Save some space in MuxTO

[WestfW]

Space in MuxTO is very tight; depending on compiler version, it can't be built to fit into the available 12k of flash.
This piece of code is (should be) a drop-in replacement for the CRC function in crc16.cpp. It does bitwise calculation of the CRC instead of the table-driven byte-at-a-time algorithm that is currently used. This is presumably about 8x slower (which I believe shouldn't matter), but it eliminates the 512byte crc_table, making it about 1/40th the flash size.

(CRC Algorithm tested on a desktop, and code compiles in Arduino. Not yet tested in an actual Nano every...)

/*                                                                                                                     
 * "reflected" (LSB first) CCITT CRC-16                                                                                
 */                                                                                                                    
uint16_t POLY = 0x8408;  // bit reversed 0x1021                                                                        
                                                                                                                       
uint16_t CRC::next(uint8_t newchar, uint16_t crc) {                                                                 
  int j;          
  uint_fast16_t crcshadow = crc ^ newchar;  // move some uxth instructions outside the bit loop.                                                                                                                                                                                                       
  for (j = 0; j < 8; j++) {                                                                                            
    if (crcshadow & 1) { // Check if LSB is set                                                                              
      crcshadow = (crcshadow >> 1) ^ POLY;                                                                                         
    } else {                                                                                                           
      crcshadow >>= 1;                                                                                                       
    }                                                                                                                  
  }                                                                                                                    
  return crcshadow;                                                                                                          
}

[WestfW]

(Also tested on SAMD21, for actual ARM CM0+ compatibility. Works.)

Test sketch:

// From AVR067
namespace CRC {
uint16_t next (uint8_t newchar, uint16_t previous = 0xFFFF);    // 'previous' defaults to CRC seed value, 0xFFFF
uint16_t nextbits (uint8_t newchar, uint16_t previous = 0xFFFF);    // 'previous' defaults to CRC seed value, 0xFFFF
}

static const uint16_t crc_table[256] =
{ 0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
  0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
  0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
  0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
  0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
  0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
  0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
  0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
  0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
  0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
  0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
  0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
  0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
  0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
  0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
  0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
  0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
  0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
  0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
  0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
  0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
  0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
  0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
  0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
  0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
  0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
  0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
  0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
  0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
  0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
  0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
  0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};

uint16_t CRC::next(uint8_t newchar, uint16_t previous) {
  union {
    uint16_t word;
    struct {
      uint8_t  low;
      uint8_t high;
    } byte;
  } crc;
  crc.word = previous;
  crc.word = crc_table[crc.byte.low ^ newchar];
  crc.byte.low ^= (previous >> 8);
  return crc.word;
}

/*
 * Bitwise version by BillW (WestfW)
 * * "reflected" (LSB first) CCITT CRC-16
 */
uint16_t POLY = 0x8408;  // bit reversed 0x1021

uint16_t CRC::nextbits(uint8_t newchar, uint16_t crc) {
  int j;
  uint_fast16_t crcshadow = crc ^ newchar;  // move some uxth instructions outside the bit loop.
  for (j = 0; j < 8; j++) {
    if (crcshadow & 1) { // Check if LSB is set
      crcshadow = (crcshadow >> 1) ^ POLY;
    } else {
      crcshadow >>= 1;
    }
  }
  return crcshadow;
}

void setup() {
  Serial.begin(115200);
  while (!Serial)
    ;
  Serial.println("CRC Test Program");
  Serial.setTimeout(60000);
}

uint8_t buffer[100] = "this is a test";

void loop() {
  uint8_t c;
  uint8_t *p = buffer;
  uint16_t fastcrc = 0xFFFF, slowcrc = 0xFFFF;

  Serial.println("\n\nPrompt:");
  Serial.readBytesUntil('\n', buffer, sizeof(buffer));
  Serial.print("Read String: ");
  Serial.println((char*)buffer);
  Serial.println("Calculating CRCs");
  do {
    c = *p++;
    fastcrc = CRC::next(c, fastcrc);
    slowcrc = CRC::nextbits(c, slowcrc);
  } while (c != 0);
  Serial.printf("\nFastCRC: %x", fastcrc);
  Serial.printf("\nSlowCRC: %x", slowcrc);
}