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Asked by: Test User 100300
Created at: 2025-05-19 17:42:13
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### **Step 1: Introduction to Entropy Encoding Compression Methods**
Entropy encoding is a lossless data compression scheme that assigns shorter codes to more frequent symbols and longer codes to less frequent symbols, reducing the average code length per symbol and thus compressing the data efficiently. Two common entropy encoding methods are **Shannon-Fano encoding** and **Huffman encoding**.
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### **Step 2: Answer (a) - Difference between Shannon-Fano and Huffman Encoding**
- **Shannon-Fano Encoding:**
- Constructs the code by recursively dividing the set of symbols into two groups with (almost) equal total probabilities and assigning 0 or 1 to each group.
- May not always produce an optimal prefix code.
- **Huffman Encoding:**
- Builds a binary tree by repeatedly combining the two least probable symbols/nodes until only one node is left.
- Always produces an optimal prefix code with the shortest average code length.
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### **Step 3: Answer (b) - Generate Codewords using Huffman Encoding**
Let's use **Huffman Encoding** for this step, as it's more optimal.
#### **Given Probabilities:**
| Symbol | Probability |
|--------|-------------|
| A | 0.25 |
| B | 0.10 |
| C | 0.15 |
| D | 0.05 |
| E | 0.30 |
| F | 0.05 |
| S | 0.10 |
#### **Step-by-step Huffman Encoding Process:**
1. **List Symbols (Lowest to Highest Probability):**
- D (0.05), F (0.05), B (0.10), S (0.10), C (0.15), A (0.25), E (0.30)
2. **Combine Lowest Probabilities:**
- D + F = 0.10
- New list: (D,F) 0.10, B 0.10, S 0.10, C 0.15, A 0.25, E 0.30
3. **Combine next lowest:**
- (D,F) + B = 0.20
- New list: (D,F,B) 0.20, S 0.10, C 0.15, A 0.25, E 0.30
4. **Combine next lowest:**
- S + C = 0.25
- New list: (D,F,B) 0.20, (S,C) 0.25, A 0.25, E 0.30
5. **Combine (D,F,B) + A = 0.45**
- New list: (S,C) 0.25, (D,F,B,A) 0.45, E 0.30
6. **Combine (S,C) + E = 0.55**
- New list: (D,F,B,A) 0.45, (S,C,E) 0.55
7. **Combine remaining:**
- (D,F,B,A) + (S,C,E) = 1.00
#### **Assign Huffman Codes:**
- Assign 0 to left, 1 to right at each branch.
| Symbol | Code |
|--------|---------|
| D | 000 |
| F | 001 |
| B | 01 |
| A | 10 |
| S | 110 |
| C | 1110 |
| E | 1111 |
(You can adjust the code assignment based on tree drawing, but this is one valid assignment.)
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### **Step 4: Answer (c) - Encode and Decode "CAEES"**
**Encoding:**
- C: 1110
- A: 10
- E: 1111
- E: 1111
- S: 110
Encoded string: **1110 10 1111 1111 110**
**Decoding:**
- Read bits and match against codewords:
- 1110 → C
- 10 → A
- 1111 → E
- 1111 → E
- 110 → S
Decoded string: **CAEES**
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### **Step 5: Answer (d) - Calculate Compression Ratio**
- **Original encoding:** ASCII uses 8 bits per character.
- "CAEES" = 5 characters × 8 bits = **40 bits**
- **Compressed encoding:**
- C (1110) = 4 bits
- A (10) = 2 bits
- E (1111) = 4 bits (×2 for two Es) = 8 bits
- S (110) = 3 bits
- **Total = 4 + 2 + 4 + 4 + 3 = 17 bits**
**Compression Ratio = Original size / Compressed size = 40 / 17 ≈ 2.35**
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### **Final Step: Very Short Final Answer**
Huffman encoding provides an optimal prefix code for entropy compression. Using Huffman encoding on the symbol probabilities, we encoded "CAEES" from 40 bits (ASCII) to 17 bits, achieving a compression ratio of about 2.35.
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### **Summary of Steps**
1. Introduced entropy encoding and its types.
2. Compared Shannon-Fano and Huffman encoding.
3. Used Huffman encoding to generate codewords for the symbols.
4. Encoded and decoded the string "CAEES" using the generated codes.
5. Calculated the compression ratio of the process.
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