🗃️ Database Implementation using Extendible Hash Table in C — Block-Level Record Management

This project implements an extendible hash table over a block-level storage manager, simulating the internal mechanics of a database system. Developed for the course K18 - Υλοποίηση Συστημάτων Βάσεων Δεδομένων, it focuses on efficient storage, retrieval, and indexing using extendible hashing principles and a provided block file (BF) layer.

Note:

• Check the report.txt and DB_askisi_1_2021-2022.pdf files for more details.

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📅 Course Info

• Course: K18 - Υλοποίηση Συστημάτων Βάσεων Δεδομένων
• Semester: Χειμερινό 2021–2022
• Instructor: Καθηγητής Ι. Ιωαννίδης

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🧑‍💻 Team

• Δημήτριος Χρυσός – 1115202100275
• Αναστάσιος Μουμουλίδης – 1115202100108
• Παναγιώτης Τσελίκας – 1115202100192

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🔧 Helper Functions

• uint32_t hash(char* data, size_t bytes): FNV-1a hash
• int hash_function(int id, int global_depth): Trims hash to match directory
• void double_hash(void* header_info): Doubles hash table, increases global depth
• void save_Hash_table(void* header_info): Persists hash table across blocks
• void print_HashTable(int* table, int size): Debug tool
• void dec2bin_string(unsigned int, char*) & int bin_string2dec(char*): Conversions

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✅ Completed Functionality

We implemented all core HT_* functions:

• HT_Init, HT_CreateIndex, HT_OpenIndex, HT_CloseFile
• HT_InsertEntry, HT_PrintAllEntries, HashStatistics
• Includes modular testable implementation, robust unpinning, and LRU buffer handling.

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📚 Project Description

The system supports:

• Creation and management of extendible hash files
• Insertion, lookup, and statistics collection on stored records
• Low-level control using BF_Block memory primitives
• Handling records of type:

typedef struct {
  int id;
  char name[15];
  char surname[20];
  char city[20];
} Record;

It utilizes a block cache system (BF layer) with LRU/MRU replacement and operates within strict memory management constraints.

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🗂️ Project Structure

project/
├── bin/                     # Compiled executables
├── build/
│   └── runner/             # Build artifacts
├── examples/
│   ├── bf_main.c           # Example usage of BF layer
│   └── ht_main.c           # Main driver for testing hash table functions
├── include/
│   ├── bf.h                # Given BF block manager header
│   ├── hash_file.h         # API for high-level hash table operations
│   └── hash_table.h        # Internal hash table structures and helpers
├── lib/
│   └── libbf.so            # Provided shared library for BF system
├── src/
│   ├── hash_file.c         # Implementation of HT_* functions
│   └── hash_table.c        # Lower-level hash logic and bucket management

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⚙️ Compilation

Build the entire project using:

make

To clean compiled binaries:

make clean

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📈 Hashing & Storage Logic

• The global depth determines directory size.
• Buckets split when full, and local depths are updated.
• File's first block stores metadata.
• All reads/writes go through BF_GetBlock, BF_UnpinBlock, and BF_SetDirty.

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🧩 Design Internals & Assumptions

• Global Depth Initialization: We initialize global_depth = 1.
• Hash Function: We use FNV-1a hash function. After hashing:
  • Convert result to binary string.
  • Take the global_depth least significant bits.
  • Convert to decimal — this is the hash value.
• Hash Table Management:
  • Stored in custom blocks with HT_blocks metadata.
  • When full, double_hash() is called to double the table size and increase global depth.
• Custom Assumptions:
  • In HashStatistics, we pass int* indexDesc instead of char* filename, per eclass clarification.
  • Block 0 (header) holds hash table metadata and root configuration.
  • Each record block has its own local depth.
  • Hash table entries are stored across multiple blocks if needed.

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