Does anyone have a crc32 implementation they could share that does not use tables?
I have been searching all over the net, and ive tried to adopt the few examples ive found but unfortunately I dont understand the concept well enough for that to be successful ! lol
// ----------------------------- crc32b --------------------------------
/* This is the basic CRC-32 calculation with some optimization but no
table lookup. The the byte reversal is avoided by shifting the crc reg
right instead of left and by using a reversed 32-bit word to represent
the polynomial.
When compiled to Cyclops with GCC, this function executes in 8 + 72n
instructions, where n is the number of bytes in the input message. It
should be doable in 4 + 61n instructions.
If the inner loop is strung out (approx. 5*8 = 40 instructions),
it would take about 6 + 46n instructions. */
unsigned int crc32b(unsigned char *message) {
int i, j;
unsigned int byte, crc, mask;
i = 0;
crc = 0xFFFFFFFF;
while (message[i] != 0) {
byte = message[i]; // Get next byte.
crc = crc ^ byte;
for (j = 7; j >= 0; j--) { // Do eight times.
mask = -(crc & 1);
crc = (crc >> 1) ^ (0xEDB88320 & mask);
}
i = i + 1;
}
return ~crc;
}
Why don't you want to use a table?
If you change your mind, this looks good: http://forum.arduino.cc/index.php?topic=91179.0
John, that link is mangled. Try this: http://forum.arduino.cc/index.php?topic=91179.0
Pete
First I have to say thanks! I managed to get the code working!
Originally I wanted a simple routine that was highly portable, and i didnt want to deal with tables , KISS principle, Im ok that it takes a little longer to calculate 
I had to change a few thing but now it works !!!!
Thanks again!
unsigned char test[] = {"HELLO"};
void setup() {
// put your setup code here, to run once:
Serial.begin(19200);
}
void loop() {
// put your main code here, to run repeatedly:
Serial.println(crc32b(test),HEX);
delay(1000);
}
// ----------------------------- crc32b --------------------------------
/* This is the basic CRC-32 calculation with some optimization but no
table lookup. The the byte reversal is avoided by shifting the crc reg
right instead of left and by using a reversed 32-bit word to represent
the polynomial.
When compiled to Cyclops with GCC, this function executes in 8 + 72n
instructions, where n is the number of bytes in the input message. It
should be doable in 4 + 61n instructions.
If the inner loop is strung out (approx. 5*8 = 40 instructions),
it would take about 6 + 46n instructions. */
unsigned long crc32b(unsigned char *message) {
int i, j;
unsigned long Byte, crc, mask;
i = 0;
crc = 0xFFFFFFFF;
while (message[i] != 0) {
Byte = message[i]; // Get next byte.
Serial.println(message[i],HEX);
crc = crc ^ Byte;
for (j = 7; j >= 0; j--) { // Do eight times.
mask = -(crc & 1);
crc = (crc >> 1) ^ (0xEDB88320 & mask);
}
i = i + 1;
}
return ~crc;
}
Here is the python3 code to check it
import binascii
#print(binascii.crc32(b"hello world"),)
crc = binascii.crc32(b"HELLO") & 0xffffffff
print('crc32 = {:#010x}'.format(crc))
Firstly, thanks for the code!
I've tried to modify it for numerical data, which can contain a zero, but it has to be tested.
unsigned long crc32c ( unsigned char *message, int bytes )
{
// CRC-32 Calculation - No Table Lookup - by Billhowl
// https://forum.arduino.cc/index.php?topic=342371.0
// Modified for Numerical Data by Specifying Length in Bytes
// Usage:
// unsigned char test[6] = {"HELLO"};
// int bytes = 6;
// Serial.println ( crc32b ( test, bytes ), HEX);
unsigned long Byte, mask;
unsigned long crc = 0xFFFFFFFF;
int index, shift;
for ( index = 0 ; index < bytes ; index++ )
{
// Get Next Byte
Byte = message[index];
Serial.println( message[index], HEX );
// XOR with Next Byte
crc = crc ^ Byte;
// Shift
for (shift = 7; shift >= 0; shift--)
{
mask = -(crc & 1);
crc = (crc >> 1) ^ (0xEDB88320 & mask);
}
}
return ~crc;
}