checked in with -k by eggert at 2006/11/05 06:59:41

diff --git a/sample/makecrc.c b/sample/makecrc.c
new file mode 100644 (file)
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--- /dev/null
+++ b/sample/makecrc.c
@@ -0,0 +1,63 @@
+/* Not copyrighted 1990 Mark Adler */
+
+#ifndef lint
+static char rcsid[] = "$Id$";
+#endif
+
+#include <stdio.h>
+
+main()
+/*
+  Generate a table for a byte-wise 32-bit CRC calculation on the polynomial:
+  x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1.
+
+  Polynomials over GF(2) are represented in binary, one bit per coefficient,
+  with the lowest powers in the most significant bit.  Then adding polynomials
+  is just exclusive-or, and multiplying a polynomial by x is a right shift by
+  one.  If we call the above polynomial p, and represent a byte as the
+  polynomial q, also with the lowest power in the most significant bit (so the
+  byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
+  where a mod b means the remainder after dividing a by b.
+
+  This calculation is done using the shift-register method of multiplying and
+  taking the remainder.  The register is initialized to zero, and for each
+  incoming bit, x^32 is added mod p to the register if the bit is a one (where
+  x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
+  x (which is shifting right by one and adding x^32 mod p if the bit shifted
+  out is a one).  We start with the highest power (least significant bit) of
+  q and repeat for all eight bits of q.
+
+  The table is simply the CRC of all possible eight bit values.  This is all
+  the information needed to generate CRC's on data a byte at a time for all
+  combinations of CRC register values and incoming bytes.  The table is
+  written to stdout as 256 long hexadecimal values in C language format.
+*/
+{
+  unsigned long c;      /* crc shift register */
+  unsigned long e;      /* polynomial exclusive-or pattern */
+  int i;                /* counter for all possible eight bit values */
+  int k;                /* byte being shifted into crc apparatus */
+
+  /* terms of polynomial defining this crc (except x^32): */
+  static int p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
+
+  /* Make exclusive-or pattern from polynomial (0xedb88320) */
+  e = 0;
+  for (i = 0; i < sizeof(p)/sizeof(int); i++)
+    e |= 1L << (31 - p[i]);
+
+  /* Compute and print table of CRC's, five per line */
+  printf("  0x00000000L");
+  for (i = 1; i < 256; i++)
+  {
+    c = i;
+    /* The idea to initialize the register with the byte instead of
+     * zero was stolen from Haruhiko Okumura's ar002
+     */
+    for (k = 8; k; k--)
+      c = c & 1 ? (c >> 1) ^ e : c >> 1;
+    printf(i % 5 ? ", 0x%08lxL" : ",\n  0x%08lxL", c);
+  }
+  putchar('\n');
+  return 0;
+}