/**
 * @file crc32.c
 * @brief CRC32 Implementation (IEEE 802.3)
 */

#include <stdint.h>
#include <stddef.h>
#include <stdbool.h>

/* IEEE 802.3 CRC32 polynomial (reversed) */
#define CRC32_POLY  0xEDB88320UL

/* CRC32 lookup table (256 entries) */
static uint32_t crc32_table[256];
static bool crc32_table_init = false;

/**
 * Initialize CRC32 lookup table
 */
static void crc32_init_table(void) {
    if (crc32_table_init) return;
    
    for (uint32_t i = 0; i < 256; i++) {
        uint32_t crc = i;
        for (int j = 0; j < 8; j++) {
            if (crc & 1) {
                crc = (crc >> 1) ^ CRC32_POLY;
            } else {
                crc >>= 1;
            }
        }
        crc32_table[i] = crc;
    }
    crc32_table_init = true;
}

/**
 * Calculate CRC32 (IEEE 802.3 polynomial)
 * 
 * @param data Input data
 * @param len Input length
 * @return CRC32 value
 */
uint32_t chunk_crc32(const uint8_t *data, size_t len) {
    crc32_init_table();
    
    uint32_t crc = 0xFFFFFFFFUL;  /* Initial value */
    
    for (size_t i = 0; i < len; i++) {
        uint8_t index = (crc ^ data[i]) & 0xFF;
        crc = (crc >> 8) ^ crc32_table[index];
    }
    
    return crc ^ 0xFFFFFFFFUL;  /* Final XOR */
}

/**
 * Update CRC32 with additional data
 * 
 * @param crc Previous CRC value (already XORed with 0xFFFFFFFF)
 * @param data New data
 * @param len Data length
 * @return Updated CRC value
 */
uint32_t chunk_crc32_update(uint32_t crc, const uint8_t *data, size_t len) {
    crc32_init_table();
    
    /* Undo final XOR from previous calculation */
    crc ^= 0xFFFFFFFFUL;
    
    for (size_t i = 0; i < len; i++) {
        uint8_t index = (crc ^ data[i]) & 0xFF;
        crc = (crc >> 8) ^ crc32_table[index];
    }
    
    return crc ^ 0xFFFFFFFFUL;  /* Final XOR */
}