Migrate COBOL merge sort functionality to Java

[devin-ai-integration]

Migrate COBOL merge sort functionality to Java

Summary

This PR implements a complete Java migration of the existing COBOL merge sort functionality in merge_sort_test.cbl. The migration maintains exact file-based sorting behavior and produces identical output to the original COBOL implementation.

Key Components:

• CustomerRecord.java: Data class with fixed-width string formatting (135 chars: 5+50+50+5+25)
• MergeSortMigration.java: Main application implementing merge by ascending customer ID, then sort by descending contract ID
• LargeDatasetTest.java: Validation test with 1000+ records to verify streaming behavior
• README.md: Comprehensive documentation with validation results

Processing Flow:

1. Create test data in two files (11 total customer records)
2. Merge files by ascending customer ID → merge-output.txt
3. Sort merged file by descending contract ID → sorted-contract-id.txt

Review & Testing Checklist for Human

• Critical: Verify output files match COBOL exactly - Run both programs and diff the output files byte-for-byte
• Test edge cases - Empty files, malformed records, very large datasets (10K+ records)
• Review fixed-width formatting logic - The toFixedWidthString() and fromFixedWidthString() methods are critical for compatibility
• Validate sort correctness - Ensure merge sorts by customer ID ascending, final sort by contract ID descending
• Test error handling - Verify graceful handling of file I/O errors, missing files, invalid data

Recommended Test Plan:

1. Run original COBOL program: cobc -x merge_sort_test.cbl && ./merge_sort_test
2. Run Java migration: javac *.java && java MergeSortMigration
3. Compare outputs: diff merge-output.txt and diff sorted-contract-id.txt
4. Test large dataset: java LargeDatasetTest
5. Test with custom data files to verify parsing logic

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    COBOL["merge_sort_test.cbl<br/>(Original COBOL)"]:::context
    CR["CustomerRecord.java<br/>(Data Model)"]:::major-edit
    MSM["MergeSortMigration.java<br/>(Main Logic)"]:::major-edit
    LDT["LargeDatasetTest.java<br/>(Validation)"]:::major-edit
    README["README.md<br/>(Documentation)"]:::major-edit
    
    TF1["test-file-1.txt"]:::context
    TF2["test-file-2.txt"]:::context
    MO["merge-output.txt"]:::context
    SO["sorted-contract-id.txt"]:::context
    
    COBOL --> TF1
    COBOL --> TF2
    COBOL --> MO
    COBOL --> SO
    
    MSM --> CR
    MSM --> TF1
    MSM --> TF2
    MSM --> MO
    MSM --> SO
    
    LDT --> CR
    
    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:#F5F5F5

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Notes

• Validation Status: All outputs match COBOL program exactly (console output, merge file, sort file)
• Performance: Successfully tested with 1000+ records using streaming approach
• Compatibility: Maintains COBOL's line-sequential file organization and fixed-width record format
• Session Info: Link to Devin run: https://app.devin.ai/sessions/c5f5ce48539748d78f3f92581b689db1, Requested by: Samir Chaudhry (@schaudhry123)

Key Risk Areas for Review:

• Fixed-width string parsing logic could have subtle bugs with padding/trimming
• Sort stability and order correctness (especially with duplicate values)
• File I/O exception handling compared to COBOL file status checks
• Memory efficiency claims need verification with production-sized datasets

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