optimize parity calculations in SerialPIO (#2932)

use LUT for faster parity checks, reduces CPU load with multiple instances

Author: Ahmed ARIF

cores/rp2040/SerialPIO.cpp

@@ -68,13 +68,32 @@ static PIOProgram *_getRxProgram(int bits) {
 }
 // ------------------------------------------------------------------------
 
-// TODO - this works, but there must be a faster/better way...
 static int __not_in_flash_func(_parity)(int bits, int data) {
-    int p = 0;
-    for (int b = 0; b < bits; b++) {
-        p ^= (data & (1 << b)) ? 1 : 0;
+    static const uint8_t parity_lut[256] = {
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1,
+        0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0
+    };
+    
+    switch (bits) {
+        case 5: return parity_lut[data & 0x1F];
+        case 6: return parity_lut[data & 0x3F];
+        case 7: return parity_lut[data & 0x7F];
+        default: return parity_lut[data & 0xFF];
     }
-    return p;
 }
 
 // We need to cache generated SerialPIOs so we can add data to them from