Entrance Test Assignment

Project Description

This project implements the Gehtsoft Entrance Test Assignment with two console applications:

1. • Caesar Cipher - Text encryption/decryption with support for Russian (Cyrillic) and English (Latin) alphabets.
   • Caesar Cipher Breaker - Frequency analysis-based decryption without shift value

2. Arithmetic Expression Evaluator - Mathematical expression parser with PEMDAS/BODMAS precedence

The implementation follows modular architecture:

• cipher/ - Caesar cipher logic with language detection
• calculator/ - Expression parsing and evaluation engine
• ui/ - Console interface with menu-driven workflow
• utils/ - Input/output utilities and stream handling

How to Compile and Run

Compilation

Requirements: Java 11 or higher

javac -encoding UTF-8 -d bin/main -sourcepath src/main/java $(find src/main/java -type f -name "*.java")

Execution

java -cp bin/main assessment.Main

Approach and Implementation Details

Caesar Cipher

The Cipher class implements the classic Caesar Cipher , supporting both English (Latin) and Russian (Cyrillic) alphabets. It correctly handles:

• Case preservation
• Wrap-around logic
• Positive/negative shifts
• Non-alphabetic characters are left unchanged

File Operations:

• Supports file input/output with automatic _out.txt suffix
• Uses FileLineStreamIO for line-by-line processing (BufferedReader, BufferedWriter)

Cipher Breaking

The CipherBreaker class extends Cipher and adds functionality to automatically decrypt text without knowing the shift value by using:

• Language detection based on character frequency

• Statistical scoring of decrypted candidates

• Frequency analysis against known language profiles (English/Russian)

• Vowel percentage checks to evaluate likelihood of meaningful text File Operations:

• Uses FileIO, that provides convenience methods for reading/writing entire file contents at once. Useful for small files. Assumptions

• Only Cyrillic (Russian) and Latin (English) alphabets are processed; other characters remain unchanged.

• Input may be a mix of both languages, but output assumes one dominant language.

• Shift can be any integer (positive, negative, or zero).

• If input is too short or appears random, automatic decryption might fail.

• No external dictionaries or NLP tools are used — only statistical models (LanguageProfile) are used.

File I/O Limitations and Constraints

• Input and output file paths must be valid and accessible from the current working directory. Encoding : The code assumes that input files are encoded in the UTF-8 encoding. There is no explicit encoding specification during file operations.
• Line endings : Line breaks are preserved during file-based encryption/decryption.
• Large files : While FileLineStreamIO is suitable for large files, there's no progress indicator or cancellation mechanism. FileIO is useful for small files
• Overwriting files : Writing to an existing file will overwrite its contents without warning.
• Empty files : Attempting to encrypt or decrypt an empty file results in an empty output file.
• Binary files : This implementation is intended for text files only . Processing binary files will not work as expected.

Expression Evaluator

The ExpressionEvaluator module uses a recursive descent parser to evaluate arithmetic expressions. It supports:

• Basic operations: +, -, *, /
• Parentheses for precedence control
• Decimal numbers and negative values

Key components:

• TokenAnalyzer : Tokenizes the input string into numbers, operators, and parentheses.
• TokenBuffer : Provides sequential access to tokens with backtracking.
• Recursive parsing methods :
  • expression() – top-level entry point
  • plusminus() – addition and subtraction
  • multdiv() – multiplication and division
  • factor() – number, parenthesis, unary minus

Assumptions

• Expressions follow standard infix notation.
• All numbers are valid decimal numbers (with optional negative sign).
• Division by zero throws an exception.
• Unary minus is allowed anywhere a factor is expected.
• Whitespace is ignored during tokenization.
• Nested parentheses are fully supported.
• Only basic operators and parentheses are supported; advanced functions like sqrt, sin, etc., are not included.

Examples of Usage

Caesar Cipher Encryption

Enter text to encrypt: Hello World
Enter shift value: 3
Encrypted text: Khoor Zruog

Caesar Cipher Decryption

Enter ciphertext: Тулпзу
Enter shift value: 3
Decrypted text: Пример

Cipher Breaking (Automatic)

Enter encrypted text: Drkxu iye kxigki!
[Warning] The input is too short or contains too few letters.
Frequency analysis may not work correctly and result could be incorrect.
Decrypted text: Thank you anyway!

Arithmetic Evaluation

Enter expression: (2 + 3) * 4 / 2 - 1
Result: 9.0

File Processing

Read input from file? (y/n): y
Enter input file path: test.txt
Enter output file path (default test_out.txt):
Processing complete. Check test_out.txt for results.

Sample Menu Flow

Welcome to Gehtsoft Technical Assessment
Please choose an option:
1. Caesar Cipher Encryption
2. Caesar Cipher Decryption
3. Caesar Cipher Breaking (Frequency Analysis)
4. Arithmetic Expression Evaluation
5. Exit

Project Structure

src/
├── main/
│   └── java/
│       └── assessment/
│           ├── Main.java
│           ├── cipher/
│           │   ├── Cipher.java        # Core encryption/decryption
│           │   ├── CipherBreaker.java # Frequency analysis-based decryption
│           │   ├── LanguageProfile.java # Language-specific frequency data
│           │   └── tests/             # Test files and resources
│           ├── calculator/
│           │   ├── ExpressionEvaluator.java # Main evaluator logic
│           │   ├── Token.java         # Token representation
│           │   ├── TokenAnalyzer.java # Tokenization engine
│           │   ├── TokenBuffer.java   # Token stream management
│           │   └── tests/             # Test files and resources
│           ├── ui/
│           │   └── ConsoleUI.java     # Menu-driven interface
│           └── utils/
│               ├── ScannerUtils.java  # Input validation
│               ├── FileIO.java        # File handling utilities
│               └── FileLineStreamIO.java # File handling utilities