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--- tanszek:oktatas:techcomm:information [2024/08/27 13:11] – [Entropy] knehez
+++ tanszek:oktatas:techcomm:information [2024/10/15 06:30] (current) – [Example of Entropy calculation] knehez
@@ Line 15: / Line 15: @@
 If an event space consist of two equal-probability event \(p(E_1) = p(E_2) = 0.5 \) then,
-$$ I_{E_1} = I_{E_2} = \log_2 \frac{1}{0.5} = - \log_2 2 = 1 [bit] $$
+$$ I_{E_1} = I_{E_2} = \log_2 \frac{1}{0.5} = - \log_2 0.5 = 1 [bit] $$
 So the unit of the information means the news value which is connected to the simple, less likely, same probability choice.
-If the event system consist of 'n' number of events and all these events have the same probability then the probability of any event is the following:
+If the event system consist of ''n'' number of events and all these events have the same probability then the probability of any event is the following:
 $$ p_E = \frac{1}{n} $$
@@ Line 48: / Line 48: @@
 This concept is crucial in various fields, including //data compression//, //cryptography//, and //machine learning//, where understanding and managing entropy can lead to more efficient algorithms and systems. For example, in data compression, reducing redundancy (and thus reducing entropy) can lead to more compact data representations. Similarly, in cryptography, managing entropy ensures that keys and encrypted messages are less predictable and more secure.
+==== Redundancy ====
+The average information content of a message set describing an equally probable, completely random set of events is the highest. In contrast, the average information content of a message set describing a completely ordered, i.e., fully known event set, is the lowest.
+A probability distribution that deviates from the maximum possible entropy leads to a message set that is redundant.
+The measure of redundancy is:
+$$ R = \frac{H_{max} - H}{H_{max}} = 1- \frac{H}{H_{max}} $$
+If the event space consists of //n// equally probable events:
+$$ H_{max} = \log_2 n \;\; \text{and} \;\; R = 1 - \frac{H(p_1, ... , p_n)}{\log_2 n} $$
+Redundancy plays a significant role in information theory. Redundancy enables the secure communication of messages over a noisy channel. The redundancy of human verbal communication is typically more than 30%. The changes in redundancy for a two-event message set are illustrated in the figure below:
+{{:tanszek:oktatas:techcomm:pasted:20240827-131817.png}}
+Thus, redundancy is minimal when the probabilities of the events are equal.
+**Example:** The occurrence probabilities of a system consisting of four events are as follows:
+$$ E = \{E_1, E_2, E_3, E_4 \}, $$
+and the probabilities of the individual events are as follows:
+$$ p = \{0.5, 0.25, 0.2, 0.05\}. $$
+The individual information content for the states of the system are:
+$$ I_{E_1} = -\log_2 0.5 = 1 \, \text{[bit]}, $$
+$$ I_{E_2} = -\log_2 0.25 = 2 \, \text{[bit]}, $$
+$$ I_{E_3} = -\log_2 0.2 = 2.32 \, \text{[bit]}, $$
+$$ I_{E_4} = -\log_2 0.05 = 4.32 \, \text{[bit]}, $$
+**What is the entropy of the message set?**
+$$ H_E = \sum_{i=1}^{4} p_i \cdot I_{E_i} = 0.5 \cdot 1 + 0.25 \cdot 2 + 0.2 \cdot 2.32 + 0.05 \cdot 4.32 = 1.68 \, \text{[bit]}. $$
+**What is the redundancy of the message set?**
+Let's calculate the maximum entropy:
+$$ H_{\text{max}} = \log_2 n = \log_2 4 = 2 [bit]$$
+Then, substituting into the formula for redundancy:
+$$ R = 1 - \frac{H}{H_{\text{max}}} = 1 - \frac{1.68}{2} = 0.16, $$
+which means the redundancy of the event system is approximately 16%.
+==== Example of Entropy calculation ====
+Why do not store raw password strings in compiled code?
+<sxh c>
+#include <stdio.h>
+#include <math.h>
+float calculateEntropy(unsigned int bytes[], int length);
+char sample[] = "Some poetry types are unique to particular cultures and  genres and respond to yQ%v?FY}ZT=/5cJ.m~A{9^8Lse characteristics of the language in which the poet writes. Readers accustomed to identifying poetry with Dante, Goethe, Mickiewicz, or Rumi may think of it as written in lines based on rhyme and regular meter. There are, however, traditions, such as Biblical poetry and alliterative verse, that use other means to create rhythm and euphony. Much modern poetry reflects a critique of poetic tradition,[6] testing the principle of euphony itself or altogether forgoing rhyme or set rhythm";
+int main()
+{
+    unsigned int byteCounter[256];
+    const int windowWidth = 20;
+    for(int i = 0; i < sizeof(sample) - windowWidth; i++)
+    {
+        memset(byteCounter, 0, sizeof(unsigned int) * 256);
+        char *p = &sample[i];
+        char *end = &sample[i + windowWidth];
+        while(p != end)
+        {
+            byteCounter[(unsigned char)(*p++)]++;
+        }
+        float entropy = calculateEntropy(byteCounter, windowWidth);
+        printf("%d - %.3f\n", i, entropy);
+    }
+}
+float calculateEntropy(unsigned int bytes[], int length)
+{
+    float entropy = 0.0f;
+    for (int i = 0; i < 256; i++)
+    {
+        if (bytes[i] != 0)
+        {
+            float freq = (float) bytes[i] / (float) length;
+            entropy += -freq * log2f(freq);
+        }
+    }
+    return entropy;
+}
+</sxh>