Diff: crc2.cpp

Revision:

3:aaa92c61931a

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/crc2.cpp	Mon Jul 13 13:14:34 2015 +0000
@@ -0,0 +1,219 @@
+
+ 
+#include "crc2.h"
+
+
+/*
+ * Derive parameters from the standard-specific parameters in crc.h.
+ */
+#define WIDTH    (8 * sizeof(crc))
+#define TOPBIT   (1 << (WIDTH - 1))
+
+#if (REFLECT_DATA == TRUE)
+#undef  REFLECT_DATA
+#define REFLECT_DATA(X)         ((unsigned char) reflect((X), 8))
+#else
+#undef  REFLECT_DATA
+#define REFLECT_DATA(X)         (X)
+#endif
+
+#if (REFLECT_REMAINDER == TRUE)
+#undef  REFLECT_REMAINDER
+#define REFLECT_REMAINDER(X)    ((crc) reflect((X), WIDTH))
+#else
+#undef  REFLECT_REMAINDER
+#define REFLECT_REMAINDER(X)    (X)
+#endif
+
+
+/*********************************************************************
+ *
+ * Function:    reflect()
+ * 
+ * Description: Reorder the bits of a binary sequence, by reflecting
+ *              them about the middle position.
+ *
+ * Notes:       No checking is done that nBits <= 32.
+ *
+ * Returns:     The reflection of the original data.
+ *
+ *********************************************************************/
+static unsigned long
+reflect(unsigned long data, unsigned char nBits)
+{
+    unsigned long  reflection = 0x00000000;
+    unsigned char  bit;
+
+    /*
+     * Reflect the data about the center bit.
+     */
+    for (bit = 0; bit < nBits; ++bit)
+    {
+        /*
+         * If the LSB bit is set, set the reflection of it.
+         */
+        if (data & 0x01)
+        {
+            reflection |= (1 << ((nBits - 1) - bit));
+        }
+
+        data = (data >> 1);
+    }
+
+    return (reflection);
+
+}   /* reflect() */
+
+
+/*********************************************************************
+ *
+ * Function:    crcSlow()
+ * 
+ * Description: Compute the CRC of a given message.
+ *
+ * Notes:       
+ *
+ * Returns:     The CRC of the message.
+ *
+ *********************************************************************/
+crc
+crcSlow(unsigned char const message[], int nBytes)
+{
+    crc            remainder = INITIAL_REMAINDER;
+    int            byte;
+    unsigned char  bit;
+
+
+    /*
+     * Perform modulo-2 division, a byte at a time.
+     */
+    for (byte = 0; byte < nBytes; ++byte)
+    {
+        /*
+         * Bring the next byte into the remainder.
+         */
+        remainder ^= (REFLECT_DATA(message[byte]) << (WIDTH - 8));
+
+        /*
+         * Perform modulo-2 division, a bit at a time.
+         */
+        for (bit = 8; bit > 0; --bit)
+        {
+            /*
+             * Try to divide the current data bit.
+             */
+            if (remainder & TOPBIT)
+            {
+                remainder = (remainder << 1) ^ POLYNOMIAL;
+            }
+            else
+            {
+                remainder = (remainder << 1);
+            }
+        }
+    }
+
+    /*
+     * The final remainder is the CRC result.
+     */
+    return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);
+
+}   /* crcSlow() */
+
+
+crc  crcTable[256];
+
+
+/*********************************************************************
+ *
+ * Function:    crcInit()
+ * 
+ * Description: Populate the partial CRC lookup table.
+ *
+ * Notes:       This function must be rerun any time the CRC standard
+ *              is changed.  If desired, it can be run "offline" and
+ *              the table results stored in an embedded system's ROM.
+ *
+ * Returns:     None defined.
+ *
+ *********************************************************************/
+void
+crcInit()
+{
+    crc            remainder;
+    int            dividend;
+    unsigned char  bit;
+
+
+    /*
+     * Compute the remainder of each possible dividend.
+     */
+    for (dividend = 0; dividend < 256; ++dividend)
+    {
+        /*
+         * Start with the dividend followed by zeros.
+         */
+        remainder = dividend << (WIDTH - 8);
+
+        /*
+         * Perform modulo-2 division, a bit at a time.
+         */
+        for (bit = 8; bit > 0; --bit)
+        {
+            /*
+             * Try to divide the current data bit.
+             */         
+            if (remainder & TOPBIT)
+            {
+                remainder = (remainder << 1) ^ POLYNOMIAL;
+            }
+            else
+            {
+                remainder = (remainder << 1);
+            }
+        }
+
+        /*
+         * Store the result into the table.
+         */
+        crcTable[dividend] = remainder;
+    }
+
+}   /* crcInit() */
+
+
+/*********************************************************************
+ *
+ * Function:    crcFast()
+ * 
+ * Description: Compute the CRC of a given message.
+ *
+ * Notes:       crcInit() must be called first.
+ *
+ * Returns:     The CRC of the message.
+ *
+ *********************************************************************/
+crc
+crcFast(unsigned char const message[], int nBytes)
+{
+    crc            remainder = INITIAL_REMAINDER;
+    unsigned char  data;
+    int            byte;
+
+
+    /*
+     * Divide the message by the polynomial, a byte at a time.
+     */
+    for (byte = 0; byte < nBytes; ++byte)
+    {
+        data = REFLECT_DATA(message[byte]) ^ (remainder >> (WIDTH - 8));
+        remainder = crcTable[data] ^ (remainder << 8);
+    }
+
+    /*
+     * The final remainder is the CRC.
+     */
+    return (REFLECT_REMAINDER(remainder) ^ FINAL_XOR_VALUE);
+
+}   /* crcFast() */
+