Write to LED indices instead of raw pointer

The original implementation violates strict aliasing. It works most of the time but can potentially cause issues.

Note that the LED index counters are 32-bit instead of 16-bit because the LED count provided by the checksum has a +1, so it will not fit in a 16-bit variable.

Similarly, we need to track both the current LED index and the remaining LEDs to write because the size of the LED strip in the code may not match the size of the LED strip as set by the data protocol.

Author: dmadison

Arduino/LEDstream_FastLED/LEDstream_FastLED.ino

@@ -52,7 +52,6 @@ const uint16_t
 #include <FastLED.h>
 
 CRGB leds[Num_Leds];
-uint8_t * ledsRaw = (uint8_t *)leds;
 
 // A 'magic word' (along with LED count & checksum) precedes each block
 // of LED data; this assists the microcontroller in syncing up with the
@@ -79,8 +78,8 @@ const uint8_t magic[] = {
 
 enum processModes_t {Header, Data} mode = Header;
 
-uint16_t outPos;             // current byte index in the LED array
-uint32_t bytesRemaining;     // count of bytes yet received, set by checksum
+uint32_t ledIndex;           // current index in the LED array
+uint32_t ledsRemaining;      // count of LEDs still to write, set by checksum (u16 + 1)
 
 unsigned long lastByteTime;  // ms timestamp, last byte received
 unsigned long lastAckTime;   // ms timestamp, lask acknowledge to the host
@@ -189,10 +188,10 @@ void headerMode(uint8_t c){
 				chk = c;
 				if(chk == (hi ^ lo ^ 0x55)) {
 					// Checksum looks valid. Get 16-bit LED count, add 1
-					// (# LEDs is always > 0) and multiply by 3 for R,G,B.
+					// (# of LEDs is always > 0), save and reset data
 					D_LED(HIGH);
-					bytesRemaining = 3L * (256L * (long)hi + (long)lo + 1L);
-					outPos = 0;
+					ledIndex = 0;
+					ledsRemaining = (256UL * (uint32_t)hi + (uint32_t)lo + 1UL);
 					memset(leds, 0, Num_Leds * sizeof(struct CRGB));
 					mode = Data; // Proceed to latch wait mode
 				}
@@ -203,16 +202,34 @@ void headerMode(uint8_t c){
 }
 
 void dataMode(uint8_t c){
-	// If LED data is not full
-	if (outPos < sizeof(leds)){
-		ledsRaw[outPos++] = c; // Issue next byte
+	static uint8_t channelIndex = 0;
+
+	// if LED data is not full, save the byte
+	if (ledIndex < Num_Leds) {
+		leds[ledIndex].raw[channelIndex] = c;
 	}
-	bytesRemaining--;
- 
-	if(bytesRemaining == 0) {
-		// End of data -- issue latch:
-		mode = Header; // Begin next header search
+	channelIndex++;  // increment regardless, for oversized data
+
+	// if we've filled this LED, move to the next
+	if (channelIndex >= 3) {
+		// reset the channel index so we can get ready to write
+		// the next LED, starting on the first channel (R/G/B)
+		channelIndex = 0;
+
+		// allow this to max out at Num_Leds, so that it represents which
+		// LEDs have data in them when the strip is ready to write
+		if (ledIndex < Num_Leds) ledIndex++;
+
+		// finished writing one LED, decrement the counter
+		ledsRemaining--;
+	}
+
+	// if all data has been read, write the output
+	if (ledsRemaining == 0) {
 		FastLED.show();
+		channelIndex = 0;  // reset channel tracking
+		mode = Header;     // begin next header search
+
 		D_FPS;
 		D_LED(LOW);
 		SERIAL_FLUSH;