Fix critical bugs, optimize performance, add test suite

[ghostyappzeta]

This pull request was generated by @kiro-agent 👻

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Summary

A thorough exploration of the Huffman Coding codebase identified several gaps and optimization opportunities. This PR addresses them across three areas:

1. Critical Bug Fixes

• Decoder padding corruption: Encoder now writes total symbol count to code_table.txt; decoder stops after decoding exactly that many symbols instead of producing garbage from padding bits
• Single-symbol crash: All three build_tree_* methods detect single-symbol input and wrap the leaf in an internal node
• Empty input crash: Both Encoder and Gen_huffman_code validate for empty input gracefully
• Static count accumulation: count is now reset to 0 at the start of build_freq_table()
• Resource leaks: All BufferedReader instances wrapped in try-with-resources
• Input validation: NumberFormatException caught with line-number context; negative integers detected and reported

2. Performance and Code Quality

• Wasteful allocations removed: Eliminated new Node(0,-1) temp variables that were immediately overwritten in all heap swap/del_min operations
• Recursive to iterative: manage_heap_upwards and manage_heap_downwards in BinaryHeap and D_aryHeap converted from tail recursion to while loops (prevents StackOverflowError on large heaps)
• Comparison operators: Changed >= to > in manage_heap_upwards to avoid unnecessary swaps on equal frequencies
• String concatenation optimized: gen_codes() and fillNwrite_code_table() now use StringBuilder with backtracking instead of code+"0" / code+"1"
• Dead code removal: Removed stub() methods and commented-out debug code
• Java naming conventions: Renamed encoder to Encoder, decoder to Decoder
• Encapsulation: heap_size made private with getSize() accessor

3. Automated Test Suite (41 JUnit 5 tests, previously zero)

Test Class	Tests	Coverage
BinaryHeapTest	8	Insert/extract ordering, empty state, single element, equal frequencies, 100 random elements
D_aryHeapTest	9	Same patterns + d=2, d=4, d=8 variants
PairingHeapTest	8	Insert/extract ordering, all heap operations
HuffmanTreeTest	8	Prefix-free property (all 3 builders), frequency-code-length relationship, single/two-symbol trees
RoundtripTest	4	Full encode/decode pipeline for various inputs
EdgeCaseTest	4	Empty input, single line, large numbers, all-same-symbol

Testing

• Build compiles cleanly with make
• All 41 JUnit 5 tests pass
• Encode/decode roundtrip verified for small, large (10M lines), single-symbol, two-symbol, and multi-symbol cases
• Edge cases (empty file, non-integer input, negative numbers) produce user-friendly messages

Files Changed

16 files changed, 1,060 insertions, 232 deletions