CRC16 Size and Speed Part 1

// crc16_comparison.c
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>

// CRC-16-CCITT polynomial: 0x1021
#define CRC16_POLY 0x1021

// Pre-computed CRC-16 lookup table
static const uint16_t crc16_table[256] = {
    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
    0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
    0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
    0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
    0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
    0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
    0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
    0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
    0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
    0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
    0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
    0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
    0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
    0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
    0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
    0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
    0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
    0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
    0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
    0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
    0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
    0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
    0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
    0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
    0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
    0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
    0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
    0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
    0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
    0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
    0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
    0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0
};

// Table-based CRC-16 calculation
uint16_t crc16_table_based(const uint8_t *data, size_t length) {
    uint16_t crc = 0xFFFF;  // Initial value
    
    for (size_t i = 0; i < length; i++) {
        uint8_t index = (crc >> 8) ^ data[i];
        crc = (crc << 8) ^ crc16_table[index];
    }
    
    return crc;
}

// Algorithmic CRC-16 calculation
uint16_t crc16_algorithmic(const uint8_t *data, size_t length) {
    uint16_t crc = 0xFFFF;  // Initial value
    
    for (size_t i = 0; i < length; i++) {
        crc ^= (uint16_t)data[i] << 8;
        
        for (int bit = 0; bit < 8; bit++) {
            if (crc & 0x8000) {
                crc = (crc << 1) ^ CRC16_POLY;
            } else {
                crc = crc << 1;
            }
        }
    }
    
    return crc;
}

// Benchmark function
void benchmark(const char *name, uint16_t (*crc_func)(const uint8_t*, size_t),
               const uint8_t *data, size_t length, int iterations) {
    clock_t start, end;
    double cpu_time_used;
    uint16_t crc = 0;
    
    start = clock();
    for (int i = 0; i < iterations; i++) {
        crc = crc_func(data, length);
    }
    end = clock();
    
    cpu_time_used = ((double)(end - start)) / CLOCKS_PER_SEC;
    
    printf("%s:\n", name);
    printf("  CRC Result: 0x%04X\n", crc);
    printf("  Time: %.6f seconds\n", cpu_time_used);
    printf("  Iterations: %d\n", iterations);
    printf("  Time per iteration: %.9f seconds\n", cpu_time_used / iterations);
    printf("  Throughput: %.2f MB/s\n\n", 
           (double)(length * iterations) / (cpu_time_used * 1024 * 1024));
}

int main(void) {
    // Test data sizes
    const size_t small_size  = 100;
    const size_t medium_size = 10000;
    const size_t large_size  = 1000000;
    
    uint8_t *small_data = malloc(small_size);
    uint8_t *medium_data = malloc(medium_size);
    uint8_t *large_data = malloc(large_size);
    
    // Initialize with pseudo-random data
    for (size_t i = 0; i < small_size; i++) small_data[i] = (uint8_t)(i * 7);
    for (size_t i = 0; i < medium_size; i++) medium_data[i] = (uint8_t)(i * 13);
    for (size_t i = 0; i < large_size; i++) large_data[i] = (uint8_t)(i * 17);
    
    printf("=== CRC-16 Implementation Comparison ===\n\n");
    
    // Verify both implementations produce same result
    uint16_t crc_table = crc16_table_based(medium_data, medium_size);
    uint16_t crc_algo = crc16_algorithmic(medium_data, medium_size);
    
    printf("Verification (10KB data):\n");
    printf("  Table-based CRC: 0x%04X\n", crc_table);
    printf("  Algorithmic CRC: 0x%04X\n", crc_algo);
    printf("  Match: %s\n\n", (crc_table == crc_algo) ? "YES" : "NO");
    
    // Benchmark small data
    printf("=== Small Data (100 bytes) ===\n");
    benchmark("Table-based", crc16_table_based, small_data, small_size, 1000000);
    benchmark("Algorithmic", crc16_algorithmic, small_data, small_size, 1000000);
    
    // Benchmark medium data
    printf("=== Medium Data (10 KB) ===\n");
    benchmark("Table-based", crc16_table_based, medium_data, medium_size, 10000);
    benchmark("Algorithmic", crc16_algorithmic, medium_data, medium_size, 10000);
    
    // Benchmark large data
    printf("=== Large Data (1 MB) ===\n");
    benchmark("Table-based", crc16_table_based, large_data, large_size, 100);
    benchmark("Algorithmic", crc16_algorithmic, large_data, large_size, 100);
    
    free(small_data);
    free(medium_data);
    free(large_data);
    
    return 0;
}