Migrate COBOL merge sort to Java with identical output validation

[devin-ai-integration]

Migrate COBOL merge sort to Java with identical output validation

Summary

This PR migrates the COBOL merge sort functionality from merge_sort_test.cbl to Java while maintaining identical file-based sorting behavior. The original COBOL program creates test data files, merges them by customer ID (ascending), then sorts by contract ID (descending). The Java implementation replicates this exact workflow and has been validated to produce character-for-character identical output.

Key Components:

• Customer.java: Data class implementing Comparable interface with fixed-width format parsing
• MergeSortExample.java: Main implementation with merge algorithm and file operations
• README.md: Comprehensive migration documentation with validation results

Validation: Both COBOL and Java outputs verified identical using diff comparison.

Review & Testing Checklist for Human

• Verify Customer.fromString() parsing logic - Check substring indices (0-5, 5-55, 55-105, 105-110, 110-135) handle the 135-character fixed-width format correctly
• Test with larger/different datasets - The validation only used the small built-in test data; verify behavior with edge cases and larger files
• Verify toString() formatting exactness - Confirm the printf-style formatting %05d%-50s%-50s%05d%-25s produces identical output to COBOL's PIC clauses
• Review merge algorithm correctness - Check the two-pointer merge implementation in mergeFiles() for edge cases and proper handling of duplicate keys
• Test error handling - Verify behavior with malformed input files, missing files, and invalid data formats

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    COBOL["merge_sort_test.cbl<br/>Original COBOL Implementation"]:::context
    Customer["Customer.java<br/>Data class with parsing"]:::major-edit
    MergeSort["MergeSortExample.java<br/>Main implementation"]:::major-edit
    README["README.md<br/>Documentation"]:::major-edit
    
    TestFile1["test-file-1.txt<br/>East customers"]:::context
    TestFile2["test-file-2.txt<br/>West customers"]:::context
    MergedFile["merge-output.txt<br/>Merged by customer ID"]:::context
    SortedFile["sorted-contract-id.txt<br/>Sorted by contract ID desc"]:::context
    
    COBOL --> TestFile1
    COBOL --> TestFile2
    COBOL --> MergedFile
    COBOL --> SortedFile
    
    Customer --> MergeSort
    MergeSort --> TestFile1
    MergeSort --> TestFile2
    MergeSort --> MergedFile
    MergeSort --> SortedFile
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

classDef major-edit fill:#90EE90
classDef minor-edit fill:#87CEEB
classDef context fill:#FFFFFF

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Notes

• The COBOL program uses built-in MERGE and SORT statements rather than implementing traditional merge sort algorithms
• Fixed-width record format (135 characters) must be preserved exactly for compatibility
• String parsing logic is critical - any off-by-one errors in substring indices will break compatibility
• The merge algorithm assumes input files are already sorted by customer ID (matching COBOL MERGE behavior)

Link to Devin run: https://app.devin.ai/sessions/6ca2095c12764cc9baffda0489888b2e
Requested by: Samir Chaudhry (@schaudhry123)

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