ws2812: change colors to accept RGB values

Nobody would expect GRB input, and converting from RGB to GRB via Lua is too slow,
so convert it in place before sending to the hardware.

app/modules/ws2812.c

@@ -36,28 +36,35 @@ static void ICACHE_FLASH_ATTR send_ws_1(uint8_t gpio) {
 }
 
 // Lua: ws2812.write(pin, "string")
-// Byte triples in the string are interpreted as G R B values.
-// ws2812.write(4, string.char(0, 255, 0)) uses GPIO2 and sets the first LED red.
+// Byte triples in the string are interpreted as R G B values
+// and sent to the hardware as G R B.
+// ws2812.write(4, string.char(255, 0, 0)) uses GPIO2 and sets the first LED red.
 // ws2812.write(3, string.char(0, 0, 255):rep(10)) uses GPIO0 and sets ten LEDs blue.
-// ws2812.write(4, string.char(255, 0, 0, 255, 255, 255)) first LED green, second LED white.
+// ws2812.write(4, string.char(0, 255, 0, 255, 255, 255)) first LED green, second LED white.
 static int ICACHE_FLASH_ATTR ws2812_write(lua_State* L) {
   const uint8_t pin = luaL_checkinteger(L, 1);
   size_t length;
   const char *buffer = luaL_checklstring(L, 2, &length);
 
+  //ignore incomplete Byte triples at the end of buffer
+  length -= length % 3;
+
   platform_gpio_mode(pin, PLATFORM_GPIO_OUTPUT, PLATFORM_GPIO_FLOAT);
   platform_gpio_write(pin, 0);
   os_delay_us(10);
 
   os_intr_lock();
   const char * const end = buffer + length;
-  while (buffer != end) {
+
+  size_t i = 1;
+  while (buffer + i <= end) {
+
     uint8_t mask = 0x80;
     while (mask) {
-      (*buffer & mask) ? send_ws_1(pin_num[pin]) : send_ws_0(pin_num[pin]);
+      (buffer[i] & mask) ? send_ws_1(pin_num[pin]) : send_ws_0(pin_num[pin]);
       mask >>= 1;
     }
-    ++buffer;
+    i += ((i % 3) == 1) ? -1 : 2;
   }
   os_intr_unlock();