Switch to table based CRC.

More code, but much faster. Drops latency from 12 bytes to 8 bytes on the host side at 1 MHz.

Author: jonathanpallant

neotron-bmc-protocol/src/crc.rs

@@ -11,41 +11,50 @@
  *  - ReflectIn     = False
  *  - XorOut        = 0x00
  *  - ReflectOut    = False
- *  - Algorithm     = bit-by-bit
+ *  - Algorithm     = table-driven
  */
 
 // Translated from C to Rust by hand.
 
+///
+/// Static table used for the table_driven implementation.
+///
+static CRC_TABLE: [u8; 256] = [
+	0x00, 0x07, 0x0e, 0x09, 0x1c, 0x1b, 0x12, 0x15, 0x38, 0x3f, 0x36, 0x31, 0x24, 0x23, 0x2a, 0x2d,
+	0x70, 0x77, 0x7e, 0x79, 0x6c, 0x6b, 0x62, 0x65, 0x48, 0x4f, 0x46, 0x41, 0x54, 0x53, 0x5a, 0x5d,
+	0xe0, 0xe7, 0xee, 0xe9, 0xfc, 0xfb, 0xf2, 0xf5, 0xd8, 0xdf, 0xd6, 0xd1, 0xc4, 0xc3, 0xca, 0xcd,
+	0x90, 0x97, 0x9e, 0x99, 0x8c, 0x8b, 0x82, 0x85, 0xa8, 0xaf, 0xa6, 0xa1, 0xb4, 0xb3, 0xba, 0xbd,
+	0xc7, 0xc0, 0xc9, 0xce, 0xdb, 0xdc, 0xd5, 0xd2, 0xff, 0xf8, 0xf1, 0xf6, 0xe3, 0xe4, 0xed, 0xea,
+	0xb7, 0xb0, 0xb9, 0xbe, 0xab, 0xac, 0xa5, 0xa2, 0x8f, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9d, 0x9a,
+	0x27, 0x20, 0x29, 0x2e, 0x3b, 0x3c, 0x35, 0x32, 0x1f, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0d, 0x0a,
+	0x57, 0x50, 0x59, 0x5e, 0x4b, 0x4c, 0x45, 0x42, 0x6f, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7d, 0x7a,
+	0x89, 0x8e, 0x87, 0x80, 0x95, 0x92, 0x9b, 0x9c, 0xb1, 0xb6, 0xbf, 0xb8, 0xad, 0xaa, 0xa3, 0xa4,
+	0xf9, 0xfe, 0xf7, 0xf0, 0xe5, 0xe2, 0xeb, 0xec, 0xc1, 0xc6, 0xcf, 0xc8, 0xdd, 0xda, 0xd3, 0xd4,
+	0x69, 0x6e, 0x67, 0x60, 0x75, 0x72, 0x7b, 0x7c, 0x51, 0x56, 0x5f, 0x58, 0x4d, 0x4a, 0x43, 0x44,
+	0x19, 0x1e, 0x17, 0x10, 0x05, 0x02, 0x0b, 0x0c, 0x21, 0x26, 0x2f, 0x28, 0x3d, 0x3a, 0x33, 0x34,
+	0x4e, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5c, 0x5b, 0x76, 0x71, 0x78, 0x7f, 0x6a, 0x6d, 0x64, 0x63,
+	0x3e, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2c, 0x2b, 0x06, 0x01, 0x08, 0x0f, 0x1a, 0x1d, 0x14, 0x13,
+	0xae, 0xa9, 0xa0, 0xa7, 0xb2, 0xb5, 0xbc, 0xbb, 0x96, 0x91, 0x98, 0x9f, 0x8a, 0x8d, 0x84, 0x83,
+	0xde, 0xd9, 0xd0, 0xd7, 0xc2, 0xc5, 0xcc, 0xcb, 0xe6, 0xe1, 0xe8, 0xef, 0xfa, 0xfd, 0xf4, 0xf3,
+];
+
 pub(crate) const fn init() -> u8 {
 	0x00
 }
 
 /// Update a CRC with more data
 pub(crate) fn update(mut crc: u8, data: &[u8]) -> u8 {
-	for c in data.iter() {
-		for i in 0..8 {
-			let bit = (crc & 0x80) != 0;
-			crc = (crc << 1) | ((c >> (7 - i)) & 0x01);
-			if bit {
-				crc ^= 0x07;
-			}
-		}
+	for d in data.iter() {
+		let idx = crc ^ *d;
+		crc = CRC_TABLE[usize::from(idx)];
 	}
 	crc
 }
 
 /// Finish the CRC calculation
-pub(crate) fn finalize(mut crc: u8) -> u8 {
-	for _i in 0..8 {
-		let bit = (crc & 0x80) != 0;
-		crc <<= 1;
-		if bit {
-			crc ^= 0x07;
-		}
-	}
+pub(crate) fn finalize(crc: u8) -> u8 {
 	crc
 }
 
 // ============================================================================
-// End of File
-// ============================================================================
+// End of File// ============================================================================

neotron-bmc-protocol/src/lib.rs

@@ -454,6 +454,69 @@ pub fn calculate_crc(data: &[u8]) -> u8 {
 	crc::finalize(crc)
 }
 
+// ============================================================================
+// Tests
+// ============================================================================
+
+#[cfg(test)]
+mod test {
+	use super::*;
+
+	#[test]
+	fn read_request() {
+		let req = Request::new_read(false, 0x10, 0x20);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC0, 0x10, 0x20, 0x3A]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+
+	#[test]
+	fn read_request_alt() {
+		let req = Request::new_read(true, 0x10, 0x20);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC1, 0x10, 0x20, 0x51]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+
+	#[test]
+	fn short_write_request() {
+		let req = Request::new_short_write(false, 0x11, 0x22);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC2, 0x11, 0x22, 0xF7]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+
+	#[test]
+	fn short_write_request_alt() {
+		let req = Request::new_short_write(true, 0x11, 0x22);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC3, 0x11, 0x22, 0x9C]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+
+	#[test]
+	fn long_write_request() {
+		let req = Request::new_long_write(false, 0x0F, 0x50);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC4, 0x0F, 0x50, 0x52]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+
+	#[test]
+	fn long_write_request_alt() {
+		let req = Request::new_long_write(true, 0x0F, 0x50);
+		let bytes = req.as_bytes();
+		assert_eq!(bytes, [0xC5, 0x0F, 0x50, 0x39]);
+		let decoded_req = Request::from_bytes(&bytes).unwrap();
+		assert_eq!(req, decoded_req);
+	}
+}
+
 // ============================================================================
 // End of File
 // ============================================================================