From: Kovarththanan Rajaratnam <kovarththanan.rajaratnam@gmail.com>
Date: Sat, 20 Feb 2010 07:40:44 +0000 (+0100)
Subject: Precompute CRC-10 table.
X-Git-Tag: tcpdump-4.1.0~8^2~10
X-Git-Url: https://git.tcpdump.org/tcpdump/commitdiff_plain/e6c39e6db430847e7f447e90c364c74fb17681fa

Precompute CRC-10 table.

We now precompute the CRC-10 table allowing us to mark it as const. The table was precomputed using a Python snippet (included as comment)
---

diff --git a/checksum.c b/checksum.c
index f441be3d..da4718ff 100644
--- a/checksum.c
+++ b/checksum.c
@@ -31,17 +31,73 @@ static const char rcsid[] _U_ =
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <assert.h>
 
 #include "interface.h"
 
-#define CRC10_POLYNOMIAL 0x633
-static u_int16_t crc10_table[256];
+/*
+ * CRC-10 table generated using the following Python snippet:
+
+import sys
+
+crc_table = []
+for i in range(256):
+	accum = i << 2
+	for j in range(8):
+		accum <<= 1
+		if accum & 0x400:
+			accum ^= 0x633
+	crc_table.append(accum)
+
+for i in range(len(crc_table)/8):
+	for j in range(8):
+		sys.stdout.write("0x%04x, " % crc_table[i*8+j])
+	sys.stdout.write("\n")
+
+ */
+static const u_int16_t crc10_table[256] =
+{
+	0x0000, 0x0233, 0x0255, 0x0066, 0x0299, 0x00aa, 0x00cc, 0x02ff,
+	0x0301, 0x0132, 0x0154, 0x0367, 0x0198, 0x03ab, 0x03cd, 0x01fe,
+	0x0031, 0x0202, 0x0264, 0x0057, 0x02a8, 0x009b, 0x00fd, 0x02ce,
+	0x0330, 0x0103, 0x0165, 0x0356, 0x01a9, 0x039a, 0x03fc, 0x01cf,
+	0x0062, 0x0251, 0x0237, 0x0004, 0x02fb, 0x00c8, 0x00ae, 0x029d,
+	0x0363, 0x0150, 0x0136, 0x0305, 0x01fa, 0x03c9, 0x03af, 0x019c,
+	0x0053, 0x0260, 0x0206, 0x0035, 0x02ca, 0x00f9, 0x009f, 0x02ac,
+	0x0352, 0x0161, 0x0107, 0x0334, 0x01cb, 0x03f8, 0x039e, 0x01ad,
+	0x00c4, 0x02f7, 0x0291, 0x00a2, 0x025d, 0x006e, 0x0008, 0x023b,
+	0x03c5, 0x01f6, 0x0190, 0x03a3, 0x015c, 0x036f, 0x0309, 0x013a,
+	0x00f5, 0x02c6, 0x02a0, 0x0093, 0x026c, 0x005f, 0x0039, 0x020a,
+	0x03f4, 0x01c7, 0x01a1, 0x0392, 0x016d, 0x035e, 0x0338, 0x010b,
+	0x00a6, 0x0295, 0x02f3, 0x00c0, 0x023f, 0x000c, 0x006a, 0x0259,
+	0x03a7, 0x0194, 0x01f2, 0x03c1, 0x013e, 0x030d, 0x036b, 0x0158,
+	0x0097, 0x02a4, 0x02c2, 0x00f1, 0x020e, 0x003d, 0x005b, 0x0268,
+	0x0396, 0x01a5, 0x01c3, 0x03f0, 0x010f, 0x033c, 0x035a, 0x0169,
+	0x0188, 0x03bb, 0x03dd, 0x01ee, 0x0311, 0x0122, 0x0144, 0x0377,
+	0x0289, 0x00ba, 0x00dc, 0x02ef, 0x0010, 0x0223, 0x0245, 0x0076,
+	0x01b9, 0x038a, 0x03ec, 0x01df, 0x0320, 0x0113, 0x0175, 0x0346,
+	0x02b8, 0x008b, 0x00ed, 0x02de, 0x0021, 0x0212, 0x0274, 0x0047,
+	0x01ea, 0x03d9, 0x03bf, 0x018c, 0x0373, 0x0140, 0x0126, 0x0315,
+	0x02eb, 0x00d8, 0x00be, 0x028d, 0x0072, 0x0241, 0x0227, 0x0014,
+	0x01db, 0x03e8, 0x038e, 0x01bd, 0x0342, 0x0171, 0x0117, 0x0324,
+	0x02da, 0x00e9, 0x008f, 0x02bc, 0x0043, 0x0270, 0x0216, 0x0025,
+	0x014c, 0x037f, 0x0319, 0x012a, 0x03d5, 0x01e6, 0x0180, 0x03b3,
+	0x024d, 0x007e, 0x0018, 0x022b, 0x00d4, 0x02e7, 0x0281, 0x00b2,
+	0x017d, 0x034e, 0x0328, 0x011b, 0x03e4, 0x01d7, 0x01b1, 0x0382,
+	0x027c, 0x004f, 0x0029, 0x021a, 0x00e5, 0x02d6, 0x02b0, 0x0083,
+	0x012e, 0x031d, 0x037b, 0x0148, 0x03b7, 0x0184, 0x01e2, 0x03d1,
+	0x022f, 0x001c, 0x007a, 0x0249, 0x00b6, 0x0285, 0x02e3, 0x00d0,
+	0x011f, 0x032c, 0x034a, 0x0179, 0x0386, 0x01b5, 0x01d3, 0x03e0,
+	0x021e, 0x002d, 0x004b, 0x0278, 0x0087, 0x02b4, 0x02d2, 0x00e1
+};
 
 static void
 init_crc10_table(void)
 {   
+#define CRC10_POLYNOMIAL 0x633
     register int i, j;
     register u_int16_t accum;
+    u_int16_t verify_crc10_table[256];
     
     for ( i = 0;  i < 256;  i++ )
     {
@@ -50,9 +106,12 @@ init_crc10_table(void)
         {
             if ((accum <<= 1) & 0x400) accum ^= CRC10_POLYNOMIAL;
         }
-        crc10_table[i] = accum;
+        verify_crc10_table[i] = accum;
     }
-    return;
+    assert(memcmp(verify_crc10_table,
+				  crc10_table,
+				  sizeof(verify_crc10_table)) == 0);
+#undef CRC10_POLYNOMIAL
 }
 
 u_int16_t