From 531b2e8f52bebb9eb4a6f0c3521c23469c40b2c7 Mon Sep 17 00:00:00 2001
From: Daniel Peter Chokola <daniel.chokola@cobham.com>
Date: Tue, 21 Apr 2020 05:55:23 -0400
Subject: [PATCH] Generate CRC LUT and test CRC calculation against reference
 implementation using a golden packet.

---
 Makefile |   9 +++++
 crc.c    | 117 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 126 insertions(+)
 create mode 100644 Makefile
 create mode 100644 crc.c

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..c6c394e
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,9 @@
+CC=gcc
+OBJ=crc.o
+CFLAGS=-Wall -Wextra -Werror -pedantic
+
+%.o: %.c $(DEPS)
+	$(CC) -c -o $@ $< $(CFLAGS)
+
+crc: $(OBJ)
+	$(CC) -o $@ $^ $(CFLAGS)
\ No newline at end of file
diff --git a/crc.c b/crc.c
new file mode 100644
index 0000000..4bbe123
--- /dev/null
+++ b/crc.c
@@ -0,0 +1,117 @@
+/*
+ * crc.c - reference CRC implementations and LUT generator
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+
+/*
+ * CRC-16-IBM -- reversed reciprocal representation
+ * https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks
+ */
+#define CRCPOLY (0xc002)
+
+static uint16_t crc16_lut[256] = { 0x0000 };
+
+uint16_t crc16(uint8_t *data, uint32_t len);
+void crc16_gen();
+uint16_t crc16_ref(uint8_t *Pkt, uint32_t PktCnt);
+
+int main()
+{
+	uint8_t i;
+	uint8_t golden_packet[32] = { 
+		0xf2, 0x22, 0x22, 0x22,
+		0x20, 0x00, 0x00, 0x00, 
+		0x78, 0x56, 0x34, 0x12, 
+		0xd2, 0xc3, 0xb4, 0xa5, 
+		0x10, 0x00, 0x00, 0x00, 
+		0x01, 0x5a, 0x00, 0x00, 
+		0x42, 0x43, 0x44, 0x45, 
+		0x46, 0x47, 0x00, 0x00
+	};
+
+	/* generate lookup table */
+	crc16_gen();
+
+	/* print out the table (for embedding into code data sections) */
+	printf("CRC byte lookup table:\n0x0000");
+	for(i = 0; i < 255; i++)
+	{
+		printf(", 0x%04x", crc16_lut[i + 1]);
+	}
+	printf("\n\n");
+
+	/* compute the golden checksum using the LUT and a reference algorithm */
+	printf("Golden Checksum: 0x%04x\n", crc16(golden_packet, 32));
+	printf("Reference Golden Checksum: 0x%04x\n", crc16_ref(golden_packet, 32));
+
+	return(0);
+}
+
+/* fast CRC calculation using LUT */
+uint16_t crc16(uint8_t *data, uint32_t len)
+{ 
+	uint32_t i;
+	uint16_t rem = 0;
+
+	for (i = 0; i < len; i++)
+	{
+		rem = (rem >> 8) ^ crc16_lut[data[i] ^ (rem & 0xff)];
+	}
+
+	return(rem);
+}
+
+/* fast CRC LUT generation */
+void crc16_gen()
+{
+	uint16_t crc = 1;
+	uint8_t i;
+	uint16_t j;
+
+	for(i = 0x80; i > 0; i >>= 1)
+	{
+		if(crc & 1)
+		{
+			crc = (crc >> 1) ^ CRCPOLY;
+		}
+		else
+		{
+			crc >>= 1;
+		}
+
+		for(j = 0; j < 256; j += 2 * i)
+		{
+			crc16_lut[i + j] = crc ^ crc16_lut[j];
+		}
+	}
+}
+
+/* reference CRC implementation */
+uint16_t crc16_ref(uint8_t *buf, uint32_t len)
+{
+	uint16_t rem = 0;
+	uint16_t dat;
+	uint16_t tmp;
+	uint32_t i, j;
+
+	for(i = 0; i < len; i++)
+	{
+		dat = buf[i];
+		for(j = 0; j < 8; j++)
+		{
+			tmp = (rem ^ dat) & 0x0001;
+
+			rem >>= 1;
+			dat >>= 1;
+
+			if(tmp)
+			{
+				rem ^= CRCPOLY;
+			}
+		}
+	}
+
+	return(rem);
+}