--- /dev/null
+/* Copyright (c) 2009 The Board of Trustees of The Leland Stanford Junior
+ * University
+ *
+ * We are making the OpenFlow specification and associated documentation
+ * (Software) available for public use and benefit with the expectation
+ * that others will use, modify and enhance the Software and contribute
+ * those enhancements back to the community. However, since we would
+ * like to make the Software available for broadest use, with as few
+ * restrictions as possible permission is hereby granted, free of
+ * charge, to any person obtaining a copy of this Software to deal in
+ * the Software under the copyrights without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * The name and trademarks of copyright holder(s) may NOT be used in
+ * advertising or publicity pertaining to the Software or any
+ * derivatives without specific, written prior permission.
+ */
+
+#include <config.h>
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "hash.h"
+
+#undef NDEBUG
+#include <assert.h>
+
+static void
+set_bit(uint32_t array[3], int bit)
+{
+ assert(bit >= 0 && bit <= 96);
+ memset(array, 0, sizeof(uint32_t) * 3);
+ if (bit < 96) {
+ array[bit / 32] = UINT32_C(1) << (bit % 32);
+ }
+}
+
+int
+main(void)
+{
+ int i, j;
+
+ /* Check that all hash functions of with one 1-bit (or no 1-bits) set
+ * within a single 32-bit word have different values in all 11-bit
+ * consecutive runs.
+ *
+ * Given a random distribution, the probability of at least one collision
+ * in any set of 11 bits is approximately
+ *
+ * 1 - ((2**11 - 1)/2**11)**C(33,2)
+ * == 1 - (2047/2048)**528
+ * =~ 0.22
+ *
+ * There are 21 ways to pick 11 consecutive bits in a 32-bit word, so
+ * if we assumed independence then the chance of having no collisions in
+ * any of those 11-bit runs would be 0.22**21 =~ 1.5e-14. Obviously
+ * independence must be a bad assumption :-)
+ */
+ for (i = 0; i <= 32; i++) {
+ uint32_t in1 = i < 32 ? UINT32_C(1) << i : 0;
+ for (j = i + 1; j <= 32; j++) {
+ uint32_t in2 = j < 32 ? UINT32_C(1) << j : 0;
+ uint32_t out1 = hash_words(&in1, 1, 0);
+ uint32_t out2 = hash_words(&in2, 1, 0);
+ const int min_unique = 11;
+ const uint32_t unique_mask = (UINT32_C(1) << min_unique) - 1;
+ int ofs;
+ for (ofs = 0; ofs < 32 - min_unique; ofs++) {
+ uint32_t bits1 = (out1 >> ofs) & unique_mask;
+ uint32_t bits2 = (out2 >> ofs) & unique_mask;
+ if (bits1 == bits2) {
+ printf("Partial collision:\n");
+ printf("hash(%08"PRIx32") == %08"PRIx32"\n", in1, out1);
+ printf("hash(%08"PRIx32") == %08"PRIx32"\n", in2, out2);
+ printf("%d bits of output starting at bit %d "
+ "are both 0x%"PRIx32"\n", min_unique, ofs, bits1);
+ exit(1);
+ }
+ }
+ }
+ }
+
+ /* Check that all hash functions of with one 1-bit (or no 1-bits) set
+ * within three 32-bit words have different values in their lowest 12
+ * bits.
+ *
+ * Given a random distribution, the probability of at least one collision
+ * in 12 bits is approximately
+ *
+ * 1 - ((2**12 - 1)/2**12)**C(97,2)
+ * == 1 - (4095/4096)**4656
+ * =~ 0.68
+ *
+ * so we are doing pretty well to not have any collisions in 12 bits.
+ */
+ for (i = 0; i <= 96; i++) {
+ for (j = i + 1; j <= 96; j++) {
+ uint32_t in1[3], in2[3];
+ uint32_t out1, out2;
+ const int min_unique = 12;
+ const uint32_t unique_mask = (UINT32_C(1) << min_unique) - 1;
+
+ set_bit(in1, i);
+ set_bit(in2, j);
+ out1 = hash_words(in1, 3, 0);
+ out2 = hash_words(in2, 3, 0);
+ if ((out1 & unique_mask) == (out2 & unique_mask)) {
+ printf("Partial collision:\n");
+ printf("hash(1 << %d) == %08"PRIx32"\n", i, out1);
+ printf("hash(1 << %d) == %08"PRIx32"\n", j, out2);
+ printf("The low-order %d bits of output are both "
+ "0x%"PRIx32"\n", min_unique, out1 & unique_mask);
+ exit(1);
+ }
+ }
+ }
+ return 0;
+}
projects / openvswitch / commitdiff