/* -*-c-*-
*
- * $Id: maurer.c,v 1.1 2000/06/17 11:29:49 mdw Exp $
+ * $Id: maurer.c,v 1.4 2004/04/08 01:36:15 mdw Exp $
*
* Maurer's universal statistical test
*
* (c) 2000 Straylight/Edgeware
*/
-/*----- Licensing notice --------------------------------------------------*
+/*----- Licensing notice --------------------------------------------------*
*
* This file is part of Catacomb.
*
* it under the terms of the GNU Library General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or (at your option) any later version.
- *
+ *
* Catacomb is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Library General Public License for more details.
- *
+ *
* You should have received a copy of the GNU Library General Public
* License along with Catacomb; if not, write to the Free
* Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
* MA 02111-1307, USA.
*/
-/*----- Revision history --------------------------------------------------*
- *
- * $Log: maurer.c,v $
- * Revision 1.1 2000/06/17 11:29:49 mdw
- * Maurer's universal statistical test.
- *
- */
-
/*----- Header files ------------------------------------------------------*/
#include <assert.h>
/*----- Main code ---------------------------------------------------------*/
-/* --- @more@ --- *
+/* --- @maurer_init@ --- *
*
- * Arguments: @bitscan *b@ = pointer to a bitscanner structure
- * @unsigned l@ = number of bits wanted
+ * Arguments: @maurer_ctx *m@ = pointer to a context to initialize
+ * @unsigned l@ = number of bits to take at a time
*
- * Returns: An integer representing the next @l@ bits of the stream.
+ * Returns: Minimum recommended amount of data to test.
*
- * Use: Fetches the next @l@ bits of a data stream. The bitscanner
- * has no way of knowing whether it's run out of data, so make
- * sure that doesn't happen.
+ * Use: Initializes a Maurer testing context. Note that @l@ values
+ * larger than 16 are not supported, and values less than 6 can
+ * give bizarre results.
*/
-static unsigned more(bitscan *b, unsigned l)
+unsigned long maurer_init(maurer_ctx *m, unsigned l)
{
- unsigned x, m;
+ size_t i;
+
+ assert(((void)"(maurer_init): chunks larger than 16 bits not supported",
+ 0 < l && l <= 16));
- while (b->b < l) {
- b->a |= U8(*b->p++) << b->b;
- b->b += 8;
+ m->l = l;
+ m->a = 0;
+ m->b = 0;
+ m->n = 0;
+ m->x = 0;
+
+ m->t = xmalloc(sizeof(unsigned long) << l);
+ for (i = 0; i < (1 << l); i++)
+ m->t[i] = 0;
+ return (((1000ul << l) * l + 7)/8);
+}
+
+/* --- @bits@ --- *
+ *
+ * Arguments: @maurer_ctx *m@ = pointer to testing context
+ * @const octet **p@ = address of a buffer pointer
+ * @const octet *q@ = limit of the buffer pointer
+ * @unsigned *x@ = address to store next chunk
+ *
+ * Returns: Nonzero if some more bits arrived.
+ *
+ * Use: Reads some bits from a buffer.
+ */
+
+static int bits(maurer_ctx *m, const octet **p, const octet *q, unsigned *x)
+{
+ while (m->b < m->l) {
+ if (*p >= q)
+ return (0);
+ m->a |= U8(*(*p)++) << m->b;
+ m->b += 8;
}
- m = (1 << l) - 1;
- x = b->a & m;
- b->a >>= l;
- b->b -= l;
- return (x);
+ *x = m->a & ((1 << m->l) - 1);
+ m->a >>= m->l;
+ m->b -= m->l;
+ m->n++;
+ return (1);
}
-/* --- @maurer@ --- *
+/* --- @maurer_test@ --- *
*
- * Arguments: @const octet *p@ = pointer to a buffer of data
+ * Arguments: @maurer_ctx *m@ = pointer to a Maurer testing context
+ * @const void *buf@ = pointer to a buffer of data
* @size_t sz@ = size of the buffer
- * @unsigned l@ = number of bits to take at a time
+ *
+ * Returns: ---
+ *
+ * Use: Scans a buffer of data and updates the testing context.
+ */
+
+void maurer_test(maurer_ctx *m, const void *buf, size_t sz)
+{
+ const octet *p = buf, *l = p + sz;
+ unsigned long q = 10 << m->l;
+ unsigned x;
+
+ /* --- Initialize the table --- */
+
+ while (m->n < q) {
+ if (!bits(m, &p, l, &x))
+ return;
+ m->t[x] = m->n;
+ }
+
+ /* --- Update the statistic --- */
+
+ while (bits(m, &p, l, &x)) {
+ m->x += log(m->n - m->t[x]);
+ m->t[x] = m->n;
+ }
+}
+
+/* --- @maurer_done@ --- *
+ *
+ * Arguments: @maurer_ctx *m@ = pointer to a Maurer testing context
*
* Returns: The statistic %$Z_u = (X_u - \mu)/\sigma$%, which should be
- * normally distributed with a mean of zero and standard
- * deviation of 1.
+ * normally distributed with a mean of 0 and variance of 1.
*
- * Use: Performs Maurer's universal statistical test on a buffer of
- * data. You must ensure that the buffer size @sz@ is larger
- * than %$11 l \cdot 2^l$% bits long, and ideally much larger.
+ * Use: Returns the result of Maurer's universal statistical test.
+ * Also frees the memory allocated during initialization.
*
- * Note that, under Unix systems, this function requires the
- * maths library.
+ * If insufficient data was received, the value @HUGE_VAL@ is
+ * returned.
*/
-double maurer(const octet *p, size_t sz, unsigned l)
+double maurer_done(maurer_ctx *m)
{
- size_t *t; /* Table of occurrences */
- size_t q, k; /* Block counts */
- size_t i; /* Loop counter */
- bitscan b; /* Bitscanning context */
- double x; /* Statistic accumulator */
- double mu, c, sigma; /* Other useful things */
+ double x, mu, c, sigma;
+ unsigned long q = 10 << m->l, k;
/* --- Table for means and variances --- */
{ 15.167379 , 3.421 }
};
- assert(((void)"bit chunk size out of range", 0 < l && l <= 16));
-
- /* --- Initialize everything --- */
-
- t = xmalloc(sizeof(size_t) << l);
- for (i = 0; i < (1 << l); i++)
- t[i] = 0;
-
- b.p = p;
- b.a = 0;
- b.b = 0;
-
- k = (sz * 8)/l;
- q = 10 << l;
- assert(((void)"buffer too small for test", k > q && k - q > (1 << l)));
-
- /* --- Set up the table --- */
-
- for (i = 0; i < q; i++) {
- unsigned a = more(&b, l);
- t[a] = i;
- }
-
- /* --- Now compute the statistic --- */
+ /* --- Check for bogus requests --- */
- x = 0;
- for (; i < k; i++) {
- unsigned a = more(&b, l);
- x += log(i - t[a]);
- t[a] = i;
+ if (m->n < q) {
+ x = HUGE_VAL;
+ goto done;
}
- /* --- Twiddle at the end --- *
+ /* --- Do the maths --- *
*
* This produces %$X_u$%, which should be normally distributed with a mean
* and variance we can compute.
*/
- k -= q;
- x /= k * log(2);
+ k = m->n - q;
+ x = m->x/(k * log(2));
/* --- Now translate this into %$Z_u$% distributed as %$N(0, 1)$% --- */
- mu = vtab[l - 1].mu;
- c = 0.7 - 0.8/l + (1.6 + 12.8/l) * pow(k, -4.0/l);
- sigma = sqrt(c * c * vtab[l - 1].var_1 / k);
-
+ mu = vtab[m->l - 1].mu;
+ c = 0.7 - 0.8/m->l + (1.6 + 12.8/m->l) * pow(k, -4.0/m->l);
+ sigma = sqrt(c * c * vtab[m->l - 1].var_1 / k);
x = (x - mu)/sigma;
/* --- Done (phew!) --- */
- xfree(t);
+done:
+ xfree(m->t);
return (x);
}
+/* --- @maurer@ --- *
+ *
+ * Arguments: @const octet *p@ = pointer to a buffer of data
+ * @size_t sz@ = size of the buffer
+ * @unsigned l@ = number of bits to take at a time
+ *
+ * Returns: The statistic %$Z_u$%.
+ *
+ * Use: Simple interface to Maurer's universal statistical test. For
+ * large %$l$% you should use the more complicated restartable
+ * interface.
+ */
+
+double maurer(const octet *p, size_t sz, unsigned l)
+{
+ maurer_ctx m;
+
+ maurer_init(&m, l);
+ maurer_test(&m, p, sz);
+ return (maurer_done(&m));
+}
+
/*----- That's all, folks -------------------------------------------------*/
projects / u / mdw / catacomb / blobdiff