2_temp = gravity.cv2.Read();
+
+  noInterrupts();
+  cv1_val = cv1_temp;
+  cv2_val = cv2_temp;
+  interrupts();
+
+  // Trigger redraw if we're not editing and the CV modulating the selected param changes
+  if (!editing_param && selected_slot > 0) {
+    if ((int)cv1_dest == (int)current_param + 1) {
+      if (abs(cv1_val - last_cv1_val) >= 4) {
+        needs_redraw = true;
+        last_cv1_val = cv1_val;
+      }
+    } else {
+      last_cv1_val = cv1_val;
+    }
+
+    if ((int)cv2_dest == (int)current_param + 1) {
+      if (abs(cv2_val - last_cv2_val) >= 4) {
+        needs_redraw = true;
+        last_cv2_val = cv2_val;
+      }
+    } else {
+      last_cv2_val = cv2_val;
+    }
+  }
 
   if (eeprom_needs_save && (millis() - last_param_change > SAVE_DELAY_MS)) {
     SaveState();