// Standard CRC64-ECMA reference implementation in C
// Compiles with: gcc -std=c99 crc64_ref.c -o crc64_ref

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>

#define CRC64_ECMA 0x42F0E1EBA9EA3693ULL

static uint64_t crc64_table[256];

void crc64_init_table(void) {
    for (int i = 0; i < 256; i++) {
        uint64_t crc = (uint64_t)i << 56;
        for (int j = 0; j < 8; j++) {
            if (crc & 0x8000000000000000ULL) {
                crc = (crc << 1) ^ CRC64_ECMA;
            } else {
                crc <<= 1;
            }
        }
        crc64_table[i] = crc;
    }
}

uint64_t crc64_checksum(const uint8_t *data, size_t len) {
    uint64_t crc = 0ULL;
    for (size_t i = 0; i < len; i++) {
        uint8_t byte = data[i];
        crc = crc64_table[(uint8_t)((crc >> 56) ^ byte)] ^ (crc << 8);
    }
    return crc;
}

int main(int argc, char *argv[]) {
    crc64_init_table();

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <action> [data...]\n", argv[0]);
        fprintf(stderr, "  checksum <hexstring>  - compute CRC64 of hex data\n");
        fprintf(stderr, "  table                 - print first 16 table entries\n");
        return 1;
    }

    const char *action = argv[1];

    if (strcmp(action, "table") == 0) {
        printf("CRC64_ECMA table (first 16):\n");
        for (int i = 0; i < 16; i++) {
            printf("  [%d] = 0x%016llx\n", i, (unsigned long long)crc64_table[i]);
        }
        return 0;
    }

    if (strcmp(action, "checksum") == 0) {
        const char *hexstr = (argc > 2) ? argv[2] : "";
        size_t hexlen = strlen(hexstr);

        if (hexlen == 0) {
            // Empty input
            uint64_t crc = crc64_checksum(NULL, 0);
            printf("%016llx\n", (unsigned long long)crc);
            return 0;
        }

        if (hexlen % 2 != 0) {
            fprintf(stderr, "Error: hex string must have even length\n");
            return 1;
        }

        size_t datalen = hexlen / 2;
        uint8_t *data = malloc(datalen);
        if (!data) {
            fprintf(stderr, "Error: memory allocation failed\n");
            return 1;
        }

        for (size_t i = 0; i < datalen; i++) {
            unsigned int byte;
            if (sscanf(&hexstr[i * 2], "%2x", &byte) != 1) {
                fprintf(stderr, "Error: invalid hex character\n");
                free(data);
                return 1;
            }
            data[i] = (uint8_t)byte;
        }

        uint64_t crc = crc64_checksum(data, datalen);
        printf("%016llx\n", (unsigned long long)crc);
        free(data);
        return 0;
    }

    fprintf(stderr, "Error: unknown action '%s'\n", action);
    return 1;
}