Centralize I2S mux slot-to-bit calculations

Add helper functions in i2s_constants.h for consistent bit position
calculations across step, direction, and enable pins in I2S mux mode.

Author: gin66

src/pd_esp32/esp32_queue.cpp

@@ -209,8 +209,8 @@ StepperQueue* StepperQueue::tryAllocateQueue(FasDriver driver,
     StepperQueue* q = new StepperQueue();
     q->use_i2s = true;
     q->i2s_mgr = StepperQueue::_i2s_mux_manager;
-    q->_i2s_mux_step_byte_offset = slot / 8;
-    q->_i2s_mux_step_bit_mask = 1 << (7 - (slot % 8));
+    q->_i2s_mux_step_byte_offset = i2s_mux_byte_offset(slot);
+    q->_i2s_mux_step_bit_mask = i2s_mux_bit_mask(slot);
     return q;
   }
 

src/pd_esp32/i2s_constants.h

@@ -43,5 +43,22 @@
 #define I2S_DMA_DESC_NUM 2
 #define I2S_DMA_FRAME_NUM I2S_FRAMES_PER_BLOCK
 
+// Convert I2S mux slot (0-31) to 32-bit bit position.
+// Matches byte/bit access: slot S → byte S/8, bit (7 - S%8)
+// slot 0 -> bit 7, slot 7 -> bit 0, slot 8 -> bit 15, slot 15 -> bit 8
+static inline uint8_t i2s_mux_slot_to_bit_pos(uint8_t slot) { return slot ^ 7; }
+
+static inline uint8_t i2s_mux_byte_offset(uint8_t slot) {
+  return i2s_mux_slot_to_bit_pos(slot) >> 3;
+}
+
+static inline uint8_t i2s_mux_bit_mask(uint8_t slot) {
+  return 1 << (i2s_mux_slot_to_bit_pos(slot) & 7);
+}
+
+static inline uint32_t i2s_mux_bit_mask_u32(uint8_t slot) {
+  return 1UL << i2s_mux_slot_to_bit_pos(slot);
+}
+
 #endif  // SUPPORT_ESP32_I2S
 #endif  // PD_ESP32_I2S_CONSTANTS_H

src/pd_esp32/i2s_manager.cpp

@@ -105,9 +105,7 @@ void I2sManager::i2sMuxSetBit(uint8_t slot, bool value) {
   if (slot >= 32) {
     return;
   }
-  uint8_t byte_offset = slot / 8;
-  uint8_t bit_within_byte = 7 - (slot % 8);
-  uint32_t bit = 1UL << (byte_offset * 8 + bit_within_byte);
+  uint32_t bit = i2s_mux_bit_mask_u32(slot);
   if (value) {
     _mux_state |= bit;
   } else {
@@ -119,9 +117,7 @@ bool I2sManager::i2sMuxGetBit(uint8_t slot) {
   if (slot >= 32) {
     return false;
   }
-  uint8_t byte_offset = slot / 8;
-  uint8_t bit_within_byte = 7 - (slot % 8);
-  return (_mux_state & (1UL << (byte_offset * 8 + bit_within_byte))) != 0;
+  return (_mux_state & i2s_mux_bit_mask_u32(slot)) != 0;
 }
 
 void IRAM_ATTR I2sManager::handleTxDone(uint8_t* buf) {