added shannon_H entropy metric for each revision

[wikiq] / disorder.c

diff --git a/disorder.c b/disorder.c
new file mode 100644 (file)
index 0000000..a5f7c35
--- /dev/null
+++ b/disorder.c
@@ -0,0 +1,192 @@
+/***************************************************************************
+ *  libdisorder: A Library for Measuring Byte Stream Entropy
+ *  Copyright (C) 2010 Michael E. Locasto
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program 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
+ *  General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the:
+ *       Free Software Foundation, Inc.
+ *       59 Temple Place, Suite 330
+ *       Boston, MA  02111-1307  USA
+ *
+ * $Id$
+ **************************************************************************/
+
+#include <math.h> //for log2()
+#include <stdio.h> //for NULL
+#include "disorder.h"
+
+#if defined(__FreeBSD__)
+#define        log2(x) (log((x)) * (1./M_LN2))
+#endif
+
+/** Frequecies for each byte */
+static int m_token_freqs[LIBDO_MAX_BYTES]; //frequency of each token in sample
+static float m_token_probs[LIBDO_MAX_BYTES]; //P(each token appearing)
+static int m_num_tokens = 0; //actual number of `seen' tokens, max 256 
+static float m_maxent = 0.0;
+static float m_ratio = 0.0;
+static int LIBDISORDER_INITIALIZED = 0;
+
+static void
+initialize_lib()
+{
+  int i = 0;
+  if(1==LIBDISORDER_INITIALIZED)
+    return;
+
+  m_num_tokens = 0;
+
+  for(i=0;i<LIBDO_MAX_BYTES;i++)
+  {
+    m_token_freqs[i]=0;
+    m_token_probs[i]=0.0;
+  }
+
+  LIBDISORDER_INITIALIZED = 1;
+}
+
+/**
+ * Set m_num_tokens by iterating over m_token_freq[] and maintaining
+ * a counter of each position that does not hold the value of zero.
+ */
+static void
+count_num_tokens()
+{
+  int i = 0;
+  int counter = 0;
+  for(i=0;i<LIBDO_MAX_BYTES;i++)
+  {
+    if(0!=m_token_freqs[i])
+    {
+      counter++;
+    }
+  }
+  m_num_tokens = counter;
+  return;
+}
+
+/**
+ * Sum frequencies for each token (i.e., byte values 0 through 255)
+ * We assume the `length' parameter is correct.
+ *
+ * This function is available only to functions in this file.
+ */
+static void
+get_token_frequencies(char* buf, 
+                     long long length)
+{
+  int i=0;
+  char* itr=NULL;
+  unsigned char c=0;
+
+  itr = buf;
+
+  //reset number of tokens
+  m_num_tokens = 0;
+
+  //make sure freqency and probability arrays are cleared
+  for(i=0;i<LIBDO_MAX_BYTES;i++)
+  {
+    m_token_freqs[i] = 0;
+    m_token_probs[i] = 0.0;
+  }
+
+  for(i=0;i<length;i++)
+  {
+    c = (unsigned char)*itr;
+    //assert(0<=c<LIBDO_MAX_BYTES);
+    m_token_freqs[c]++;
+    itr++;
+  }
+}
+
+/**
+ * Return entropy (in bits) of this buffer of bytes. We assume that the
+ * `length' parameter is correct. This implementation is a translation
+ * of the PHP code found here:
+ *
+ *    http://onlamp.com/pub/a/php/2005/01/06/entropy.html
+ *
+ * with a helpful hint on the `foreach' statement from here:
+ *
+ *    http://php.net/manual/en/control-structures.foreach.php
+ */
+float
+shannon_H(char* buf, 
+         long long length)
+{
+  int i = 0;
+  float bits = 0.0;
+  char* itr=NULL; //values of itr should be zero to 255
+  unsigned char token;
+  int num_events = 0; //`length' parameter
+  float freq = 0.0; //loop variable for holding freq from m_token_freq[]
+  float entropy = 0.0; //running entropy sum
+
+  if(NULL==buf || 0==length)
+    return 0.0;
+
+  if(0==LIBDISORDER_INITIALIZED)
+    initialize_lib();
+
+  itr = buf;
+  m_maxent = 0.0;
+  m_ratio = 0.0;
+  num_events = length;
+  get_token_frequencies(itr, num_events); //modifies m_token_freqs[]
+  //set m_num_tokens by counting unique m_token_freqs entries
+  count_num_tokens(); 
+
+  if(m_num_tokens>LIBDO_MAX_BYTES)
+  {
+    //report error somehow?
+    return 0.0;
+  }
+
+  //iterate through whole m_token_freq array, but only count
+  //spots that have a registered token (i.e., freq>0)
+  for(i=0;i<LIBDO_MAX_BYTES;i++)
+  {
+    if(0!=m_token_freqs[i])
+    {
+      token = i;
+      freq = ((float)m_token_freqs[token]); 
+      m_token_probs[token] = (freq / ((float)num_events));
+      entropy += m_token_probs[token] * log2(m_token_probs[token]);
+    }
+  }
+
+  bits = -1.0 * entropy;
+  m_maxent = log2(m_num_tokens);
+  m_ratio = bits / m_maxent;
+
+  return bits;
+}
+
+int 
+get_num_tokens()
+{
+  return m_num_tokens;
+}
+
+float 
+get_max_entropy()
+{
+  return m_maxent;
+}
+
+float 
+get_entropy_ratio()
+{
+  return m_ratio;
+}